essay on migratory birds

Bird migration is one of nature’s great wonders. Here’s how they do it.

Some fly 11 days nonstop. Others trek 8,000 miles. Each year, thousands of bird species leave home in search of food.

Every spring and fall, a spectacle unfolds in the night sky as millions of birds attempt long, perilous journeys between their summer breeding and wintering grounds.

Most of the thousands of bird species that engage in this annual migration travel at night, when wind currents are smoother and the moon and stars guide their way.

The birds typically follow established flyways , generally north-south routes that offer the best opportunities for rest and refueling along the way.   Multiple bird species share these flight paths as they contend with rough weather, dehydration, starvation, and the threat of predation. ( Read more about the legendary treks of migratory birds .)

Arctic terns , for instance, undertake pole-to-pole roundtrips spanning more than 60,000 miles —a record, believed to be   the world’s longest migration of any animal . Other migrations involve birds flying east-west or up and down mountains. Even flightless birds migrate, such as the Adélie penguin , which makes a nearly 8,000-mile trek through frigid Antarctica.

Because migration is such an integral part of the avian life cycle, it was likely almost as prevalent thousands of years ago as it is today, says Martin Wikelski , director of the Max Planck Institute for Ornithology and a National Geographic Explorer .

Why some birds migrate and others don’t is the focus of a complex and active field of research. Finding food generally is believed to be the main driver. Additional motivations could include to escape from inclement weather and to reduce exposure to predators or parasites, especially during breeding season.

New technological advances, such as sophisticated GPS tags and radar-detection systems, are giving scientists unprecedented opportunities to observe bird migration.

As part of his ICARUS project , for instance, Wikelski has outfitted some birds with Fitbit-like devices that track their movements and the environmental conditions they encounter.

These miniature solar-powered satellite transmitters could one day reveal animal migrations and behavior at a global scale from space.  

“There’s just so much to learn,” Wikelski says. “I’ve been tracking birds for over two decades, and the ease with which birds seamlessly migrate between worlds is absolutely astounding.”

Which birds migrate?

Roughly half of the world’s nearly 10,000 known bird species migrate, including several songbirds and seabirds, waterfowl and waders, as well as some raptors. The Northern Hemisphere has the most diverse array of migratory birds .

Among the most well known are Arctic-breeding bar-tailed godwits, champions of endurance. In 2020, scientists recorded a godwit undertaking the longest-known nonstop migratory flight between Alaska and New Zealand, traveling more than 7,500 miles across the Pacific Ocean for 11 days straight. ( Learn why birds matter, and are worth protecting.)

There are also feathered migrants that fly far and fast. The great snipe, for instance, covers distances exceeding 4,200 miles and reaches speeds of up to 60 miles per hour when traveling nonstop between Europe and sub-Saharan Africa, making it the fastest flying migratory bird.

Even tiny birds embark on gargantuan journeys.   Calliope hummingbirds—North America’s smallest bird—make 5,600-mile roundtrips between the high-elevation meadows and open forests of the northern Rockies and the pine-oak forests of Mexico.

Most species of migratory birds may be partial migrants , meaning that some populations or individuals within the species migrate while others stay put. A fraction of American robins, for example, remain near their breeding grounds across seasons while others travel south and then return north.

Yellow-eyed juncos breeding at high elevations along southeastern Arizona’s mountains are most likely to migrate up to a mile downslope during severe snowy winters, compared to those at lower elevations facing fewer food constraints. Even tropical birds , especially insectivores, undertake short-distance elevational trips.

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How do they know where to go  .

In addition to following celestial cues, such as the position of the sun, stars, and the moon, adult birds use a magnetic compass to navigate. Even when there are no landmarks, this internal “GPS system” can prevent them from getting lost.

Such navigational acumen can enable individual birds to move through regions not typically traveled. In experiments, when solo-flying common cuckoos were transported nearly 1,500 miles away from their breeding grounds prior to migration, they often steered back to their normal migratory routes.

But what about inexperienced birds migrating for the first time? In one experiment, geographically displaced young common cuckoos navigated back to roughly the same flight path used by those birds that weren't displaced from their home.   ( Read about amazing animal navigators .)

Whether this navigational capacity is inherited and innate or learned is an ongoing debate . “I think it’s a combination of innate tendency, but you learn from others on the way,” says Wikelski, who has been tracking common cuckoos since 2012.

One way to learn might be tuning into nocturnal flight calls from other migrating birds. Distinct from a bird species’ regular vocalizations, these acoustic signals could especially guide the inexperienced, sometimes even those of other species, Wikelski says.

How do they know it’s time to go?  

For some birds, changes in environmental conditions, such as the length of the day, may trigger migration by stimulating hormones, telling the birds it’s time to fly.

Birds’ internal biological clocks can also detect when a season shifts, using cues such as changes in light and possibly air temperature.

Once the birds are in migration mode, a feeding frenzy ensues. This allows the birds to accumulate fat to power their journeys, says Lucy Hawkes , a migration scientist at the U.K.’s University of Exeter who currently tracks Arctic terns.

“Somehow, [the birds] know that they have to migrate soon and get massive,” Hawkes says.

Local and regional weather conditions , such as rain, wind, and air temperatures can also influence decisions about when migratory birds take to the skies.

Migrating in a changing world  

Overall, migration schedules seem to be shifting, as a result of climate change . “It looks like bird migrations are commencing a little earlier in the spring,” says   Kyle Horton, an aeroecologist at the University of Colorado who uses radar technology to map realtime and historical bird migrations in the United States.

Black-throated blue warblers, for example, are migrating almost five days earlier now, on average, than they did in the 1960s. Canada-bound American robins are arriving 12 days earlier in the spring than they did in 1994. Migrating whooping cranes are showing up nearly 22 days earlier at their stopover site in Nebraska in the spring and leaving almost 21 days later in the fall than they did in the 1940s. ( Learn how climate change has affected the annual migration of the yellow warbler .)

Such early starts to migration may benefit birds if plant and insect productivity at the breeding grounds mirror the trend. However, not all migratory birds may be able to adapt to a warming world, and if they did, the full costs of doing so remain unclear.

As scientists continue to unravel the mysteries of bird migration, the phenomenon remains one of nature’s great wonders.  

“They’re flying all night, feeding all day, and doing it again,” Horton says. “That’s sort of remarkable."

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• Migratory Routes and Patterns
• Communication
• Feeding Behaviors
• Mating and Breeding
• Types of Migration
• Navigational Techniques
• Migration Challenges and Conservation
• Social Structures
• Learning and Intelligence
• Sleeping Habits and Behavior
• Bathing & Preening
• Human Interaction
• Understanding Migratory Routes
• Major Migratory Routes
• Patterns of Migration
• Conservation of Migratory Routes

Journeys Across the Globe

Each year, billions of migratory birds switch between spring/summer breeding grounds and overwintering grounds, although their movements vary greatly between species and populations. Their journey may be just a few miles into a nearby valley or thousands of miles across the breadth of an ocean, but no matter the route, these birds rely on seasonal migrations for survival.

While some birds undergo somewhat random and nomadic movements, many follow highly predictable routes year after year, so identifying and protecting these pathways is increasingly important for bird conservation in the modern world.

This guide introduces the fascinating secrets of bird migration routes and patterns. Read along with us to learn about the eight major flyways and much more!

Pictured: A small flock of Barnacle Geese in-flight

Definition and Importance

A migratory route is the pathway birds choose to move between their breeding and overwintering grounds. These may be very direct where long-distance water crossings are involved, but many other factors come into play for birds migrating over land.

Many birds select routes that avoid major obstacles like ocean crossings, high mountains, and habitats that don’t provide their natural food sources.

Other birds choose routes that follow specific habitats, and large soaring birds like Golden Eagles may take advantage of certain features like mountain ridges to assist their flight.

Pictured: A Golden Eagle. A migratory route is the pathway birds choose to move between their breeding and overwintering grounds

Most medium and long-distance migrants move between high and low latitudes to take advantage of the changing seasons. The major migratory routes between these areas are simplified into units known as flyways. Each flyway includes the breeding range, migratory path, and overwintering range of the latitudinal migrants that use them.

Continue reading to learn about the world’s eight major flyways.

North & South American Flyways

Many North and South American birds are latitudinal migrants that move north and south, either crossing between the continents or remaining on either one. These migratory birds typically use the following important migratory routes or flyways:

Pacific Americas Flyway

The Pacific Americas Flyway passes through 18 countries, running from Alaska in the north and along the west coast of the United States, passing through Mexico and reaching Chile in South America. This flyway is used by many bird species, including the Surfbird and the Violet-green Swallow.

Central Americas Flyway

The Central America’s Flyway extends through 27 countries from the Arctic zones of Canada in North America to the Southern tip of South America, passing through Central America and the central regions of each continent. This flyway is used by over 380 different bird species, including the Whooping Crane and the Buff-breasted Sandpiper.

A Surfbird standing on a rocky mountain close to the shore

A family of Whooping Cranes, two adults, and one juvenile (middle), preparing for take-off

Atlantic Americas Flyway

The Atlantic Americas Flyway is used by nearly 400 bird species, including familiar birds like the Baltimore Oriole and Summer Tanager.

This important flyway extends along the East coast of North and South America, from Greenland to the southernmost tip of South America.

Crossing between the continents involves following the coast of the Gulf of Mexico or crossing the open ocean, with or without stops in the Caribbean.

The four flyways of North America

While the three flyways mentioned above link migrants between North and South America, four flyways are typically recognized in the United States. From west to east, these are the Pacific Flyway, the Central Flyway, the Mississippi Flyway, and the Atlantic Flyway.

Close-up of a Baltimore Oriole perching on a branch

The Summer Tanager is a medium-sized songbird

Eurasian Flyways

Over in Europe, Asia, Africa, and Australia, birds follow broadly similar north/south migration patterns to those seen in the Americas. These migrations may occur within each continent or see birds travel extraordinary distances between East Asia and the Southern tip of Africa or even from Western Alaska to New Zealand.

Continue reading to learn about the major Old-World flyways.

East Atlantic Flyway

The East Atlantic Flyway is used by about 300 hundred bird species and connects about 75 countries on four different continents.

This migratory pathway brings birds from the Arctic of Canada and Greenland in the west and Northern Russia in the east to the United Kingdom and other parts of Europe. It also connects Europe with Western and Southern Africa.

Black Sea & Mediterranean Flyway

This flyway connects Northern Asia and Central and Southern Europe with much of Africa and Madagascar. About 300 bird species use this massive flyway, which includes roughly 100 countries.

Long-distance migrants using this flyway may cross the Mediterranean Sea into Africa or use the Isthmus of Suez in Egypt to avoid a water crossing.

Pictured: A European Honey Buzzard - this species migrate south for the winter to sub-Saharan and southern Africa

East Asia/East Africa Flyway

About 331 bird species use this flyway linking Southern, Central, and Eastern Africa with Asia and even Alaska. Birds that use this route include long-distance migrants that travel the entire length of the flyway, as well as intra-African migrants that move between Eastern and Southern Africa.

Central/South Asian Flyway

This short flyway links central and northern Asia with the Indian Subcontinent in the south, remaining within the boundaries of the world’s largest continent.

Birds that migrate along this flyway must negotiate the Himalayas, which are the highest mountains on the globe. Some of the over 300 species that use this route circle around the Himalayas, while others fly directly over this formidable barrier.

East Asian/Australasian Flyway

Nearly 500 species migrate along this far eastern flyway that extends from the Arctic zones of Alaska and Eastern Russia in the north and New Zealand in the south.

Species that move between these continents must make ocean crossings ranging from a few hundred to several thousand miles!

Nearly 500 species migrate along this far eastern flyway which include the Bar-tailed Godwit (pictured)

Bird Corridors

Natural habitats vary across the landscape due to various factors, including altitude, local weather patterns, geology, drainage, and, increasingly, human development.

Bird corridors link areas of suitable habitat and create suitable pathways for migratory birds at a much finer scale than flyways.

Migrating birds may follow natural corridors such as mountain ranges or wooded river courses in arid areas. Managed corridors of natural habits through fragmented urban, agricultural, and industrial areas may provide safe passage for migratory birds passing over-developed areas.

Pictured: A Prairie Merlin. Migrating birds may follow natural corridors such as mountain ranges or wooded river courses in arid areas

Cyclical Patterns

Bird migrations are generally highly cyclical, with well-defined nesting seasons, overwintering periods, and predictable migration times and routes between these important life stages. These natural rhythms and routes have evolved to maximize the bird's chances of successfully reproducing and then surviving the periods of migration and overwintering.

The timing of their movements may vary slightly due to natural variations, but bird movements are generally guided by hormonal changes, day length, and predictable patterns like snowmelt, the thawing of waterbodies, the budding of deciduous plants, and the emergence of insects.

Navigational Patterns

How birds stay on course during migration has long fascinated ornithologists, and through clever scientific study, some of the secrets of bird navigation have been revealed.

Birds navigate using a combination of cues, including:

• Position of the Sun
• The center of rotation in the night sky
• The Earth’s magnetic fields and intensity
• Memory of landmarks
• An innate sense of direction

While rapid changes in the environment may impact a bird's ability to navigate by landmarks, they have less impact on their other navigational techniques. Failing to adapt could cause birds to stick to migratory routes that pass through areas with fewer and fewer resources.

Pictured: Flock of Canada Geese during migration

Threats to Migratory Birds

Migration is an essential behavior for a large proportion of the world’s birds, although their journey is filled with dangers like predation, exhaustion, and extreme storms.

The impacts of human population growth and development have put increasing pressure on birds by damaging their habitats. Habitat destruction and fragmentation are particularly harmful to migratory birds because many rely on specific environments throughout their migration paths.

Climate change also affects migratory birds negatively by altering their habitats and the timing of seasonal events like insect emergence and plant budding.

These events are critical in the breeding cycle of migratory birds, and arriving late at their breeding grounds can increase competition and decrease their chance of successful reproduction.

Conservation Efforts

At a local level, conserving migratory birds requires focused efforts to protect important habitats along flyways and corridors. This includes everything from growing native plants that support hungry Hummingbirds to engaging with landowners and the proclamation and management of protected areas.

On a broader scale, the conservation of migratory birds requires collaboration between various nations, both neighboring and distant.

International conventions like the Migratory Bird Treaty Act and the CMS (Convention on the Conservation of Migratory Species of Wild Animals) promote cross-border conservation.

Pictured: A Rufous Hummingbird. Conserving migratory birds requires focused efforts to protect important habitats along flyways and corridors

Whether skirting mountains, following river courses, or committing to long open-water crossings, migratory birds are on the move all across the globe.

Various bird species have evolved to use different migratory pathways and corridors, both within political boundaries and across continents.

However challenging it may be, it’s vital that natural habitats all along the length of these flyways are protected if migrating birds are to survive.

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Audubon Adventures

Background for Teachers

Note: This Audubon Adventures topic focuses on the Bird Migration Explorer, the digital tool created by Audubon and its partners to visually document the migration journeys of North American birds. For additional background on bird migration, please read the Background for Teachers essay for the topic “Birds on the Move,” which covers bird migration in general. That Audubon Adventures unit is referred to as you and your students engage with the Bird Migration Explorer and the related classroom activity.

Humans have long been captivated by migratory birds, awed by the animals’ biannual treks between their breeding and wintering grounds. The Bird Migration Explorer is a tool that allows anyone to follow hundreds of species on their epic migratory journeys. Users can pore over the movements of individual species, discover the birds that spend time at a specific location, and learn about the challenges these far-flying creatures face. The Explorer was created by Audubon and nine founding partners using science contributed by hundreds of researchers and institutions. It paints the most complete picture ever of the journeys of 458 bird species that breed in the United States and Canada.

The Explorer homepage features a colorful map composed of routes of more than 9,300 birds captured by tracking devices and shared by scientists across the Western Hemisphere. As Melanie Smith, program director for the project, says, “You can see how birds trace the outlines of continents, rivers, lakes, mountain ridges.” The Explorer will help conservationists who are seeking to identify and protect the places migratory birds need as well as members of the public who are curious about their seasonal neighborhood visitors. For educators like you, it is a powerful way to help students understand migration through classroom participation as well as self-directed explorations in which they follow their curiosity, pose questions, analyze data, test hypotheses, and identify answers. These are the hallmarks of the work scientists do. In other words, the Explorer offers a STEM-infused approach to learning that will serve students well throughout their time in school and in their lives beyond as engaged and informed members of society.

Which Birds, Where, and When

When you choose a species in the Explorer, you can see an animated map that shows where these birds are at any given time throughout the year. You can also see a brief description of that species and its habitat. A key takeaway is the ability to see how birds that spend time near you rely on an array of habitats across the hemisphere. Another view of the map lets users see how a single tagged bird connects people and regions across time and space. And yet another lets users explore the various conservation challenges a species faces in different places. These attributes of the Explorer offer many opportunities for further investigation by your class as a whole and/or for projects for small groups or individual students.

Why It Matters

Birds play a significant role in every habitat and ecosystem. They are connected to the plants and other animals that share a place in various ways. Birds need plants for places to rest, hide, and nest. Some depend on plants for food. Some plants depend on them for pollination or seed dispersal. Some birds feed on other wild animals—from insects and worms to fish, snakes, and even other birds. Birds are healthy when the places they pass through or live are healthy. Other livings things, including humans, need those places to be healthy, too. Being able to see which birds are going where, how many of them there are, and whether their patterns are changing, is important information scientists can use to evaluate the environmental health of a given place, a region, a continent, and even planet Earth. That same information is valuable to policy makers at all levels, farmers and other land managers, civic planners, and individuals as they make decisions that support the well-being of those to whom they are accountable or for whom they are responsible or concerned. For detailed help using the Bird Migration Explorer, click the “How to Use the Explorer” button on the Teacher’s Guide page for this topic.

New text/images tk.

Our planet runs on energy. Energy is defined as the ability to do work, to make something happen. Energy allows flowers to bloom, birds to sing, cars to move, televisions and lights to turn on, and the wind to blow. We—and all living things—wouldn’t exist without energy.

Most of Earth’s energy originates from the sun. The sun lights our planet and heats land, water, and air. Warming caused by sunlight makes the wind blow and ocean currents circulate. Energy from the sun also powers solar cells and wind turbines to produce electricity. Solar energy powers the planet’s carbon cycle, where energy flows from the sun to plants to animals. Plants turn solar energy into carbohydrates (e.g., sugars and starches) through photosynthesis, and in the process they absorb carbon dioxide from the atmosphere and release oxygen. Animals—including humans—eat the sugars and starches stored in plant tissues and require oxygen to convert carbohydrates into energy to move and grow. This conversion is called cellular respiration. So, plants absorb carbon dioxide and release oxygen and animals absorb oxygen and release carbon dioxide. In that sense they are “breathing buddies.” When plants and animals die and decompose, the energy returns to the ground or oceans. Burning releases it back into the atmosphere.

Energy Sources: Renewable or Not?

America’s primary and secondary schools spend more on energy each year ($6 billion) than on textbooks and computers.

The sun is a natural, renewable energy source. It won’t burn out anytime soon! Other renewable sources of energy include wind (which results from the sun warming the Earth’s surface), biomass (energy stored in wood, some crops, and animal and plant waste), geothermal (heat deep within the earth), and moving water.

Renewable energy is all around us, but most of the energy we consume to grow our food, power our technology, drive our transportation systems, and fuel our buildings and industries comes from nonrenewable energy sources—sources that can’t be replaced. Coal, oil, natural gas, and nuclear energy are our main nonrenewable energy sources. The first three are fossil fuels—formed from plants and animals that died millions of years ago. Over millions of years, their remains were covered by sediments whose weight compressed and heated these layers of organic matter into coal (a shiny black rock), crude oil (a thick, tarry liquid), or natural gas (a bubbling gas).

Fossil Fuels = Warming Planet

Burning fossil fuels is a main cause of global warming. Just as we exhale carbon dioxide during cellular respiration, burning coal, oil, and natural gas releases carbon dioxide. Carbon dioxide is the main contributor to the “greenhouse effect,” in which solar energy reaches the Earth’s surface but the resulting heat is prevented from escaping back into outer space. Burning fossil fuels that release carbon dioxide into the atmosphere has caused Earth’s average temperature to rise faster than ever before in recorded history. (Fourteen of the fifteen hottest years on record have occurred since 2000, with 2015 the hottest so far.) This increase in global average temperature is why we call it global warming . That warming trend, in turn, affects other aspects of climate, such as precipitation, storm patterns and the severity of storms, and so on. Taken together, those changes are referred to as climate change .

Electricity is a convenient way to distribute energy to our houses, stores, factories, offices and other buildings. The electricity that turns on the lights in the classroom when you flip a switch comes via wires, and may be generated by power plants burning fossil fuels. Burning these fuels to produce electricity is directly responsible for climate change as well as contributing to air pollution. Coal-burning power plants emit 2.5 billion tons of carbon dioxide to the atmosphere each year, making them the number one source of atmospheric carbon in the United States. Gas-powered vehicles are number two, adding nearly 1.5 billion tons of carbon dioxide.

Food Production Eats Energy

Between 10 and 20 percent of the energy used in the United States goes into food production. Thus, what we eat can greatly affect climate. Much of our food comes from industrial-style farms dependent on fossil-fuel-burning equipment and intensive use of synthetic fertilizers and pesticides, which themselves are made using fossil fuels. Transporting food between farm and table—an average journey of 1,500 miles—also requires fossil fuels. One of the most energy-intensive foods is grain-fed beef: Each calorie of meat requires 35 calories of energy to grow the grain, process and transport the meat, and refrigerate it in stores and houses. Food packaging also gobbles energy, both in production and disposal. It accounts for a third of household trash by weight and 10 percent of the average grocery bill.

Throwing Away Oil

Americans dispose of 1.7 million plastic bottles every hour, 24 hours a day. Plastic is made from petroleum, a fossil fuel. Manufacturing the plastic requires electricity, which uses more fossil fuels; transporting the bottles uses gasoline or diesel, also fossil fuels. So every time we “trash” a plastic bottle, we’re literally throwing away energy and contributing to climate change in a big way. One good way to reduce this wastefulness is to stop buying bottled water and start carrying refillable containers of good old tap water! Almost all tap water in the United States is pure, tastes good, and is completely safe to drink.

Facing Reality

The facts are clear, and it’s important to face them: Climate change is real and produces negative consequences for life on Earth right now. Audubon’s 2014 “Birds and Climate Change Report” uses empirical observations to identify and project the effects of a changing climate on North American birds. (See “The Audubon Report at a Glance.” ) The findings are sobering. Audubon’s study found that 314 North American bird species, nearly half of those studied, are at risk from climate change. Of those, 126 are projected to lose more than 50 percent of their current range by 2050. Many birds will need to either adapt or move in response, and we don’t know for sure if those that move will find all the resources they need to survive and reproduce.

Birds are like the canary in the coal mine. Climate change is warming the oceans, raising sea levels, and melting polar ice, which means it is affecting the lives of people and countless animals and plants, right now.

Switching to renewable, nonpolluting sources of energy such as solar, wind, and geothermal energy will help slow global warming by slowing the rate greenhouse gases are added to the atmosphere. Energy conservation helps, too: The less energy we use, the less greenhouse gases we add to the atmosphere and the less damage we do to our planet and the other species we share it with.

Saving Pikas and Other Wildlife

Helping Children Understand Climate Change

The National Audubon Society agrees with the United Nations and the vast majority of scientists worldwide that climate change is a fact and that it is caused by human activities. The dual nature of climate change—people caused it; people can take steps to remedy it—can empower children rather than simply scaring them. In other words, yes, there’s trouble, but there’s also hope for the future because human beings are smart and resourceful. Having recognized the problem, we can work toward solving it.

Children (and adults, too!) are often confused about exactly what climate change is. One source of confusion is the difference between weather and climate. It’s not uncommon to hear someone blame an unusually hot day on global warming or climate change, for example. One hot day is a temporary weather phenonemon. Years of increasingly higher average temperatures over a large region point to global warming and climate change. Addressing misconceptions and distinguishing fact from opinion are both important aspects of teaching and learning. Because climate change is a phenomenon that will affect life on Earth in increasingly apparent ways, preparing yourself with the latest and most authoritative information will help you prepare students for the future they will inherit.

Photo: Tara Tanaka/Audubon Photography Awards

The Basics: How Birds Navigate When They Migrate

Related Stories

Staying On Course

Birds have a remarkable homing instinct, allowing them to return to the same area year after year, even when their migration takes them halfway around the world. How this remarkable feat is accomplished has been the topic of many studies.

Young birds

Research indicates that young birds that do not migrate with their parents have an innate knowledge of the direction and distance they should travel, but lack a specific goal. After it arrives at its wintering grounds, the young bird will select a winter range to which it imprints during that winter. After the first year the bird has the ability to return to the same area, even if blown off course during migration.

Adult birds

Adults seem to have even more homing skills. Two classic experiments illustrate this point.

Manx Shearwaters were flown by plane from their nesting island off the coast of Great Britain to two different locations. One group was released near Boston, MA, and another near Venice, Italy. Shearwaters do not fly over land so both groups must have taken an over water route, which would be especially convoluted from Venice. Both groups of birds returned to their nesting burrows within 14 days, covering approximately 250 miles per day. How they were able to achieve this remarkable return is not fully understood.

In another experiment, several hundred White-crowned Sparrows were captured in their winter grounds near San Jose, California. One group was flown to Baton Rouge, Louisiana, and released, while a second group was flown to Laurel, Maryland, and released. The following winter thirty-four of the birds were recaptured in the same 1/4 acre plot in California they had been captured in originally, presumably after having visited their northern breeding grounds during the summer.

Homing Pigeon Studies

Homing pigeons have been used extensively as test subjects in order to develop a better understanding of migration and homing abilities. They have exhibited almost unbelievable navigation skills.

In one noted experiment, German scientist Hans Wallraff transported homing pigeons to a very distant location. To ensure that the birds did not receive any external navigational information, they were transferred under stringent conditions. The pigeons were transported in closed, airtight cylinders and provided bottled air. Light was turned on and off at random times and loud white noise was played. The cylinders were enclosed in magnetic coils that provided a changing magnetic field. Finally, the cylinders were mounted on a tilting turntable connected to a computer that varied both the rotation and tilt of the cylinders. After release at the distant and completely unknown area, the birds were able to fly home to their roost, apparently without trouble (other than an initial case of nausea).

The pigeons’ ability to fly home from a totally strange and distant location indicates that somehow the birds have both an internal compass and an internal map. A compass by itself would not be helpful, since the bird would not know if it were north, south, east or west of its home. The question of how a bird has a map of a location to which it has never been before (and was transferred to under such isolated conditions in the above test) and the sense of the direction it must take to return home remains a puzzle. Some possible explanations have been proposed, as follows:

Internal Maps

The nose knows theory.

How could a bird possibly have a map of places it has never been? One very surprising theory suggests that homing pigeons may use an olfactory map.

Visualize a pigeon in its home loft with the smell of pine trees from one direction and the smell of an onion farm in another. If the bird moves closer to the pine trees, the odor of pine will presumably grow stronger while the odor of onions grows weaker. In theory, a gradient map of odors could be created that would provide some directional information, even if the pigeon were suddenly dropped into a new location. Floriano Papi and others from the University of Pisa initiated this theory and have some evidence that olfactory navigation may extend to a distance of 310 miles. This theory remains controversial.

Magnetic map theory

A second theory suggests that birds use the earth’s magnetic field to obtain at least a partial map of its position. The earth’s magnetic field becomes stronger as you travel away from the equator and toward the poles. In theory, a bird might be able to estimate its latitude based on the strength of the magnetic field. While the change in strength is very small from one location to the next, there is some indication that homing pigeons have the sensitivity to detect even tiny changes in the strength of the magnetic field. Even if true, this would provide only a limited indication of the bird’s latitude.

At this time there is no clear evidence that either of these theories is the complete story and the mapping skills of birds remains largely unexplained.

The Compass

The other half of the navigation requirement is the compass. The internal map provides a bird with the general location of where it is relative to its homing or migration goal and its internal compass guides its flight and keeps it on course. Migrating birds are apparently utilizing several different compasses.

The sun compass

In 1951 Gustav Kramer discovered the sun compass. He performed his experiments by placing European Starlings in orientation cages and then used mirrors to shift the apparent location of the sun. In response, the birds shifted their migratory restlessness to match the compass direction indicated by the apparent new position of the sun.

Further research revealed that the bird’s sun compass is tied to its circadian rhythm. It seems birds have a time compensation ability to make allowances for changes in the sun’s position over the course of the day. This theory is supported by another experiment in which pigeons were placed in a closed room with an altered cycle of light and dark. Over a period of a few days their circadian rhythm was reset. The birds were then released on a sunny day. Because their “internal clock” had been reset, they misinterpreted the position of the sun and made a predictable error in their homing direction. The pigeons actually ignore the position of the sun relative to its position in the sky, relying on its azimuth direction, i.e. the compass direction at which a vertical line from the sun intersects the horizon.

Further study has also revealed that pigeons have to learn the sun’s path to use it in navigation. Young pigeons allowed to see the sun only in the morning lack the ability to use the sun for navigation in the afternoon.

The star compass

The sun compass plays a role in homing and may be used by birds that migrate during the day. Many songbird species, however, migrate at night. For many years scientist suspected that birds use the stars for navigation. In 1957 Franz and Eleanor Saur collected data from a series of experiments in which birds were placed inside an enclosed planetary dome. The Saurs were able to demonstrate that birds do use the stars for migration but not, as it turns out, in the way they thought. The common belief at the conclusion of the Saur experiments was that birds have a genetically coded map of the stars. In 1967 Cornell scientist Stephen Emlen used Indigo Buntings to prove that the actual story was a little different.

Dr. Emlen also used a closed planetarium for his tests. He started by collecting young birds and then hand raising them in a lab.  His research included the following:

A. One group of birds was raised in a windowless room and was never exposed to a point source of light.

B. A second group also never saw the sun but was exposed on alternate nights to a simulated night sky in the planetarium, with normal rotation around the North Star.

C. A third group was also raised in a windowless room, but on alternate nights was exposed to a simulated night sky in the planetarium. In this case, the sky was manipulated to rotate about a different star, Betelgeuse.

When the fall migration period started, the birds were released into a special cage inside the planetarium.

Group A was placed in the planetarium under a normal fixed sky. The birds oriented themselves in random directions, showing no ability to recognize a southerly migration direction.

Group B was placed in the planetarium with a normal rotation around the North Star. The birds oriented themselves away from the North Star, in the appropriate southern direction for migration.

Group C was also placed into the planetarium. They had been raised with Betelgeuse as the central point of rotation. When exposed to a normal sky these birds oriented themselves away from Betelgeuse.

This research indicates that young birds do not learn star patterns themselves but learn a north-south orientation from a rotational star pattern.

The Magnetic Compass

Another German team did research with the European Robin in the early 1960s. In their tests, robins showing migratory restlessness were placed in covered cages to eliminate sun, star and other light clues. Despite the lack of visual clues, the robins were observed hopping in the correct migratory direction.

As an additional refinement to the test, a Helmholtz coil was placed around the covered cages. The coil allowed the researchers to shift the direction of the earth’s magnetic field. When the direction of the magnetic field was changed, the robins changed their hopping direction.

Further research indicates that while birds can sense the north and south ends of a compass, they cannot tell the difference between the two. To determine which direction is north, the birds apparently have the capability to sense that the magnetic lines of force align toward the poles of the earth. They can also detect the dip in the lines of force as they approach the earth and, through some currently unknown method, seem to be able to detect and make navigational decisions based on the dip angle.

The Sunset Cue

Patterns of polarized light also appear to play a key role in navigation. Many of the nocturnal migrants start their flights at sunset or a little after. Birds apparently use the polarized light patterns to provide information on initial migratory flight directions.

Birds that migrate during the day often follow, and may recognize, natural landforms such as mountain ranges, rivers, and lakes.

There is some indication that birds use multiple compass methods and calibrate them against each other. Some species use one type of compass as the primary navigational aid while others rely on a different primary system. The complexity of migration and the skill with which it is accomplished is one of the many marvels that make birds so interesting to study.

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Why migratory birds?

Avian migration is a natural miracle. Migratory birds fly hundreds and thousands of kilometres to find the best ecological conditions and habitats for feeding, breeding and raising their young. When conditions at breeding sites become unfavourable, it is time to fly to regions where conditions are better.

There are many different migration patterns. The majority of birds migrate from northern breeding areas to southern wintering grounds. However, some birds breed in southern parts of Africa and migrate to northern wintering grounds, or horizontally, to enjoy the milder coastal climates in winter. Other birds reside on lowlands during the winter months and move up a mountain for the summer.

Migratory birds have the perfect morphology and physiology to fly fast and across long distances. Often, their journey is an exhausting one, during which they go to their limits. The Red Knot has one of the longest total migration routes of any bird, travelling up to 16,000 kilometres twice a year. It breeds in Siberia and overwinters on the west coast of Africa, some even going down to the tip of South Africa.

It is truly amazing how migratory birds can navigate with pin-point accuracy. Exactly how migrating birds find their flyways is not fully understood. It has been shown that they are able to orientate by the sun during the day, by the stars at night, and by the geomagnetic field at any time. Some species can even detect polarized light, which many migrating birds may use for navigation at night.

Why Migratory Birds Need Protection

Migration is a perilous journey and involves a wide range of threats, often caused by human activities. And as diverse as people and their habits in different countries are, so are threats the birds face. As migratory birds depend on a range of sites along their distribution area, the loss of wintering and stopover sites could have a dramatic impact on the birds’ chances of survival.

Flying long distances involves crossing many borders between countries with differing environmental politics, legislation and conservation measures. It is evident that international cooperation among governments, NGOs and other stakeholders is required along the entire flyway of a species in order to share knowledge and to coordinate conservation efforts. The legal framework and coordinating instruments necessary for such cooperation is provided by multilateral environmental agreements such as CMS and AEWA .

World Migratory Bird Day has a global outreach and is an effective tool to help raise global awareness of the threats faced by migratory birds, their ecological importance, and the need for international cooperation to conserve them.

This site is maintained by the UNEP/CMS Secretariat and UNEP/AEWA Secretariat © 2006 - 2022     Disclaimer | Impressum UNEP/CMS and UNEP/AEWA Secretariat | Platz der Vereinten Nationen 1, 53113 Bonn, Germany | Tel. (+49 228) 815 2454, Fax. (+49 228) 815 2450 |  Contact

Essay on Migration of Birds

Students are often asked to write an essay on Migration of Birds in their schools and colleges. And if you’re also looking for the same, we have created 100-word, 250-word, and 500-word essays on the topic.

Let’s take a look…

100 Words Essay on Migration of Birds

Introduction.

Bird migration is a fascinating natural event. It is the regular seasonal journey undertaken by many species of birds.

Why Birds Migrate

How birds migrate.

Birds use a combination of the sun, stars, earth’s magnetic field, and landmarks to navigate during migration.

Challenges in Migration

Migration is not an easy task. Birds face threats like predators, harsh weather, and exhaustion.

250 Words Essay on Migration of Birds

Migration of birds is a complex and fascinating natural phenomenon. It involves the regular seasonal movement of birds, often north and south along a flyway, between breeding and wintering grounds.

The Process of Migration

Birds migrate to optimize their survival. During cold seasons, they move to warmer regions where food is abundant. The process is guided by several factors: genetic predisposition, day length, and changes in temperature. Birds navigate using celestial cues, the earth’s magnetic field, and landmarks.

Challenges and Adaptations

Migration is not without challenges. Birds face threats such as habitat destruction, climate change, and predation. To overcome these, they have evolved various adaptations. For instance, they accumulate fat reserves to fuel their long journeys and some species even sleep while flying.

Importance of Bird Migration

Bird migration has significant ecological implications. Migratory birds contribute to pollination, seed dispersal, and control of pests. Moreover, their migration patterns can indicate environmental changes, acting as bio-indicators.

Understanding bird migration is crucial for conservation efforts. As climate change disrupts migration patterns, studying and protecting these avian travelers becomes even more important. Indeed, bird migration is a testament to nature’s resilience and complexity, a spectacle that continues to captivate us.

500 Words Essay on Migration of Birds

Birds migrate primarily for two interconnected reasons: food availability and breeding. Many birds feed on insects, nectar, or other food sources that are abundant in certain seasons but scarce in others. To survive, they must move to areas where food is plentiful. Similarly, birds often migrate to specific locations to breed, driven by factors such as food abundance for their offspring, fewer predators, and suitable nesting sites.

Patterns of Migration

Bird migration is not a random occurrence but follows specific patterns. These patterns are influenced by geographical features, weather conditions, and the Earth’s magnetic field. Birds generally migrate along established routes known as flyways, which include coastal routes, mountain passes, and river valleys. These routes provide the necessary resources such as food and resting spots for the birds during their journey.

Despite the evolutionary advantages, bird migration is fraught with numerous challenges. Birds face threats from predators, harsh weather conditions, and exhaustion. Additionally, human activities such as habitat destruction, climate change, and light pollution pose significant threats. Many birds die during their migratory journey, making it a high-risk, high-reward strategy from an evolutionary perspective.

The Science Behind Bird Migration

Implications of bird migration.

Bird migration has significant ecological implications. Migratory birds can act as pollinators, seed dispersers, and even as a form of pest control. They also play a crucial role in the food chain. Additionally, bird migration has cultural and economic implications. Many societies celebrate the arrival and departure of migratory birds, and birdwatching is a popular and economically significant activity in many regions.

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A Brief History of How Scientists Have Learned About Bird Migration

Bird migration is one of the most fascinating and inspiring natural phenomena—but how do scientists figure out where all those birds are going?

From the earliest origins of bird banding to high-tech approaches involving genomic analysis and miniaturized transmitters, the history of bird migration research is almost as captivating as the journeys of the birds themselves. My book Flight Paths , forthcoming in 2023, will take a deep dive into the science behind these techniques and the stories of the people who developed them, but in the meantime, below you can read a selection of milestones that trace our unfolding understanding of migration.

Early History

Indigenous cultures develop a range of legends and stories about migratory birds. Athabascan peoples in Alaska, for example, tell the story of “Raven and Goose-wife,” in which Raven falls in love with a beautiful goose but cannot stay with her because he can’t keep up when the family of geese migrates south over the ocean.

While Aristotle correctly recognized some aspects of bird migration in his Historia Animalium in the 4th century, BC, he hypothesizes that swallows hibernate in crevices and that some winter and summer residents are actually the same birds in different plumages.

Inspired by Aristotle, Swedish priest Olaus Magnus suggests that swallows hibernate in the mud at the bottom of lakes and streams. This misconception will persist into the 1800s.

English minister and educator Charles Morton theorizes that birds migrate to the moon for the winter. Although this sounds ridiculous today, he correctly conjectured that birds may be spurred to move to new areas by changing weather and a lack of food and even noted that body fat might help sustain them on their journey.

John James Audubon ties silver thread to the legs of Eastern Phoebe nestlings and identifies them when they return to the same area the following spring—or, at least, so will later claim. Biologist and historian Matthew Halley cast doubt on this in 2018 when he noted that Audubon was actually in France in spring 1805 when the phoebes would have returned.

German villagers shoot down a White Stork that had a spear made of African wood impaled in its side. Dubbed the “pfeilstorch” (or “arrow stork”), this unfortunate bird provides some of the first concrete evidence of migration between continents.

Ornithologist William Earl Dodge Scott is touring the Princeton University astronomy department when he’s offered a view of the full moon through a telescope. Astonished to see migrating birds silhouetted against the face of the moon, he is able to use his observations to calculate a rough estimate of how high they must be flying.

Climbing a hill outside Madison, Wisconsin, historian and amateur ornithologist Orin Libby counts 3,800 calls by migrating birds over the course of five hours on one September night. Many of the calls seemed “almost human,” he will later write, “and it was not difficult to imagine that they expressed a whole range of emotions from anxiety and fear up to good-fellowship and joy.” These calls will eventually be dubbed “nocturnal flight calls" and be used as one way of monitoring bird migration.

Hans Christian Cornelius Mortensen places metal rings around the legs of starlings in Denmark to study their movements, the beginning of the scientific use of bird banding.

At a meeting in New York City, members of the American Ornithologists’ Union vote to form the American Bird Banding Association, the direct forerunner of today’s USGS Bird Banding Laboratory. Its mission is to oversee and coordinate bird-banding efforts at a national scale.

The U.S. Bureau of Biological Survey assumes authority over the bird banding program after the Migratory Bird Treaty Act passes in 1918. The agency's Frederick Charles Lincoln will use banding records from waterfowl to develop the concept of “migratory flyways”—four major North America flight routes around which bird conservation is still organized today.

David Lack and George Varley, biologists working for the British government, use a telescope to visually confirm that a mysterious military radar signal is being generated by a flock of gannets. It’s the first concrete proof that radar can detect flying birds, but the idea is not immediately embraced: “At one meeting,” Lack later writes, “after the physicists had again gravely explained that clouds of ions must be responsible, Varley with equal gravity accepted their view, provided that the ions were wrapped in feathers.”

Louisiana State University ornithologist George Lowery’s moon-watching observations in the Yucatan, using techniques inspired by Scott’s original full moon observations in 1880, provide evidence that some birds do indeed migrate across the Gulf of Mexico instead of taking a land route over Mexico.

Oliver Austin, an ornithologist leading wildlife management in Japan under the Allied occupation that followed World War II, describes the traditional Japanese method of catching birds for food using silk nets strung between bamboo poles. Mist nets will soon become the primary method for capturing songbirds for ornithological research. 

George Lowery and his collaborator Bob Newman oversee a massive effort to recruit volunteers across the continent to record moon-watching observations during fall migration. “Telescopes swung into operation at more than 300 localities as people by the thousands took up the new form of bird study,” writes Newman. “By the end of the season, reports had been received from every state in the United States and all but one of the provinces of Canada.” Due to the difficulties in analyzing such large amounts of data without computers, Lowery and Newman will not publish the full results until 1966. Their work provides the first continent-wide snapshot of migration patterns.

Illinois Natural History Survey ornithologist Richard Graber and engineer Bill Cochran record nocturnal flight calls for first time, rigging up a tape recorder with bicycle axles to hold the six thousand feet of tape needed to record a full night of migration.

Richard Graber tags a migrating Gray-cheeked Thrush in Illinois with a miniature radio transmitter developed by Bill Cochran. That night, he follows it for 400 miles in an airplane as it continues its migratory journey. “Each of us, at times, must stand in awe of mankind, of what we have become, what we can do,” Graber will write in Audubon . “The space flights, the close-up lunar photographs, the walks in space—all somehow stagger our imagination. I was thinking about this as I flew south from Northern Wisconsin [the next morning], having just witnessed an achievement of another kind by another species.”

Ornithologist Sidney Gauthreaux, who studied for his PhD under George Lowery, publishes “Weather radar quantification of bird migration,” the first systematic study of bird migration patterns using the relatively new technology of weather radar.

Bill Cochran tracks a radio-tagged Swainson’s Thrush for 930 miles on its migration, following it from Illinois to Manitoba over the course of a week in a modified station wagon with a radio receiver sticking out of the top.

Johns Hopkins University's Applied Physics Lab carries out the first field tests of satellite transmitters on birds using the  Argos satellite system —launched in 1978 for the purpose of tracking oceanic and atmospheric data. Swans and eagles are early subjects. 

British seabird biologist Rory Wilson tracks the movements of foraging penguins using a device of his own invention that he calls a Global Location Sensor. It uses ancient navigation principles to calculate and record a bird’s location using only a tiny light sensor and clock. These devices will later be better known as light-level geolocators.

Canadian scientist Keith Hobson and his colleagues publish a paper demonstrating that it’s possible to determine where a migrating songbird originated by analyzing the amount of deuterium—a rare isotope of hydrogen that occurs in varying amounts across the landscape—in its feathers.

“Selective availability,” a U.S. government practice which intentionally limits the accuracy of GPS technology available for non-military use, is switched off. Ornithologists quickly begin creating GPS devices for tracking the movements of birds.

The Cornell Lab of Ornithology launches eBird, a community science platform that lets birdwatchers upload records of what they observe to a database that is accessible to ornithologists, ecologists, and other researchers. Today more than one billion sightings have been contributed from around the world. 

A satellite transmitter implanted in a Bar-tailed Godwit dubbed “E7” tracks the bird’s astonishing nonstop 7,000-mile migration from Alaska to New Zealand over the open water of the Pacific Ocean—“the equivalent,” according to a USGS press release , “of making a roundtrip flight between New York and San Francisco, and then flying back again to San Francisco without ever touching down.”

Ornithologists Kristen Ruegg and Tom Smith launch the Bird Genoscape Project, an effort to map genetic diversity across the ranges of 100 migratory species. It will enable ornithologists to identify where in North America a migrating bird came from by analyzing its DNA.

The Cornell Lab of Ornithology scientists kick off the second iteration of BirdCast , a project that uses weather radar data to predict nights of especially intense bird migration activity. (The original BirdCast, started in 2000 by Sidney Gauthreaux, was discontinued after a year due to the limits of the technology available at the time.) One major result of the project is initiatives that encourage cities to shut off disruptive nighttime lighting when large numbers of migrating birds are likely to be on the wing.

The Motus Wildlife Tracking System , which uses miniature radio transmitters and an automated network of ground-based receiver towers, is launched in Canada. More than 30,000 animals (mostly birds) will be tracked by the system in the next decade.

Light-level geolocators  confirm  long-held suspicions that Blackpoll Warblers, songbirds that weigh roughly the same as a ballpoint pen, make a nonstop 1,400-mile, three-day flight over the eastern Atlantic Ocean during their fall migration from New England to South America.

Project Night Flight,  the largest nocturnal flight call monitoring project to date, operates more than 50 recording stations in Montana’s Bitterroot Valley. Spearheaded by Kate Stone and Debbie Leick, staff members at private research and conservation property MPG Ranch, Project Night Flight will record more than 100,000 hours of data in the next two years.

Icarus,  a new space-based wildlife tracking system with receivers on the International Space Station, begins operations. The initiative's overseers aim to provide transmitters that are lighter, lower-cost, and provide better-quality data than any trackers used before.

This piece originally ran in the Spring 2022 issue as “A Brief History of Discovery.” To receive our print magazine, become a member by  making a donation today .

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The Sorrow of Homecoming: On Mariana Oliver’s “Migratory Birds”

By farah abdessamad september 30, 2021.

Migratory Birds by Mariana Oliver

Authors who write in languages that are not their own are frequently interrogated about their motivations, as though words were also private property. Perhaps hidden behind this line of questioning lies a suspicion of betrayal or assault, an aversion to things illegitimate in appearance that can only be expressed through relentless probing. Perhaps people believe deep down that authors who do not write in the language of their mothers are taking something that is not theirs, that they are writing where they don’t belong, that they are word thieves.

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Bird Migration: Definition, Types, Causes and Guiding Mechanisms

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In this article we will discuss about the Migration of Birds:- 1. Definition of Bird Migration 2. Types of Bird Migration 3. Causes 4. Guiding Mechanisms 5. Disadvantages.

• Disadvantages of Bird Migration

1. Definition of Bird Migration:

The word “migration” has come from the Latin word migrara which means going from one place to another. Many birds have the inherent quality to move from one place to another to obtain the advantages of the favourable condition.

In birds, migration means two-way journeys—onward journey from the ‘home’ to the ‘new’ places and back journey from the ‘new’ places to the ‘home’. This move­ment occurs during the particular period of the year and the birds usually follow the same route. There is a sort of ‘internal biological clock’ which regulates the phenomenon.

Definition :

According to L. Thomson (1926), bird migration may be described as “changes of habitat periodically recurring and alter­nating in direction, which tend to secure optimum environmental conditions at all times” .

Bird migration is a more or less regular, extensive movements between their breeding regions and their wintering regions.

2. Types of Bird Migration:

All birds do not migrate, but all species are subject to periodical movements of vary­ing extent. The birds which live in northern part of the hemisphere have greatest migra­tory power.

Migration may be:

(i) Latitudinal,

(ii) Longitudinal,

(iii) Altitudinal or Vertical,

(iv) Partial,

(vi) Vagrant or Irregular,

(vii) Seasonal,

(viii) Diurnal and

(ix) Noctur­nal.

(i) Latitudinal migration:

The latitudinal migration usually means the movement from north to south, and vice versa. Most birds live in the land masses of the northern temperate and subarctic zones where they get facilities for nesting and feeding during summer. They move towards south during winter.

An opposite but lesser movement also occurs in the southern hemisphere when the seasons are changed. Cuckoo breeds in India and spends the summer at South-east Africa and thus covers a distance of about 7250 km.

Some tropical birds migrate during rainy season to the outer tropics to breed and return to the central tropics in dry season. Many marine birds also make considerable migra­tion. Puffinus (Great shearwater) breeds on small islands and migrates as far as Greenland in May and returns after few months.

It covers a distance of 1300 km. Penguins migrate by swimming and cover a considerable distance of few hundred miles. Sterna paradisaea (Arctic tern) breeds in the northern temperate region and migrates to the Antarctic zone along the Atlantic. It was observed that Sterna covers a distance of 22 500 km during migration!

(ii) Longitudinal migration :

The longitudinal migration occurs when the birds migrate from east to west and vice- versa. Starlings (Sturnus vulgaris), a resident of east Europe and west Asia migrate towards the Atlantic coast. California gulls, a resident and breed in Utah, migrate westward to winter in the Pacific coast.

(iii) Altitudinal migration :

The altitudinal migration occurs in moun­tainous regions. Many birds inhabiting the mountain peaks migrate to low lands during winter. Golden plover (Pluvialis) starts from Arctic tundra and goes up to the plains of Argentina covering a distance of 11 250 km (Fig. 9.54).

Birds migrate either in flocks or in pairs. Swallows and storks migrate a distance of 9650 km from northern Europe to South Africa. Ruff breeds at Siberia and travels to Great Britain, Africa, India and Ceylon thus travelling a distance of 9650 kilometers.

(iv) Partial migration:

All the members of a group of birds do not take part in migration. Only several members of a group take part in migration. Blue Jays of Canada and northern part of United States travel southwards to blend with the sedentary populations of the Southern States of U.S.A. Coots and spoon bills (Platalea) of our country may be example of partial migration.

(v) Total migration :

When all the members of a species take part in the migration, it is called total migration.

(vi) Vagrant or irregular migration :

When some of the birds disperse to a short or long distance for safety and food, it is called vagrant or irregular migration. Herons may be the example of vagrant or irregular migration. Other examples are black stork (Ciconia nigra), Glossy ibis (Plegadis falcinellus), spotted eagle (Aquila clanga), and bee eater (Merops apiaster).

(vii) Daily migration :

Some birds make daily journey from their nests by the influence of environmental factors such as temperature, light, and humidity also. Examples are crows, herons and starlings.

(viii) Seasonal migration :

Some birds migrates at different seasons of the year for food or breeding, called seasonal migration, e.g., cuckoos, swifts, swallows etc. They migrate from the south to the north during summer. These birds are called summer visitors. Again there are some birds like snow bunting, red wing, shore lark, grey plover etc. which migrate from north to south during winter. Th ey are called winter visitors.

Nocturnal and Diurnal Flight :

(i) Diurnal migration :

Many larger birds like crows, robins, swal­lows, hawks, jays, blue birds, pelicans, cranes, geese, etc. migrate during daytime for food.

These birds are called diurnal birds and gene­rally migrate in flocks.

(ii) Nocturnal birds :

Some small-sized birds of passerine groups like sparrows, warblers, etc. migrate in darkness, called nocturnal birds. The darkness of the night gives them protection from their enemies.

3. Causes of Migration :

Most species of birds migrate more or less on schedule and follow the routes in a regular fashion. The actual causative factors deter­mining the course and direction of migration are not clearly known.

The following factors may be related to the problems of migration:

i. Instinct and Gonadal changes :

It is widely accepted that the impulse to migrate in birds is possibly instinctive and the migration towards the breeding grounds is associated with gonadal changes.

ii. Scarcity of food and day length:

Other factors, viz., scarcity of food, shortening of daylight and increase of cold are believed to stimulate migration. Migration in birds depends upon two important factors— stimulus and guidance.

Scarcity of food and fall of daylight are believed to produce endocrinal changes which initiate bird migration.

iii. Photoperiodism:

The increase of day length (Photoperiodism) induces bird’s migration. The day length affects pituitary and pineal glands and also caused growth of gonads which secret sex hormones that are the stimulus for migration. In India, Siberian crane, geese, swan those come from central Asia, Himalayas, begin to return from March and onwards with the increase of day length.

iv. Seasonal variation:

The north-to-south migrations of birds take place under stimulus from the internal condition of the gonads which are affected by seasonal variation.

The experiments of Rowan with Juncos (summer visitor to Canada) have esta­blished that light plays an important role in the development of gonads, which has indirect role on migration. If the gonads undergo regression, the urge for migration is not felt. So the seasonal changes in illumination appear to be a crucial factor for determining migration.

Despite all these suggestions, it is not clear how birds — through successive generations — follow the same route and reach the same spot. The instinctive behaviours like migration, breeding, moulting are phasic occurrences in the annual cycle which are possibly controlled by the endocrine system. In all migratory birds, accumulation of fat takes place for extra fuel during prolonged flight in migration.

4. Guiding Mechanisms in Bird Navigation :

For more than a century the celestial navigations of birds have fascinated the ornithologists. Different explanations have been advanced to explain how birds navigate. It is difficult to generalize on the means of orientation and navigation in migration. The different groups of birds with different modes of existence have evolved different means of finding their way from one place to another (Pettingill, 1970).

The other reasons may be:

Fat deposition :

Migratory birds become greedy and fat is deposited in the subcutaneous region of the body. The fat deposition plays an important role in the migration of birds. Birds, those migrate a long distance, reserve enough fat which provides energy in their arduous jour­ney and helps the birds to reach its desti­nation, following a particular route. After fat deposition, restlessness (Zugunruhe) is seen among birds for migration.

Inherited instinct :

Birds that take part in migration or follow a more or less definite goal, evidently possess an inherited instinct. Both the direction and the goal must have been implanted in the bird’s genetic code when a population can adjust to a particular location or environment.

Experienced Lead the Flock :

The theory is sometimes advanced that old and experienced birds lead the way and thereby lead the whole route and show the whole route the younger generation. This the­ory may be applicable to some birds like swans, geese and cranes because they fly in flocks but not applicable in all species where old and youngs migrate at different times and mainly youngs start ahead of the adult.

Werner Ruppell of Germany, a leading experimenter on avian migration, found that Starlings of Berlin find their way back to their nestling places from about 2000 km away. A sea bird named Manx shearwater collected from the western coast of England after being flown by plane to Boston was found back in its nest in England within 12 days.

The shearwa­ter had flown its own way about 4940 km across the unknown Atlantic Ocean! The gold­en plover of North America migrates from its winter home in the Hawaiian islands to its breeding place in northern Canada.

This bird lacks webbed feet and it is quite natural that it must fly for several weeks over thousands of kilometers of ocean to reach its destination. The birds have wonderful power of navigation and orientation to find their destination even under odd conditions.

There are many theories regarding the phenomenon of migration in birds.

Various theorists propose that birds are guided by a number of agencies:

a. Earth’s magnetic field—as the guiding factor:

Some ornithologists believed about the existence of a “magnetic sense” as the impor­tant factor in the power of “geographical orientation”. The theory was conceived as early as 1885 but conducted by Yeagley in 1947 and 1951. Yeagley suggested that birds are sensitive and guided by the earth’s mag­netic field.

The Coriolis force arising from rotation of the earth plays the guiding role in migration of birds. The basic question of the theory may be asked — “can birds detect such minute differences in the earth’s magnetic field and can these forces affect bird’s behaviour?”

Attempts to demonstrate by experimental evidences have not supported Yeagley’s experiment. Experiments, in which the earth’s magnetic field was changed, had no effect on the direc­tion which the birds undertook.

b. Sun—the guiding agent in diurnal migration:

The concept that birds are guided by the position of the sun was advanced by Gustav Kramer in Germany and G. V. T. Matthews in England. They have shown by intensive experimentations those homing pigeons and many wild birds use the sun as the compass and that they possess a ‘time sense’ or ‘internal clock’ which allows them to take account of motion of the sun across the sky.

Kramer (1949, 1957, 1961) performed experiments on Starlings (diurnal migrants) and showed that these birds use the sun for setting their migratory course. When the sky remains clear, the Starlings succeed in taking the right direction.

If the sky remains overcast they become bewildered and fail to orient themselves. Mechanical placement of a mirror which deflects rays of the sun result into con­siderable deviation of orientation to a pre­dictable extent. The experiments of Kramer and others failed to explain the navigation and orienta­tion of night migrants. This aspect was exten­sively worked out by E.G.F. Sauer (1958).

c. Stars—the guiding agent in nocturnal migration:

The warblers and many other birds orient themselves during navigation by the sun during daytime. But the warblers as well as many other birds navigate mainly at night. What sorts of system do these birds use to the pathways during navigation at night?

Sauer performed experiments on white throat warblers to give an insight to the prob­lem. Sauer put the birds in a cage placed in a planetarium having an artificial replica of natural sky. When the light of the planetarium was poorly illuminated, i.e., when the stars were not visible, the warbelers failed to orient themselves.

When the illumination was better and the planetarium sky matched with the natural night sky, the birds followed up the proper direction. It has also been shown by Sauer that a warbler which has spent its life in a cage (i.e., never navigated in natural sky) has an inborn ability to follow the stars to navigate along the usual route the members of the species follow.

Sauer has suggested that the warblers possess hereditary mechanism to ori­ent themselves by the stars during nocturnal migration. The warbler can adjust the direc­tion perfectly at the latitude.

Suggestions have been advanced by many workers that the configuration of the coastline possibly helps in navigation, but Sauer has dis­proved the idea and advocated that the birds are exclusively guided by the stars during night.

d. The ‘compass’ and the ‘internal clock’ in bird migration:

It is a known fact that mil­lions of birds fly to their winter ‘home’ in every autumn. In doing so they cover often thou­sands of kilometers from their native ‘home’. In the following spring they again return to their breeding grounds. This is a regular bio­logical phenomenon in avian life.

It has been established that the young birds caught during migration, when released afterwards, follow exactly the original route their undisturbed fellows followed. This phe­nomenon suggested the presence of a sort of ‘compass’ the birds use during navigation.

But Kramer’s experiment gave a clue to the problem. The position of the sun is vital in con­trolling the navigation pathways. During the day the position of the sun in the sky is changed from east to west via the south. Despite such changes birds tried to navigate in the same direction. This means they have the inherent ability to make appropriate allowance for the time of day.

How do the birds know the time of day? They have possibly a built-in time­keeping mechanism (internal clock) which is synchronized with the earth’s rotation. The ‘internal clock’ can be made to synchronize with external happenings.

Existence of biological clocks is a pro­perty of living organisms. It is not confined to animals, it is found in plants and even in sim­ple cells too. It is a common experience that if we are in the habit of getting up every day at a particular time, we frequently wake up at the same time. Besides, many of our bodily func­tions have a rhythm of their own. These are possibly controlled by an ‘internal clock’ of which we are normally unaware.

Telemetry means methods of tracking of the movement of birds or other migratory ani­mals by using radio. This is the most promising method that has been applied to trace the route of bird’s migration. The method consists of attaching a small radio transmitter, weighing about 2-3 gm. that sends periodic signals or “beeps”.

The miniature transmitter can be placed on birds and it does not interfere flight and the signals can be detected by means of a receiving set mounted on vehicles or aero planes that can detect the routes of migratory birds.

Though there are some limitations of telemetry but this technology gives encoura­ging results. More recently researchers are engaged largely to track the routes of the migratory birds with the aid of satellites and radar tracking instruments.

5. Disadvantages of Bird Migration:

i. Many youngs are not, able to reach the destination because they die during the course of the continuous and tiresome journey.

ii. Sudden changes in the climate such as storms and hurricanes, strong current of wind, fog are the causes for the death of a sizeable number of migrants.

iii. Sometimes man-made high tours and light houses cause the death of migratory birds.

iv. Man themselves are responsible for the death of the migrants. They shoot at these poor birds just for their own leisure and amusement.

Related Articles:

• Essay on Bird Flu: History, Symptoms and Prevention
• Migration in Fishes (With Diagram)

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Migratory Birds

About the basics of bird migration.

Our world is full of biodiversity, we can see different kinds of birds, animals and plants all around us. There are different species of animals and plants that use various strategies to survive and keep the cycle of life going. One such strategy that birds use is called migration. Migration is a regular seasonal movement of birds in large groups. 

It happens in the case of migratory birds when they have to leave their home place to migrate to some other favourable place and for that, they have to adopt a long journey in which there is no guarantee whether they will be able to return back or not but if they do not migrate, in that case as well, their survival is not possible, thus they used to have migrated in any case. In this article, we will be talking about migratory birds and all about their migration. This article will help you to understand one of the most important behavioural patterns of the animal world, and will increase your subject knowledge as well.

What is Migration?

Migration of birds is one of the most fascinating phenomena in which birds travel from one habitat to another in search of favourable conditions and increased resources for survival and it also involves the journey to return to the original place. It also happens during seasonal change or movement between breeding and non-breeding locations. Migration is not an easy process, as birds have to cover long distances in order to reach their destinations and during these journeys, they need a lot of energy, food, water, sufficient rest, etc and not all the migration journeys become successful and some of the birds die as well in these journeys.

What are Migratory Birds?

Those birds who migrate from one location to another location in order to breed, feed, and raise their offspring, are known as migratory birds. They usually migrate from unfavourable locations to some favourable places which are having suitable conditions along with sufficient food and water resources and are also safe as well. The majority of the birds migrate during the breeding season and others migrate for food resources and because of change in seasons.

Types of Migrating Birds

The types of migrating birds can be judged through the type of migration they adopt which can be cleared from the following:

Seasonal Migration: It happens with the change in seasons. Birds migrate from a location when they are not able to survive in harsh conditions.

Latitudinal or Longitudinal: This kind of migration happens between different latitudinal or longitudinal locations. Either North to South or East to West or vice - versa.

Altitudinal: It generally happens for those birds who give birth at high altitude areas, and when they have to migrate again because of the harsh conditions over there.

Loop: Those who follow this kind of migration, those birds usually follow annual migration in a cycle again and again to enjoy the resources of two locations.

Nomadic: Understanding exact patterns and their timings are not easy, they stay in one place until sufficient resources are available otherwise they will migrate.

LeapFrog: It is a kind of skip migration in which birds migrate to long distances in order to skip a sedentary population.

Reverse: Aberration among birds is seen when they are confused and choose an unexpected path and go in the opposite direction.

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Features of Migratory Birds

These birds are known to have good morphology as well as physiology because of which they can cover long distances by flying fast and observing various other things.

They have the ability to navigate things with good accuracy. They use the sun, the stars, the Earth's magnetism, etc.

They know when to migrate and when to return. For their specific reasons, they do not hesitate to migrate to far present locations.

They can fly as far as 16000 miles and some of the birds fly at a speed of 30mph to reach their destination. With this speed, they can reach in 533 hours whereas if they fly on the Basis of 8 hours per day, they can reach the final destination in 66 days.

They fly at different speeds and at different altitudes. Some fly at low altitudes where we can see them whereas some birds fly at high altitudes as well such as Songbirds who travel at 500 to 2000 feet whereas if we talk about Geese or Vultures, they used to fly at 29,000 to 37,000 feet altitudes.

Before migration, they prepare themselves for the journey by increasing their body weight or by keeping food reserves.

Different birds migrate at different timings but most of the birds prefer to fly at night because usually, the night is much safer for them due to fewer predators or having cooler air at night with which they can fly and rest easily.

They also prepare for their return as well because, after exhaustion of their whole energy in the long-distance journey, they usually feel hungry and require food and water.

Why do Birds Migrate?

There are several reasons, a few of which are mentioned below:

Food is one of the major reasons for their migration. If they all stay at one place then food will be exhausted & scarce during the breeding time and thus breeding will be less successful. Thus, they migrate to food-abundant areas.

During the nesting season, the depletion of food will not only affect the adult birds but also attract a lot of predators because they become an easy meal for them.

Birds usually migrate for their family or we can say for healthy breeding. They always require healthy conditions for raising their offspring. These conditions depend upon different species such as sources of food, weather, habitat , adequate shelter, breeding colonies, safety, etc.

Another reason can be a change in the climatic conditions. Any severe change in these conditions can cause their migration because it makes it difficult for them to survive in harsh conditions be it extra warm or extra cold.

They also can migrate to save themselves and their offspring from predators and diseases. They usually migrate to places that are inaccessible to predators.

How do Birds Migrate?

They make different physical adaptations for the travel journey such as building extra fat supplies to provide extra energy during the journey.

Keeping food and water reserves by increasing their body weight before the migration and this phenomenon of increasing weight is known as hyperphagia and a lot of birds experience this phenomenon.

They also shed their old feathers in order to make their flight easy so that it takes less energy to fly.

They used to change the altitudes as well. They fly at higher altitudes for a speedy and fast journey.

They change their behaviour of flying as well. Sometimes those birds who used to fly in the day, during migration fly at night.

Sometimes they also fly in a V pattern or we can say in a group by following the leader who has much experience and this pattern makes the journey much easier.

Migratory Birds With Names

Let's see some of the examples of migratory birds which are mentioned below:

Siberian cranes and Greater flamingo are migratory birds that are usually seen in India in the winter season.

Asiatic Sparrow Hawk migrates to India and Myanmar during winters.

Swallow, which is a small bird, migrates from Southern England to Southern Africa.

Red Wing lives in Eurasia or the Himalayas but flies to Africa in Winter.

Sand Martin that live in Eurasia or North America usually migrate to southern areas which depend on their zones.

Whinchat who lives in Europe flies to Africa between October to March.

Common Rosefinch lives in Eurasia and flies to southern parts of Asia in Winters.

Names of other migrating birds are Black-headed gull, Green Sandpiper, Northern Lapwing, Eurasian Hobby, Gray Heron, etc.

Threats and Conservation of Migratory Birds

When birds migrate from one place to another there can be many threats to them. It took a lot of energy to cover these long-distance journeys. The major threats include exhaustion, starvation, injuries, threats from predators or hunters, diseases, pollution, natural calamities or disasters, etc.

For their conservation, we have CMS which means Convention on Migratory Species at the international level which is also famous as the Bonn convention which aims to protect migratory species such as territorial, avian, or marine,  throughout their ranges and all the countries coordinate with each other for their conservation.

To sum up we can say that avian migration is not an easy task but also important for the birds as well. They require favourable conditions in order to survive and raise their young ones for which they adopt long journeys which are exhausting for them and in these journeys they have to cross the boundaries which can lead to more problems and threats for them and for which every country should adopt conservation measures for them. In this article, we have covered everything about migratory birds, why and how birds migrate, several features, their examples, etc. We believe that this comprehensive article will help you to understand this important topic and you will also think about the conservation of these species.

Migratory Birds - Survival out of their Habitat

The birds who migrate from one unfavourable location to some favourable location in order to breed, feed, and raise their children are known as migratory birds. They migrate to some locations which are having abundant food and water resources with good climatic conditions. They make different physical adaptations for the travel journey such as building extra fat supply and keeping food and water reserves by increasing their body weight before the migration. 

Migratory birds shed their old feathers in order to make their flight easy and on the other hand, they used to change their attitudes as well. They fly at higher altitudes for a speedy and fast journey along with the change in their behaviour of flying as well. Sometimes those birds who used to fly in the day, during migration fly at night. Sometimes they also fly in a V pattern or we can say in a group by following which makes the journey much easier. Some of the examples of migratory birds with names are the Black-headed gull, Green Sandpiper, Northern Lapwing, Eurasian Hobby, Gray Heron, Siberian Cranes or Greater Flamingo, etc.

FAQs on Migratory Birds

What are the Major Reasons For Bird Migration?

There can be several reasons for the birds' migration. Food is one of the major reasons for their migration. If they all stay at one place then food will be exhausted & scarce during the breeding time. Thus, they migrate to food-abundant areas. During the breeding season, the depletion of food will not only affect the adult birds but also attract a lot of predators because they become an easy meal for them. 

Birds usually migrate for their family or we can say for healthy breeding for which they require healthy conditions for raising their offspring. Another reason can be a change in the climatic or weather conditions of the locations and if any change occurs in these conditions can cause their migration because it makes it difficult for them to survive in these harsh conditions. On the other hand, they also can migrate to save themselves and their offspring from predators and various diseases. They usually migrate to places where they are safe and are inaccessible to predators.

What is Latitudinal Migration?

There are different types of bird migration. In latitudinal migration,  birds migrate from north to south (or south to north) between their breeding and non-breeding area. Some migrant species breed in temperate North America and migrate to tropical America.

What is Altitudinal Migration?

Altitudinal migration is not as common as longitudinal Migration but has the same principle. Unlike the latitudinal migration when the migrants cover long distances, altitudinal migrants cover short distances from montane regions to lower elevations outside of their breeding season. This is usually triggered by food abundance in these areas.

How do birds navigate during migration?

The secrets of amazing navigation skills of birds aren't fully understood, they combine several different types of senses during the journey to navigate. They use information from the sun, the stars, and by sensing the earth’s magnetic field they are able to navigate easily. Birds get information from the position of the setting sun and from landmarks seen during the day. There is even proven evidence that sense of smell plays a role, at least for homing pigeons.

Biology • Class 11

World Migratory Bird Day 2024 :  Protect Insects, Protect Birds

Key messages.

Bird migration is a natural phenomenon that has captured the imagination of people across cultures for centuries. These annual journeys involve the mass movement of birds across vast distances, often spanning continents, and are driven by a combination of instinct, environmental cues, and survival strategies. Migration is a complex behavior that serves various ecological, evolutionary, and survival purposes. Yet, the very phenomenon of bird migration is under threat, particularly due to declining insect populations, upon which many migratory birds rely for sustenance.

Insects provide essential energy for migratory birds, offering a rich source of nutrients critical for their survival. They rely on insects for energy during migration and other stages of their life cycles, especially when feeding their offspring. However, the massive decline in insect populations in many parts of the world is having serious implications for migratory birds. This year’s World Migratory Bird Day will therefore shine a spotlight on the interdependence between insects and birds, urging action to protect insects as a means to safeguard avian populations.

The campaign will explore the intricate relationship between birds and insects, and highlight the challenges faced by both as well as actions people can take to address the threats and halt the decline in insects. Below you will find a set of key messages and recommended actions and conservation measures which will be promoted through this year’s campaign.

Summary of Key Messages

• Insects provide the necessary energy for migratory bird species , because they contain remarkable nutrients, providing birds with protein-dense, high-energy sources of food. Migratory bird species depend on insects during migration and at other stages of their life cycles when feeding their offspring.
• The presence of insects greatly influences the timing, duration, and overall success of bird migrations . During stopovers at various locations along their migration routes, birds actively forage for insects in fields, forests, wetlands, and other habitats. They catch flying insects in mid-air, or they search for insects among leaves, bark, and vegetation. Many birds’ journeys coincide with peak insect abundance in their stopover areas.
• This reliance on insects is especially pronounced for species such as warblers, flycatchers, swallows, and swifts . However, many other bird species such as ducks or shorebirds, and some raptors depend on insects during migration and at other stages in their lifecycle, in particular for raising their young before they are able to fly.
• Insect populations are declining globally, and this phenomenon is mirrored by a decrease in populations of birds that depend on insects for their survival.
• Migratory birds bring multiple benefits to humans as insect eaters, providing pest control for mosquitoes, and other insects that may damage crops and spread disease.
• 85 percent of flowering plants require animal pollination; in most cases this job is done by insects. If we lose those pollinators, many plants will struggle to survive, and by extension many migratory birds that depend on them. 
• Actions taken to reduce insect declines will benefit birds across their flyways. 
• Migratory bird monitoring has shown that insectivorous migrants associated with farmland ecosystems, especially grassland species, have declined considerably (13% on average in Europe during the period 1990-2015).
• Restoring grassland ecosystems , providing support to farmers to secure flower strips and fallow land is vital to restore insect and migratory bird populations globally. 
• Close monitoring of migratory birds, using citizen-science tools and raising awareness through environmental educational campaigns, can deliver vital data to restore and manage critical habitats, and ensure their connectivity, for migratory birds.
• Maintaining and/or restoring water bodies is essential to protect insectivorous aquatic communities, including many waterbird species. 
• The insect-eating bird diet encompasses a diverse range of insects including: Aerial insects such as flies and mosquitoes, moths and caterpillars, butterflies and beetles, and aquatic invertebrates.

Challenges and Threats to Insect Populations and Their Effect on Birds

The decline and disturbance in insect populations across the flyways compounds the threat to birds’ existence and overall welfare.

• Habitat Loss and Change: Where natural spaces, such as forests or grasslands, are transformed or destroyed by activities such as intensive agriculture or urban development, insect populations decline. This decline has very negative implications for both plants, migratory birds, and the entire food chain in a negative cascading effect that results in severe bird population declines across an entire flyway.
• Toxic Trouble: Pesticides and herbicides, which are intended to protect crops, often have unintended negative consequences. They often affect non-target insect species, either directly (by killing them) or indirectly (for example by polluting water bodies and affecting water invertebrates’ development). This collateral effect affects migratory birds, ending up in lower body condition at stop-over sites or lower breeding success.
• Decreased Bird Populations: Without an adequate supply of energy-rich insects, birds may struggle to complete their migrations. Insufficient nutrition can lead to weakened immune systems, reduced reproductive success, and increased mortality rates among both adult birds and their offspring.
• Ecosystem Imbalance: Birds and insects are a classic example of ecological coevolution. Altering this relationship brings negative impacts to both communities. Maintaining and/or restoring healthy ecosystems means ensuring vital ecosystem services provided by birds and insects, such as pollination, seed dispersal, pest control and human well-being.  
• Climate Change; A negative consequence of the current climate is the desynchronization of bird migration: birds arriving at their breeding ground before or after the insect peak, reducing the breeding success, and increasing the damage insect populations can cause to some agricultural or forestry products .

Facts About Insect & Bird Population Declines

• The causes of insect decline include land-use change, climate change, agriculture, introduced species, nitrification, pollution, insecticides, herbicides, urbanization and light pollution. (PNAS. 2021)
• The terrestrial bird population reliant on insects as a food source has dropped by 2.9 billion in the last 50 years (Tallamy & Shriver. 2021)
• Supporting native plant species that productively sustain insect herbivores could help reverse these declines and benefit birds and other wildlife. (Tallamy & Shriver. 2021)
• 96% of North American terrestrial birds feed their young exclusively or in part on insects. (Tallamy & Shriver. 2021)
• Caterpillars are preferred over other insect options due to size, softness and higher nutritional value. Low caterpillar availability has been linked to reduced nestling fitness; for example, smaller clutch sizes, greater mortality from starvation, fewer fledglings, slower growth rates and lower body mass have been reported. (Tallamy & Shriver. 2021)
• Recent bird population declines in North America, totaling over 3 billion individuals, are concentrated among terrestrial insectivores due to declining insect numbers. (Tallamy & Shriver. 2021)
• Studies show that insect abundance is declining by 1-2% yearly, leading to potential shifts or losses of ecosystem function. (PNAS. 2021)
• Not all insects are declining: some species are expanding in range and population size, and those associated with humans may be thriving due to human assistance. (PNAS. 2021)
• Factors contributing to the decline of honeybees include mites, viral infections, microsporidian parasites, poisoning by pesticides and overuse of artificial foods. (PNAS. 2021)

Actions to Address Insect Declines

T o preserve the delicate balance between birds and insects, it is crucial to take proactive and effective conservation measures. A range of strategies can safeguard these vital components of our ecosystems.

• Habitat restoration and management : Identifying suitable habitats and adequately managing and/or restoring them, through the creation of protected areas when needed that are adequately connected will have immediate and positive impacts for both birds and insects to flourish.
• Sustainable Agriculture: Reducing pesticide and chemical fertilizer use, crop rotation, and maintaining natural vegetation corridors within agricultural landscapes benefit both insects and birds.
• Pesticide Alternatives: Opt for organic alternatives, targeted application, and integrated pest management systems minimize negative impacts on insect populations and the birds and other species that depend on them.
• Public Awareness: Raising public awareness about the importance of birds and insects fosters an understanding and empathy that is the basis for any conservation action. Communities can support conservation efforts, from local clean-up initiatives to creating bird-friendly urban spaces.
• International Cooperation: Join forces beyond political boundaries to protect critical stopover points, coordinate conservation measures, and ensure the availability of a chain of habitats required by migratory birds.
• Corporate Accountability: Encourage the private sector, companies and industries to adopt sustainable practices that reduce pollution and minimize habitat destruction through both regulations and incentives.

7. Recognizing the Value of Insects: Insects have a critical role in our survival. They contribute to food production, serving as pollinators for numerous fruits and vegetables. Insects enrich the quality of soil for plant growth and contribute to the overall health and balance of our planet. Ensuring the protection of insects enables the sustainability of our ecosystems and the food production industry.

Actions We Can Take as Individuals

• Insect friendly gardens: plant native flowers, shrubs, and trees that provide habitat for insects.  Insects prefer plants that are native to your area and need cover to survive. These plants are accustomed to the local weather and provide a good habitat for bugs.
• Support organic farming: choose organic products whenever possible to avoid agricultural practices that use harmful pesticides and prioritize biodiversity.
• Minimize habitat destruction: when developing or landscaping your property, minimize habitat destruction, try to compensate for the loss and preserve natural areas.
• Don’t rake! Create a thriving ecosystem for insects and birds by leaving leaves in your garden or yard. The leaf litter acts as a natural shelter, food source, and breeding ground for various insect species. The decaying leaves also attract insects that are essential for insectivorous birds’ diets, promoting biodiversity and ecological balance. By refraining from raking leaves, you contribute to a healthier and more sustainable environment for both insects and birds.
• Keep water clean by using eco-friendly soaps and cleaners. Some cleaning products and salts can be harmful to insects if they get into the water. 
• Teach your friends and family about the significance of insects. Support programs that educate people about insects and speak up for conservation in your community. Make sure rules about lawns and pesticides protect insects and get involved locally to support decisions that protect natural habitats, insects, and other wildlife.
• Appreciate insects for the good things they do , like pollinating plants and being part of the food chain. Share positive pictures and stories about insects with your family and neighbors and on social media  to help others learn their importance.
• Supports pollinator-friendly initiatives: advocate for and support local policies and initiatives that protect insects and their habitats. 

We’ve handpicked seven bird species to be ambassadors, showing the important role of insects in the lives of migratory birds. These species not only demonstrate the diversity of migratory birds but also highlight their dependence on insects and healthy insect habitats. Cliff Swallows capture flying insects and provide natural pest control benefits around their colonies. Despite being primarily granivorous, the North American Bobolink relies on insects as a crucial food resource during the nesting season. The Nacunda Nighthawk, active during twilight and night, skillfully uses artificial lighting to hunt for insects. Semipalmated Sandpipers scour mudflats for aquatic insects, while the Broad-tailed Hummingbird supplements its nectar diet with insects, essential for nurturing its chicks. Even the American Kestrel relies significantly on insects as a food source. Finally, the migration of Wood Ducks to coastal areas shows just how important wetlands with plenty of insects are. Learn more about each species below.

Cliff Swallow

Petrochelidon pyrronota  

Cliff swallows are small migratory passerines that form nesting colonies across North America. In the fall, they depart for their wintering grounds, ranging from southern Brazil to Argentina and Chile. Cliff swallows migrate during the day, catching flying insects along the way. Many Cliff Swallow’s colonies are hosts for ectoparasite insects. These insects use the birds to disperse in the environment, or inhabit their body and flight feathers like Swallow Bugs, Fleas, and Louse Flies. Birds nesting in colonies often live closely with humans, and their large groups provide ecological benefits like natural pest control in urban and rural areas.

Dolichonyx oryzivorus 

 Migratory grassland birds, like the North American Bobolink, flutter and sing atop expansive grasslands. In autumn, they travel south to Bolivia, Paraguay, Brazil and Argentina. Although their diet is granivorous most of the year, during the nesting season, insects become the preferred resource to feed their chicks; prey includes grasshoppers, beetles and butterflies. Bobolink flocks can eat large quantities of grain, and the species’ scientific name, _oryzivoru_s, means “rice-eating.” They are often hunted and exposed to chemical pollution as agricultural pests, particularly in wintering grounds.

Nacunda Nighthawk

Chordeiles nacunda

 The Nacunda Nighthawk is a migratory bird found in the southern Neotropics. It is most active during twilight, night hours, and dawn. It is an aerial hunter and takes advantage of artificial lighting in urban and rural areas to capture insects. Their diet includes mayflies (Ephemeroptera) and beetles (Coleoptera). When there is a large emergence of moths (Lepidoptera: Noctuidae) or water bugs (Hemiptera: Belostomatidae), significant numbers of Nacunda Nighthawks are observed flying and hunting in groups.

Semipalmated Sandpiper

Calidris pusilla

Semipalmated Sandpipers forage in shallow water on mudflats, targeting aquatic insects like flies and larvae, particularly during the breeding season. Despite its Latin species name ‘pusilla’ which means ‘very small’, this characteristic does not limit these long-distance migratory shorebirds. They can cover vast distances, flying from North America to central and South America, including the Caribbean, to reach their preferred habitats, which include shorelines, mudflats, and sandy beaches.

Broad-tailed Hummingbird 

Selasphorus platycercus

The Broad-tailed Hummingbird, a medium-sized hummingbird, breeds in western North America and migrates south to Mexico and Central America. While their primary diet is nectar, which is low in protein, they supplement it by hunting insects in flight or perched on foliage between trees. A nesting female can capture up to 2,000 insects in a single day, a useful source of protein-rich food for raising chicks. They also hunt mosquitoes in the air or consume mites while feeding on nectar. Chemical pesticides and heavy yard cleaning can impact both hummingbirds and the insects they depend on for survival.

American Kestrel

Falco sparverius

Contrary to what many people think, we often underestimate how much birds of prey rely on insects for their diet. The American Kestrel is a species found all over the world and loves soaring over fields to snatch insects from the air. It captures the big ones like grasshoppers, cicadas, beetles, dragonflies, butterflies and moths. About 82% of the kestrel’s diet is made up of insects, especially during breeding seasons or when they stop at certain points on their migratory routes. Sadly, the use of chemicals in agricultural fields can dramatically decrease the population of these natural insect controllers.

Aix sponsa  

Wood Ducks primarily forage in shallow waters, and research on their feeding habits indicate that during the breeding season, Coleoptera and Diptera are the two main insect groups they eat. While adult Wood Ducks mainly follow a herbivorous diet, including seeds, algae, small fish, and both terrestrial and aquatic plants, young ducks rely on insects, making up about 70% of their diet. These migratory ducks travel from North America to coastal and wetland areas in Mexico and a portion of the Caribbean. Wetlands with plenty of water resources with insects are essential for Wood Ducks.

Vivid Dancer 

Argia vivida

The Vivid Dancer is a bright blue to violet colored damselfly with black markings. Argia vivida has three life stages (egg, nymph and adult) and a life cycle that is partly aquatic and partly terrestrial. Adult Vivid Dancers are aerial predators and both sexes hunt. Damselflies’ wings fold when resting, unlike dragonfly wings, which protrude.

The species is threatened by habitat loss and degradation in most hot spring habitats due to intensive recreational use (e.g., bathing, diversion of water to create pools), trampling by livestock in fresh springs, and possible predation by introduced aquatic species. Vivid Dancers  play several important ecological roles as predators, prey, and indicators of ecosystem health.

Willow Sawfly

Nematus corylus

As with all insects, Nematus corylus has several stages of development. They are: Egg, Larva and Chrysalis. The larvae resemble caterpillars and feed on young, tender leaves. Nematus corylus are related to wasps and bees; adults are 6 to 9 millimeters (mm) long. The larvae of Nematus corylu s have six or more pairs of legs.

The larvae are slug-shaped and bright olive-yellow in color. Larvae feed on leaf margins and consume the entire leaf leaving only the midrib. Each larva can eat 1 to 2 willow leaves in its short life, and a sufficient number of larvae can defoliate large willows. These trees often grow new leaves, which are then eaten by the next generation of Nematus corylus .

Giant Cicada

Quesada gigas

The giant cicada is the only species of the Q uesada genus found in North America, ranging from central Texas to central Argentina. It has a wingspan of 18 cm and a body length of 7 cm. The song of the giant cicada reaches 93 decibels at a distance of 50 cm, generated from a pair of organs in the abdomen called timbales. Its calls have been compared to various power tools such as motors, fan belts and steam engines.

Their larvae spend years underground feeding on the roots of trees. In times of mass emergence, cicadas are commonly consumed by birds, including birds of prey, as well as flycatchers, grosbeaks, wrens, and other aerial insectivorous birds. Interestingly, studies have shown that some bird populations may have larger clutches during the years when cicadas emerge. Cicadas benefit birds by providing significant amounts of food during their mass emergence events.

Buzzer Midge

Chironomus plumosus

The larvae of the species Chironomus plumosus , also known as the buzzing mosquito, is a non-biting mosquito species (Family Chironomidae) found in areas of the northern hemisphere. The larvae of this species play an important role in the aquatic food web and adult buzz gnats emerge rapidly from the water into the air. These mosquitoes are commonly known as “blind mosquitoes” because they have a mosquito-like appearance but do not bite humans. They fly in large swarms, often emitting a loud buzzing sound, and the adult male is distinguished from the female by its feathered antennae.

Buzzing midges can be found in fast-moving streams, deep slow-moving rivers, stagnant ditches, and in lakes and ponds rich in decaying organic matter. The larvae “clean” the aquatic environment by consuming and recycling organic debris. Larvae of the species Chironomus plumosus support migratory shorebirds during their arctic breeding season.

*Thenius, Ronald, et al. “Biohybrid entities for environmental monitoring.” ALIFE 2021: The 2021 Conference on Artificial Life. MIT Press, 2021.

Cinygmula ramaleyi

Ephemeroptera populations thrive in shallow, productive lakes with soft, organic-rich sediments. These adult stage insects have no mouthparts. Young mayfly nymphs prefer to burrow in soft underwater sediments. All the nutrients they need in their short adult life have been absorbed in the 2 years they spent underwater. In the short time they are, the birds feast on them. Emergent mayfly swarms in early summer are an annual event that helps migratory birds have full resources during their breeding season.

Emergent mayfly swarms are an annual event that helps birds have full resources during their breeding season. Mayfly populations do well in shallow, productive lakes with soft, organic-rich sediments. Mayflies are also useful indicators because they are highly visible, relatively easy to sample, and provide “real evidence” that restoration has been effective in wetlands, where high concentrations of contaminants are often transported in polluted areas.

Red-legged Grasshopper

Melanoplus femurrubrum

The red-legged Grasshopper , Melanoplus femurrubrum , is one of the most common leafhoppers and is distributed throughout most of North America. Members of the Orthoptera (crickets and grasshoppers) tend to contain a higher amount of protein than other insects; for example, adults of Melanoplus femurrubrum are 77% higher in protein.

Red-legged grasshoppers are found in both upland and lowland pastures, meadows, roadsides, lowlands, and cultivated fields. A grasshopper hatchling from the beginning resembles its parents, only it lacks wings and in its proportions is smaller. Grasshopper nymphs are an important source of food for birds, especially for chicks that cannot eat seeds.

Giant Water Scavenger Beetle

Hydrophilus triangularis

The water scavenger beetle is one of those types of beetles that can live both on land and in water while feeding on smaller insects and larvae. This beetle needs fresh water to reproduce and prefers to live in large, deep ponds. Larvae feed on small invertebrates such as insects and snails, but may also consume tadpoles and small fish. Adults have an adaptation that allows them to surface less frequently in search of air than their larvae. 

Adult Hydrophilus triangularis create an air bubble under their elytra (hardened wing covers) that they can use while underwater. Spiracles (tracheal openings) connect to this air bubble and oxygen can be accessed. Using their mandibles, they incapacitate prey, tear them apart and finally suck out their juices, playing an important role in controlling the population of other aquatic organisms.

Each of these creatures, insects, and birds alike, plays an essential role, from pollinating plants to being prey for larger organisms. They are threads in nature’s rich tapestry, a constant reminder of the interconnectedness of all living things.

Douglas W Tallamy , W Gregory Shriver, Are declines in insects and insectivorous birds related?, Ornithological Applications, Volume 123, Issue 1, 1 February 2021, duaa059, https://doi.org/10.1093/ornithapp/duaa059

PNAS. “A new approach for predicting the evolution of COVID-19.” Proceedings of the National Academy of Sciences of the United States of America, vol. 118, no. 1, 2021, e2023989118. https://www.pnas.org/doi/full/10.1073/pnas.2023989118 .

Both, C., R. G. Bijlsma & M. Visser (2005) : Climatic effects on timing of spring migration and breeding in 351 a long-distance migrant, the pied flycatcher Ficedula hypoleuca. J. Avian Biol. 36 : 368-373. 352

Both, C., S. Bouwhuis, C.M. Lessells, M. Visser (2006) : Climate change and population declines in a 353 long-distance migratory bird. Nature 441, 81-83 354

• Bird Migration

CCB is a leader in conservation research involving bird migration

Populations of many migratory birds depend not only on places to breed and spend the winter but also on the quality and continued availability of habitats along migration routes. The importance of identifying and protecting these non-breeding habitats has been recognized by conservation organizations throughout the world and represents a formidable international conservation challenge. CCB continues to be a leader in migration research.

The broad objectives of our research program are to determine 1) the location of migratory pathways, 2) the resource and habitat requirements of birds in passage and 3) the ecological role that geographic areas play in the lifecycle of migrant species.

Whimbrel flying over Boxtree Creek on Virginia’s Eastern Shore, heading north to their Arctic breeding grounds. Photo by Alex Lamoreaux.

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Introduction

Emerging model systems for the study of genetics and epigenetics of migration and navigation, accurate phenotyping of migratory behaviour is key, migratory traits: hard-wired or environmentally regulated, timing the seasonal migratory switch and departure, migration on the move in the genomic and epigenomic era, looking forward: exploiting the diversity of migratory species within and across taxa for comparative studies, closing remarks, acknowledgements, the genetics and epigenetics of animal migration and orientation: birds, butterflies and beyond.

Competing interests

The authors declare no competing or financial interests.

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Christine Merlin , Miriam Liedvogel , Basil el Jundi , Almut Kelber , Barbara Webb; The genetics and epigenetics of animal migration and orientation: birds, butterflies and beyond. J Exp Biol 6 February 2019; 222 (Suppl_1): jeb191890. doi: https://doi.org/10.1242/jeb.191890

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Migration is a complex behavioural adaptation for survival that has evolved across the animal kingdom from invertebrates to mammals. In some taxa, closely related migratory species, or even populations of the same species, exhibit different migratory phenotypes, including timing and orientation of migration. In these species, a significant proportion of the phenotypic variance in migratory traits is genetic. In others, the migratory phenotype and direction is triggered by seasonal changes in the environment, suggesting an epigenetic control of their migration. The genes and epigenetic changes underpinning migratory behaviour remain largely unknown. The revolution in (epi)genomics and functional genomic tools holds great promise to rapidly move the field of migration genetics forward. Here, we review our current understanding of the genetic and epigenetic architecture of migratory traits, focusing on two emerging models: the European blackcap and the North American monarch butterfly. We also outline a vision of how technical advances and integrative approaches could be employed to identify and functionally validate candidate genes and cis -regulatory elements on these and other migratory species across both small and broad phylogenetic scales to significantly advance the field of genetics of animal migration.

Migration is a common and critical behavioural adaptation for survival that has evolved in many animal taxa ranging from invertebrates to mammals. Animal migrations greatly vary in the distance travelled, from short trips as in the case of altitudinal migration (i.e. from lower to higher altitudes and back) to trans-continental migrations. All are, however, characterised by a seasonal movement to escape unfavourable environmental conditions and reach more-favourable sites during the inimical season, following precisely coordinated orientations and timing schedules. To accomplish such a remarkable navigational feat, migrants are equipped with a suite of adapted morphological, sensory, physiological and behavioural traits that are genetically encoded and turned on at the appropriate time of the year and/or under specific environmental conditions. Many studies have focused on elucidating the sensory cues and navigational strategies used for maintaining course orientation ( Reppert et al., 2010 ; Reppert et al., 2016 ; Mouritsen, 2018 ), which includes the use of celestial compass cues ( Kramer, 1950 , 1952 ; Sauer, 1957 ; Schmidt-Koenig, 1958 ; Wiltschko et al., 1987 ; Perez et al., 1997 ) and the Earth's magnetic field ( Wiltschko, 1968 ; Wiltschko and Wiltschko, 1972 ; Guerra et al., 2014 ). In contrast, the genetic architecture and molecular mechanisms that underlie the migratory phenotype, including flight direction/orientation and timing of their migration, remain poorly understood.

The explosion of technological advances in high-throughput sequencing (HTS) is starting to change this trend and holds great promise in applying non-biased approaches in the quest for de novo discovery of ‘migratory’ genes. Draft genome sequences of a few migratory species, including the monarch butterfly Danaus plexippus ( Zhan et al., 2011 ), the Swainson's thrush Catharus ustulatus ( Delmore et al., 2016 ), the willow warbler Phylloscopus trochilus ( Lundberg et al., 2017 ), the stonechat Saxicola maurus ( van Doren et al., 2017a ), the rainbow trout Oncorhynchus mykiss ( Berthelot et al., 2014 ) and the Chinook salmon Oncorhynchus tshawytscha ( Christensen et al., 2018 ) are now available, and many more are on their way. This includes those of iconic migratory birds with well-characterised behaviours and evolutionary histories such as the European blackcap Sylvia atricapilla (K. E. Delmore and M.L., personal communication; Fig. 1 ). The use of whole-genome sequencing (WGS) or other HTS techniques, such as genotyping by sequencing (GBS) or restriction site associated DNA sequencing (RAD-seq) has enabled population genomics studies characterising migratory and non-migratory populations in the monarch and avian species or populations with different inherited migratory orientation ( Zhan et al., 2014 ; Delmore et al., 2016 ). These studies revealed genomic regions with signatures of selection that may contain candidate genes for migration, cis -regulatory elements (CREs) and chromosomal inversions ( Zhan et al., 2014 ; Delmore et al., 2015 , 2016 ). RNA-seq is another approach that has been used to identify candidate migratory genes in the rainbow trout, the Swainson's thrush and the European blackbird Turdus merula by quantifying gene expression differences in brains and/or blood of individuals exhibiting variation in a migratory trait, such as migrants versus residents or populations of migrants varying in migratory orientation ( Hale et al., 2016 ; Johnston et al., 2016 ; Franchini et al., 2017 ). A comprehensive understanding of the genetic basis of migration will, however, only be achieved through the use of combinatorial genomic and epigenomic (i.e. the study of modification in the genetic material due to environmental factors) approaches and the ultimate functional validation of candidate genomic regions through in vivo genetic disruption and behavioural assays in any given migratory species.

Distribution map and migratory phenotypes of European blackcaps. (A) Adult male with its characteristic black cap. (B) Map showing the variability in migratory phenotypes with respect to distance (reflected by the length of the arrows; partial migrants as dashed lines, resident island and continental populations as green circles) and direction during autumn migration [south-west (SW) migrants red, south-east (SE) migrants blue, and a recently evolved population migrating north-west (NW) overwintering in the British Isles orange]. The approximate location of the central European migratory divide with neighbouring populations choosing distinctly different migratory orientation strategies is indicated (dashed blue line). (C) Cross-breeding experiments in blackcaps demonstrate the inherited nature of migratory direction during autumn. Offspring from selectively mated SW (red) or SE migrating parents (blue) follows the parental migratory direction, but offspring from cross-bred parents (blue, framed red) exhibit an intermediate direction. Circular plots depict directional preference of individuals (circles) and mean heading of the population (arrow). Modified from Helbig, 1991 .

Here, we provide an overview of two iconic and complementary systems with well-characterised life history strategies and migratory patterns that are poised to increase our understanding of the genetics and epigenetics of animal migration: the European blackcap and the North American monarch butterfly. We also outline our vision of how technical advances and integrative approaches could be employed on these and other migratory species across both small and broad evolutionary and phylogenetic scales to rapidly move the field of genetics of animal migration forward.

The European blackcap Sylvia atricapilla

Populations of European blackcaps, which are common breeding birds across Europe, exhibit a remarkably broad spectrum of behavioural variation in migratory status and orientation direction ( Fig. 1 A). Although selective breeding studies firmly established a genetic basis for migratory orientation decades ago ( Helbig, 1991 , 1996 ; Berthold et al., 1992 ), the recent development of genomics is starting to unlock the great potential of the extreme phenotypic variation in blackcaps to study migration genetics at the molecular level. Phenotypic variation across populations ranges from residents to short-, medium- and long-distance migrants ( Berthold et al., 1990 ). In addition, migrants exhibit variation in migratory orientation strategies and form so-called migratory divides, i.e. areas where neighbouring populations breeding in close proximity have distinct migratory routes and non-breeding grounds ( Fig. 1 B). Blackcaps are distributed continuously across the central European migratory divide, with breeding populations in Europe and overwintering areas in southern Iberia and northern Africa. In autumn, populations breeding west of the divide migrate southwest (SW) around the Mediterranean Sea, while populations east of the divide migrate southeasterly (SE) ( Fig. 1 B). In addition, since the 1960s, a growing number of birds breeding in central Europe migrate northwest in autumn to overwinter in the British Isles ( Berthold et al., 1992 ), possibly because of the milder wintering conditions and food availability in British gardens ( Plummer et al., 2015 ).

A series of common garden experiments using blackcaps with different migratory phenotypes provided clear evidence that migratory traits like distance ( Berthold and Querner, 1981 ) and direction ( Helbig, 1991 , 1996 ; Berthold et al., 1992 ) are genetically encoded and have the potential to drastically change within a few generations under strong artificial selection ( Berthold, 1991 ). In these experiments, selection lines were first generated by mating SW migrants with SW migrants and SE migrants with SE migrants for several generations in order to obtain populations that are fixed for each migratory orientation ( Fig. 1 C). In cross-breeding experiments, SW migrants were then mated with SE migrants, and the progeny was reared in isolation from their parents to exclude the possibility that migratory direction could be learned. The intermediate orientation taken by hybrids unambiguously demonstrated the genetic inheritance of this migratory trait ( Helbig, 1991 , 1996 ; Berthold et al., 1992 ).

Like most songbirds, blackcaps migrate at night and mostly by themselves. Because parents often leave the breeding grounds earlier ( Flack et al., 2018 ), young inexperienced birds on their first migratory journey have to find their way on their own. Despite heading towards a completely unfamiliar destination, they reach it with amazing precision. Behavioural experiments in the lab and in the wild have shown that migratory birds use various compasses for course orientation. Like other night navigators, blackcaps rely on a magnetic compass calibrated by sunrise/sunset polarized light cues and a star compass ( Wiltschko and Merkel, 1966 ; Wiltschko and Wiltschko, 1972 ; Wiltschko et al., 1987 ; Phillips and Moore, 1992 ; Cochran et al., 2004 ). Birds are also equipped with an inherited time schedule and at least an initial migratory direction that are integrated into a spatiotemporal strategy signalling of when to leave the breeding grounds, and how far and in which direction to fly ( Liedvogel et al., 2011 ).

The variability in migratory strategy that presumably reflects differences in direction and strength of selection make blackcaps an excellent model to study not only the genetic variation in migratory traits, but also the effects of selection on this complex phenotype ( Fig. 1 ) ( Liedvogel et al., 2011 ; Delmore and Liedvogel, 2016 ). The inability to estimate population structure using neutral markers across the genome ( Helbig, 1994 ; Pérez-Tris et al., 2004 ; Mettler et al., 2013 ) suggests that migratory traits of neighbouring populations do not correlate with strong overall genetic differentiation. Migratory traits rather seem to be controlled either by selection processes on relatively few genomic regions and/or differences in gene expression. This hypothesis is supported by the finding that few genes appear to be involved in determining the expression of migratory traits ( Helbig, 1996 ), and by the apparent strong genetic correlation between different migratory traits for which variation in one trait is often dependent on variation in another ( Pulido et al., 1996 ; Pulido and Berthold, 1998 ). With a de novo assembly of the blackcap reference genome underway, and the technological advances in HTS, characterizing the genetic basis of the variation in migratory phenotypes at the genome-wide level will now be feasible. Whole-genome re-sequencing of blackcaps exhibiting the full spectrum of migratory phenotypes is anticipated to provide insights into the origins and maintenance of variation in the migratory behaviour and the identification of genetic variants underlying focal traits in migratory behaviour, including the propensity to migrate, orientation and distances travelled.

The North American monarch butterfly Danaus plexippus

With established genomic resources that include tools for germline editing, the North American monarch butterfly is an equally compelling model and arguably the best suited to define the genetic and epigenetic basis of insect long-distance migration. Monarch butterflies from the eastern North American population undergo one of the most impressive annual long-distance migrations accomplished by any insects, have a rich and well-documented natural history, and progress has been made in understanding the neurobiological basis of their migration ( Urquhart, 1960 ; Reppert et al., 2016 ). Each autumn, coincident with decreasing day length (i.e. photoperiod), monarchs east of the Rocky Mountains depart from their northern breeding sites, migrating south to reach their overwintering sites in Mexico, where they remain in a state of reproductive quiescence (i.e. diapause) until the spring ( Fig. 2 A). Then, with increased temperatures and photoperiod, migrants become reproductive, mate and re-migrate northward to the southern United States where females lay their progeny onto milkweed plants, the unique host on which larvae feed. Both autumn migrants and spring re-migrants use a time-compensated sun compass as a primary compass system to guide their course orientation ( Perez et al., 1997 ; Mouritsen and Frost, 2002 ; Froy et al., 2003 ; Merlin et al., 2009 ) and may also take advantage of their ability to orient to the magnetic field when the sun is not visible ( Guerra et al., 2014 ). Unlike birds, the re-migrant adults do not make the return back to the breeding sites. Instead, the completion of the annual migratory cycle takes at least two successive generations of reproductively competent spring and non-oriented summer butterflies that follow the northward progression of milkweed availability ( Fig. 2 A). The butterflies that emerge by late summer in the northern range of the breeding sites are then re-programmed into autumn migrants. Just like young inexperienced birds, autumn migratory monarchs travel thousands of kilometres on their virgin migratory journey and locate their overwintering grounds with astonishing precision, sometimes even clustering on the same trees as their great-grandparents. However, unlike birds, migratory direction cannot just be an inherited trait, as autumn migrants not only share the genetic make-up of their non-migratory parents but also reverse flight orientation by 180 degrees in the spring when they become remigrants.

Annual migratory cycle of North American monarch butterflies ( Danaus plexippus ) and world-wide distribution of monarch populations. (A) Monarch butterflies in flight (left). Photo credit: MonarchWatch. The southward migration of North American monarchs coincides with autumnal decreasing photoperiod that is sensed by an endogenous seasonal timer (middle). Monarchs east of the Rocky Mountains (brown line) navigate over long distances (red arrows) to their overwintering sites in Mexico (orange dot) ( Urquhart, 1960 ; Brower, 1995 ). Some migrants fly towards Florida where they mix with resident populations (dashed grey arrow) ( Knight and Brower, 2009 ; Reppert et al., 2016 ). Monarchs west of the Rockies migrate southward in the autumn but overwinter along the Californian coast (red arrows). In the spring, eastern monarchs in Mexico become reproductive, mate and fly northward to the southern United States (right; red arrows). Subsequent generations of spring and summer butterflies progress northward following the latitudinal emergence of their host plants to repopulate the northern summer breeding grounds (black arrows). Whether monarchs overwintering in Florida make the trip back north is not known (dashed grey arrow). Western monarchs also migrate northward in the spring (black arrows). The key roles that environmental changes (photoperiod and temperature) play in the Eastern North American monarch migration suggest an epigenetic basis to the monarch migration. Modified from Reppert et al., 2016 . (B) World-wide geographic distribution of monarch populations and their migratory/non-migratory status. Populations present on both the North American and the Australian continents exhibit seasonal migrations ( Urquhart, 1960 ; Brower, 1995 ; Dingle et al., 1999 ) (circle with red border). Populations located in Central America, South America, and the Caribbean, are felt to be non-migratory ( Zhan et al., 2014 ), primarily based on wing morphology. Populations with similar genetic structures, determined by whole-genome re-sequencing ( Zhan et al., 2014 ), are shown by similar colour inside the circles.

The switch in migratory physiology and behaviour and the reversal in flight orientation that occur respectively in the autumn and spring in response to environmental changes suggest an epigenetic control of the eastern North American monarch migration ( Fig. 2 A). While seasonal changes in autumn photoperiod and temperature likely control the migratory switch, the trigger that reverses flight orientation from south in autumn migrants to north in spring remigrants has been identified as a prolonged exposure to low temperatures that mimic the coldness experienced by migrants at their overwintering sites ( Guerra and Reppert, 2013 ). The ability to experimentally re-program the southward migratory orientation of autumn migrants into a northward orientation in controlled laboratory conditions provides a unique opportunity to define the epigenetic mechanisms underlying reorientation of the sun compass in the brain ( Guerra and Reppert, 2013 ).

In fact, transient exposure of animals to environmental factors has previously been shown to induce and maintain behavioural states by changing the neuronal epigenetic landscape that regulates transcription of genome-wide gene expression over long periods of time ( Bonasio, 2015 ; Yan et al., 2015 ). Reprogramming of gene regulatory networks could involve the alteration of chromatin structure via histone post-translational modifications, and/or the activity of specific transcription factors that bind to CREs to activate or repress gene expression ( Bonasio et al., 2010 ). In contrast to birds, DNA methylation is unlikely to play a significant role in the monarch seasonal migratory behaviour, as this species appears to lack detectable patterns of de novo DNA methylation ( Zhan et al., 2011 ; Bewick et al., 2017 ). Temperature-dependent post-transcriptional mechanisms involving regulation of gene expression by microRNAs, mRNA splicing or RNA editing are alternative mechanisms by which flight orientation could also be re-programmed, as temperature-dependent regulation of these mechanisms has previously been reported in other organisms ( Low et al., 2008 ; Garrett and Rosenthal, 2012 ; Bizuayehu et al., 2015 ).

Despite being the focal population for studying the mechanisms that drive monarch migration, the North American population is not the only monarch population found around the globe. Other populations are present in Oceania, Central America, South America, the Caribbean, Europe and North Africa ( Fig. 2 B). While the Australian population also undergo a seasonal migration, although of shorter distances than the North American population ( Dingle et al., 1999 ), all the others are felt to be non-migratory ( Dockx, 2007 ; Altizer and Davis, 2010 ; Zhan et al., 2014 ). These differences in migratory behaviour between populations have also been leveraged to identify putative genes involved in monarch migration (see below).

As illustrated with the blackcap and the monarch butterfly, there are clear differences in the nature of the molecular basis (genetic and/or epigenetic) underlying migration between taxa that may be inherent to the biology of each species. The diversity of migratory phenotypes across bird species, together with the existence of populations with clearly defined and differing migratory orientations within a single species, makes them uniquely suited for studying the genetic basis of migration. The seasonal plasticity of monarch butterflies trans-generational migratory behaviour, on the other hand, offers opportunities to study the epigenetic basis of migration. With the ability to maintain colonies in the laboratory year-round, a fast generation time and its accessibility to in vivo genetic manipulation ( Merlin et al., 2013 ; Markert et al., 2016 ; Zhang et al., 2017 ), the monarch is also uniquely suited to rapidly assess the function of candidate migratory genes ( Fig. 3 B). Using these two species as focal examples, we highlight how integrating a diverse array of approaches used in behavioural ecology, sensory biology, evolutionary biology, genetics and molecular biology can be leveraged to move the field of migration genetics forward. Ultimately, the discovery of generalizable mechanisms, or lack thereof, will, however, require the diversification of strategically chosen migratory animal models across taxa.

Cutting-edge genomic approaches for identifying and functionally validating candidate migratory genes and cis -regulatory elements. (A) Sequencing-based technologies for identifying gene expression, cis -regulatory elements (CREs) and the prediction of transcription factors (TFs) controlling gene expression. ATAC-seq and DNase-seq are used to identify nucleosome-depleted chromatin regions in which TFs bind DNA enhancer sequences to activate or repress the transcription of the genes they control. The activity of enhancers can be determined using ChIP-seq of histone marks such as H3K27me3 (associated with poised/inactive enhancers) and H3K27Ac (associated with active enhancers). ChIP-seq of RNA polymerase II (Pol II) can be used to measure actual transcription rates, and the transcriptome can be explored using RNA-seq. The nature of the TF binding in enhancers can be predicted based on the motif sequences to which they leave footprints using bioinformatics tools. ATAC-seq, assay for transposase-accessible chromatin with high throughput sequencing; ChIP, chromatin immunoprecipitation; H3K27me3, trimethylation of lysine 27 on histone 3; H3K27Ac, acetylation of lysine 27 on histone 3. (B) The CRISPR/Cas9 system as a tool for editing germline and post-mitotic cells. The CRISPR/Cas9 system relies on a Cas9 protein complexed to a guide RNA that is complementary to a 20-bp target sequence harbouring a protospacer adjacent motif (PAM site; left). Upon binding to the targeted genomic sequence, the RNA-guided Cas9 induces a DNA double-stranded break (DSB) that stimulates cellular DNA repair through either error-prone non-homologous end joining (NHEJ)-mediated repair or homology-directed repair. NHEJ-mediated repair can introduce insertions/deletions (indels) that lead to frame shifts and subsequent gene disruptions through the introduction of an early stop codon. Gene correction and/or addition can also be achieved through homology-directed repair by co-injection of an exogenous DNA donor template. Germline editing is a technique classically used in insects, fish and mammals that can be achieved by the co-injection of a Cas9 mRNA or a Cas9 protein with a gRNA into a ‘one-nucleus stage’ embryo (right). The injected embryo gives rise to a mosaic founder, which once mated, can produce mutant progeny. Alternatively, CRISPR/Cas9-mediated editing can be achieved in adult post-mitotic cells through the delivery of viral vectors expressing Cas9 and the gRNA in the tissue or structure of interest. In both germline and post-mitotic editing, co-injection of a donor DNA template can stimulate gene correction or gene addition by knock-in (KI). Genetically manipulated organisms can then be subjected to assays such as flight orientation and migratory restlessness to test the function of targeted genes. AAV, adeno-associated virus; LV, lentivirus.

Regardless of the species or population of interest, accurate knowledge of the migratory phenotype and the ability to characterize and quantify it is an essential prerequisite to characterize the underlying molecular machinery that modulates the variation in migratory behaviour. Capture–mark–recapture methods and careful assessment of the individual's morphology and physiology associated with the migratory state (i.e. wing size and shape, fat stores) have provided useful information, with the exception of the detailed behavioural strategies of individual migrants.

Aided by the miniaturization of tracking devices, more advanced on-board tracking technologies can now inform both orientation and timing characteristics of migratory routes taken by individuals over long distances in their natural habitat. The most informative data come from studies using global positioning system (GPS) satellite transmitters ( Perras and Nebel, 2012 ), where real-time recording of the signal by satellites enables the long-term monitoring of migratory movement around the globe without recapture of the tagged animal. Using this technology for real-time monitoring of smaller passerines such as the European blackcap is not yet possible due to the relatively large size of transmitters. Current alternative technologies include archival tags that record light-intensity data (i.e. light-level geolocators) as well as radio-transmitters and telemetry systems ( Stutchbury et al., 2009 ; Kishkinev et al., 2016 ; Taylor et al., 2017 ). In contrast, no comparable devices exist yet for insects. Until the next revolution in miniaturization, when geolocators or GPS devices weighing only a few tens of milligrams could be developed, entomological radars will likely remain the method of choice to track flight paths and strategies of migratory insects in nature ( Chapman et al., 2008 , 2010 , 2011 ; Bruderer, 2016 ; Woodgate et al., 2016 ).

Assessing migratory behaviour in laboratory conditions will be equally important to ultimately test gene function in vivo . A number of laboratory-based methods have been developed in birds and butterflies for use as proxies of migratory status and orientation. A commonly used method in birds relies on recording the characteristic migratory restlessness exhibited during the migratory season, even when kept in cages in controlled conditions ( Kramer, 1949 , 1950 ). Quantified timing and directedness of this behaviour coincide with timing and orientation of free-flying conspecifics ( Gwinner, 1968 , 1986 ; Berthold, 1973 , 1995 ). Behavioural cages for testing bird migratory status and orientation, known as ‘Emlen funnel’ ( Emlen and Emlen, 1966 ), are generally funnel-shape arenas. The walls are covered with coated and scratch-sensitive paper such that restless migratory birds leave scratch marks on the inclined walls. These marks can be quantified to measure activity levels and mean orientation bearings ( Helbig, 1991 ; Wiltschko and Wiltschko, 1995 ; Mouritsen et al., 2009 ; Zapka et al., 2009 ). The precise monitoring of timing of migration, including onset, duration and intensity of migratory restlessness, can be achieved by fitting the cage with passive infrared detectors that record activity profiles ( Gwinner, 1967 ). Related methodological approaches have been adapted to insects, where the orientation of a tethered insect is tracked in a flight simulator apparatus ( Mouritsen and Frost, 2002 ). This cylindrical plastic barrel can be used outdoors under natural sky conditions to assess spontaneous migratory orientation of tethered insects that are free to fly in any direction on the horizontal plane ( Mouritsen and Frost, 2002 ; Froy et al., 2003 ; Dreyer et al., 2018 ). Similar circular plastic arenas have also been adapted to track orientation in aquatic animals, including fish ( Mouritsen et al., 2013a ). Even though these approaches do not fully recapitulate the natural behaviour exhibited by a migrating animal ( van Doren et al., 2017b ), they will be valuable tools to ultimately test the function of migratory genes in vivo as genetically manipulated animals cannot be released into the wild.

Evidence suggests that migratory traits are genetically inherited and/or environmentally regulated in different taxa. The impressive series of common garden experiments with European blackcaps provided evidence for a genetic basis of migratory traits in birds ( Fig. 1 C) ( Berthold et al., 1992 ; Helbig, 1991 , 1996 ). Genetic inheritance of both timing and migratory direction was further supported by elegant displacement experiments in which both experienced adults that already had successfully completed a migratory journey and naïve juvenile birds on their first journey were displaced from their original location ( Perdeck, 1958 ; Thorup et al., 2007 ). Inexperienced juveniles followed an innate clock and compass strategy (e.g. vector navigation), leaving at the right time and flying the correct distance in the inherited migratory direction. In contrast, adult birds that had the opportunity to learn landmarks on their previous trip were able to compensate for the displacement, actively navigating to their original wintering area ( Perdeck, 1958 ).

In contrast to blackcaps, genetic inheritance of migratory traits in North American monarchs has not been established using breeding experiments, and the interpretation of displacement experiments remains controversial ( Mouritsen et al., 2013b ; Oberhauser et al., 2013 ). Nevertheless, the multigenerational nature of the monarch migratory cycle in the Eastern United States suggests that genes encoding migratory traits are encoded in the monarch genome, and thus heritable, but are turned on or off in response to seasonal environmental changes in the migratory generation.

Across taxa, the onset of migratory behaviour and departure from breeding grounds is tightly linked to the changing seasons. To precisely follow timing schedules, animals keep track of the seasons using an endogenous seasonal timer that measures photoperiodic changes.

In birds, migratory timing appears to be controlled in concert with timing of reproduction and moult by a circannual clock in both wild populations and caged birds ( Gwinner, 2003 ; Visser et al., 2010 ). Correlative studies using a candidate gene approach identified the Clock gene as a possible candidate for migratory timing ( Peterson et al., 2013 ; Bazzi et al., 2015 , 2016 ; Saino et al., 2017 ; Contina et al., 2018 ). The otherwise highly conserved Clock gene shows length variation at a poly-glutamine repeat, and shorter alleles have been associated with earlier arrival times at the breeding grounds in barn swallows ( Bazzi et al., 2015 ). In the same species, elevated methylation levels responsible for reduced transcription at the Clock locus have been shown to correlate with earlier spring migration and onset of breeding ( Saino et al., 2017 ).

In monarchs, circadian clocks or clock genes in the brain have also been proposed to take part in photoperiodic measurement ( Reppert et al., 2016 ; Denlinger et al., 2017 ). They could not only trigger the photoperiodic programming of the migratory state in the autumn but also time the departure of migrants from their breeding range. Although the role of circadian clocks in triggering the migratory switch remains to be explored, their importance for monarch migration is not unprecedented. Circadian clocks located in the antennae provide the necessary timing component that allows autumn migrants and spring remigrants to maintain course orientation by compensating for the daily changes of the position of the sun in the sky – the main compass cue used for navigation in this species ( Perez et al., 1997 ; Mouritsen and Frost, 2002 ; Froy et al., 2003 ; Merlin et al., 2009 ; Guerra et al., 2012 ). Brain clocks are, however, most likely involved in the induction of the migratory traits and could impact migratory traits by regulating the transcription of clock genes and clock-controlled genes in this tissue ( Hardin and Panda, 2013 ). With the availability of several clock gene knockouts in monarchs ( Merlin et al., 2013 ; Markert et al., 2016 ; Zhang et al., 2017 ), functionally determining whether circadian clocks or clock genes play a role in the migratory switch should now be feasible, at least in this system.

The current revolution in HTS technologies and functional genomic tools and their applicability in non-model organisms provide unique opportunities to comprehensively dissect the genetic and epigenetic basis of migration ( Fig. 3 ).

The use of WGS has already enabled population genomics studies between migratory and non-migratory populations for the identification of variation in the genome associated with variation in the phenotype. In monarchs, for example, re-sequencing of more than 100 individuals from migratory and presumably non-migratory populations from around the globe ( Fig. 2 B) identified genomic regions of divergent natural selection comprising ∼500 candidate genes proposed to be associated with shifts in migratory behaviour ( Zhan et al., 2014 ). The interpretation of such genome-wide scans for a signature of selection is, however, often restricted to protein-encoding genes. Although variations within coding regions could have a fundamental impact on protein function, without functional evidence, variations in non-coding regions such as in CREs should not be excluded as they could affect differences in expression of genes located kilobases away. Quantification of differential gene expression between migratory and non-migratory forms can be achieved using RNA-seq, as exemplified by a few studies in fish and insects ( Jones et al., 2008 ; Zhu et al., 2009 ; Hale et al., 2016 ). However, differential gene expression studies based on RNA-seq usually yield long lists of candidate genes and the prioritization process for further functional studies (discussed below) is not always straightforward. More-integrated approaches that consider not only loci under selection but also gene expression and its transcriptional or post-transcriptional control within a single species could help in narrowing down the list to a smaller number of promising candidates.

At the transcriptional level, gene expression is regulated by the binding of transcription factors to specific DNA sequences in CREs located in open chromatin regions, and by the recruitment of specific cofactors mediating histone post-transcriptional modifications (hPTMs) that regulate enhancer activity ( Bonasio et al., 2010 ; Lelli et al., 2012 ) ( Fig. 3 A). The nature of the recruited cofactors, which include chromatin-modifying enzymes ( Bonasio et al., 2010 ), determines the type of hPTMs that ultimately either inactivate or stimulate transcriptional activity. Identifying CREs and correlating their differential activity with the differential expression of the genes they regulate in a given tissue between migrants and non-migrants would be a powerful approach to pinpoint important candidate migratory genes and genomic regulatory elements. Of note, genomic variation associated to variation in migratory phenotypes in population genomic studies could occur in these open chromatin regions. DNase-seq, a method that identifies genome-wide nucleosome-free DNA regions that are hypersensitive to DNase I digestion ( Song and Crawford, 2010 ), has been used extensively to identify gene regulatory elements in open chromatin regions. The large amount of starting material it requires may, however, be an important limitation on its application to migratory species. ATAC-seq (assay for transposase-accessible chromatin followed by sequencing), a technique relying on the integration of a mutated hyperactive transposase into open chromatin regions emerged as an attractive alternative as it requires 1000-times less starting material than DNase-seq ( Fig. 3 A) ( Buenrostro et al., 2013 , 2015 ). The activity status of CREs can be determined using chromatin immunoprecipitation (ChIP) of flanking histone marks followed by high-throughput sequencing ( Fig. 3 A). These are conserved across organisms, and antibodies against marks associated with active and poised/inactive promoters and enhancers ( Creyghton et al., 2010 ; Shlyueva et al., 2014 ; Heinz et al., 2015 ) are commercially available.

DNA methylation is another gene regulatory mechanism by which behavioural and physiological plasticity can be regulated ( Lyko et al., 2010 ; Foret et al., 2012 ; Herb et al., 2012 ; Pegoraro et al., 2016 ). Reprogramming of gene expression and gene regulatory networks between different migratory phenotypes could occur through changes in DNA methylation patterns, which can be studied using bisulfite-sequencing methods, even for species without a reference genome ( Klughammer et al., 2015 ). The existence of such a mechanism may, however, be taxa-, lineage- or species-specific. While genome-wide patterns of DNA methylation occur broadly in vertebrates, including birds ( Li et al., 2011 ; Laine et al., 2016 ), DNA methylation does not appear to be ubiquitous in insects ( Bewick et al., 2017 ). Some species such as monarchs seem to lack detectable patterns of de novo DNA methylation ( Zhan et al., 2011 ; Bewick et al., 2017 ).

Post-transcriptional regulation of gene expression by microRNA (miRNA), RNA editing and alternative splicing should also be considered as potential mechanisms by which steady state mRNA levels could be differentially regulated between migratory phenotypes and/or seasons. So far, with the notable exception of differential expression of miRNAs in migratory and non-migratory forms of the monarch butterfly ( Zhan et al., 2011 ), to our knowledge, none of these mechanisms has been studied in any other migratory species. The current array of HTS approaches offers unprecedented opportunities to exploit all these possibilities, and should accelerate the identification of genes, epigenetic and post-transcriptional mechanisms underlying the migratory phenotype.

Ultimately, providing proofs of causality will be imperative. Genetically manipulating identified candidate genes and regulatory regions in vivo to assess how genetic disruption affects the migratory physiology and behaviour will undoubtedly be challenging, but should be facilitated by increasingly accessible functional genomic tools. Classical approaches that use RNA interference (RNAi) to knock genes down are possible ways to test gene function, but could have limited applications, as the gene knockdown is often incomplete. Unlike RNAi, targeted genome editing allows the generation of complete gene knockouts and the creation of mutations in CREs that would render them non-functional. The use of RNA-guided nucleases derived from clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 system is arguably one of the most exciting avenues of investigation for the study of migration genetics because of its simplicity and applicability to virtually any species of interest. CRISPR/Cas9 enables site-specific genetic modifications by inducing DNA double-stranded breaks (DSBs) at genomic locations of choice. DSBs repair by the cellular machinery stimulates the introduction of small random insertions/deletions through non-homologous end joining (NHEJ) repair, or the introduction of donor constructs through homology-directed repair ( Fig. 3 B) ( Gaj et al., 2013 ; Kim and Kim, 2014 ).

Thus far, CRISPR/Cas9 has been successfully applied to generate full knockouts and deletion mutants in the migratory monarch butterfly ( Markert et al., 2016 ; Zhang et al., 2017 ), thereby unlocking its potential for the functional characterization of migratory candidate genes and CREs. Germline genome editing in this species has been achieved with high efficiency by injection of Cas9 mRNA and a single guide (sg)RNA into fertilized embryos within the first nuclei divisions ( Fig. 3 B) ( Merlin et al., 2013 ; Markert et al., 2016 ). Similar strategies are commonly used in model species such as the zebrafish and mouse, in which embryos can be collected at an early development stage, and should therefore be transferable to other migratory species such as fish and mammals. In birds, efficient germline transformation has proved to be more challenging due to constraints associated with the structure of the zygote and the opacity of the oocyte ( Dimitrov et al., 2016 ). However, the use of CRISPR/Cas9-mediated modification of primordial germ cells in vitro has been shown to overcome this limitation in chickens ( Dimitrov et al., 2016 ; Oishi et al., 2016 ; Woodcock et al., 2017 ), making it a possible approach for gene editing in migratory bird species. In the event that breeding colonies in laboratory conditions preclude the implementation of similar approaches, strategies for genome editing in post-mitotic cells using virus-mediated delivery of CRISPR/Cas9 into the appropriate tissue/organ, depending on the presumptive function of the gene of interest, are attractive alternatives ( Fig. 3 B). Expanding genetic tools in migratory species for knocking genes out both in the whole organism and in a tissue-specific manner, as well as for knocking in reporters that can be used to probe circuit function, will be important milestones for the field. Combined with diagnostic physiological and behaviour assays, genetically engineered migratory organisms will ultimately facilitate a systematic dissection of the molecular and cellular mechanisms underlying animal migration and orientation.

The identity of genes underlying migratory orientation and how they code for this remarkable behaviour remains largely unknown. Applying cutting-edge HTS technologies and powerful behavioural approaches in the European blackcap and the North American monarch butterfly hold great promise to identify and functionally validate migratory genes and molecular pathways. However, restricting genetic and molecular approaches to only a few species can limit our ability to differentiate the molecular pathways that are species- or taxa-specific from those widely generalizable across taxa. Exploiting a larger number of strategically chosen migratory species for comparative studies across evolutionary scales and taxa will be key, and will be facilitated by the increasing accessibility of genomics and molecular tools in non-model organisms.

Among other migratory insect species, the nocturnal Australian bogong moth Agrotis infusa is an ideal complement to the diurnal monarch butterfly for molecular comparative studies. Each spring, migratory moths leave the breeding grounds in southern Queensland and northwestern New South Wales and migrate south to reach the Australian Alps, where they aggregate to aestivate (i.e. spend the summer in a state of dormancy) in cool mountain caves. In the autumn, the same individuals migrate back to the breeding sites to reproduce, and die soon after, leaving their progeny to become the new generation of southward migrants the following spring ( Fig. 4 A) ( Heinze and Warrant, 2016 ; Warrant et al., 2016 ). The opposite flight orientations taken by bogongs and monarchs each autumn and spring in different hemispheres of the Earth, where seasons are reversed, may suggest that similar environmental cues (i.e. photoperiod and/or temperature changes) recalibrate compass orientation. Leveraging this parallel using genomic comparative approaches may accelerate the identification of a set of conserved molecules and pathways underlying migratory orientation in insects. All the genomic tools mentioned previously, including CRISPR/Cas9 that has been implemented in other moth species ( Koutroumpa et al., 2016 ; Chen et al., 2018 ), should be easily transferable to the bogong.

Diversifying systems from different taxa will accelerate our understanding of the genetics and epigenetics of migration. (A) An Australian bogong moth, Agrotis infusa , aestivating in a cool cave (top); photo credit: Ajay Narendra. Map shows the southward spring migration (orange arrow) and the northward autumn remigration (grey arrow). Modified from Dreyer et al., 2018 . (B) North American migratory steelhead (top) and resident rainbow (bottom) trout ( Oncorhynchus mykiss ) are the same species, although the appearance of both life-history patterns can differ as a function of the environment and food. Migratory steelheads are an anadromous form of resident rainbow trout, migrating to sea and returning to their natal stream or river to spawn. The migratory pathway of steelhead in their Pacific basins is depicted in red. Some steelhead populations continue to migrate inland after return. (C) Lesser long-nosed bats ( Leptonycteris yerbabuenae ) at the peak of the spring migration arriving in maternity roosts in the Sonoran desert; photo credit: Jens Rydell. Migratory cycle of L. yerbabuenae is shown on the right. These nectar-feeding bats, which are found in southern Arizona and northern Mexico in the summer, massively migrate south in the autumn by presumably following Agave corridors to overwinter in central Mexico. In the spring, pregnant females migrate north along a corridor of blooming columnar cacti on the western coast of Mexico and Baja California, where they give birth to their young in maternity caves.

Although the spectrum of migratory orientation is exceptionally diverse in blackcaps, other bird species also exhibit differences in migratory timing and direction. Two subspecies of willow warbler form a migratory divide in central Scandinavia, and WGS combined with SNP-chip data identified two chromosomal regions showing strong genetic differentiation between subspecies associated with differences in migratory behaviour ( Lundberg et al., 2017 ). The Swainson's thrush is another compelling model forming a migratory divide in western North America, and hybrids in this system have been shown to take intermediate migratory routes ( Delmore and Irwin, 2014 ). Variation in one genomic region linked to variation in migratory behaviour was identified, which included the Clock gene ( Delmore et al., 2016 ). However, the overall lack of consistency between identified regions and genes linked to migratory traits across species may call for caution in assuming one common and simple genetic basis of migratory traits ( Delmore and Liedvogel, 2016 ). Performing comparative analyses of gene expression and of the activity of CREs present in the regions under selection in different bird systems could help determine whether the molecular pathways responsible for flight orientation in birds are conserved or not.

Increasing the number of vertebrate taxa to include fish and mammals will be equally important to unravel molecular mechanisms underlying vertebrate migration and orientation behaviour. Mass seasonal migrations are also performed below the ocean surface by many fish species. Indirect evidence for a genetic basis of migratory orientation comes from a study on Atlantic eels. American ( Anguilla rostrata ) and European ( Anguilla anguilla ) eels both start their migratory journeys in the Sargasso Sea, but their migration differs in distance and direction. American eels migrate towards the coast of North America while European eels take a longer route to Europe. Icelandic eels, which have been characterised as possible hybrids ( Albert et al., 2006 ), migrate using an intermediate orientation. The Salmonidae are another well-suited family to study the underlying genetics of fish migration and its spatiotemporal characteristics because of the high variability in migratory life history strategies within and among species ( Dodson et al., 2013 ). In these species, the high variability in migratory phenotype is not mirrored in overall genetic differentiation ( O'Malley and Banks, 2007 ). Variation within the Clock gene has however been shown to match a cline in spawning time in the Chinook salmon O. tshawytscha ( O'Malley and Banks, 2008 ; O'Malley et al., 2010 ). The genetics of adult migration timing have been further characterized through genome-wide association mapping of migratory (steelhead) populations exhibiting two distinct timing strategies in the wild ( Hess et al., 2016 ). This study identified a 46 kb region overlapping an oestrogen receptor cofactor GREB1 , a relevant candidate gene since upstream migration happens during sexual maturation in steelhead and other salmonids ( Choi et al., 2014 ). The same gene has recently been shown to have a major effect on adult migration timing in both O. mykiss and O. tshawytscha ( Prince et al., 2017 ).

Research in O. mykiss , a species with both resident (rainbow trout) and migratory (steelhead) phenotypes, also identified several genomic regions associated with adaptations during smoltification, the process facilitating transition in preparation for seaward migration ( Fig. 4 B) ( Nichols et al., 2008 ; Hecht et al., 2012 ; Hale et al., 2013 ). Epigenetic regulation of gene expression may contribute to controlling variation in migratory behaviour during the smoltification process, as differentially DNA methylated sites were found to distinguish migratory from resident lines, including in regions associated with circadian rhythm pathways ( Baerwald et al., 2016 ). Correlating differential DNA methylation to differential gene expression in migratory versus resident forms of O. mykiss could be used to identify promising candidate genes underlying fish migration.

Bats living in temperate climate also embark on massive seasonal migrations with navigational abilities that are no less impressive than those of birds, insects and fish. Two examples of migratory bats with untapped potential for comparative genomics and molecular studies are the lesser long-nosed bat Leptonycteris yerbabuenae , and the Mexican free-tailed bat Tadarida brasiliensis . These bats, which are found in the southwestern United States and northern Mexico in the summer, migrate southward in the autumn to their respective overwintering sites in central or southern Mexico. Mating occurs in the early spring and pregnant females migrate north to give birth in maternity roosts before returning in the autumn, along with their progeny, to central Mexico ( Fig. 4 C) ( Cockrum, 1967 ; Wilkinson and Fleming, 1996 ). Variation in migratory tendency (migratory versus residents) has also been reported in T. brasiliensis populations ( Cockrum, 1967 ). Both the seasonal change in migratory direction and the variation in migratory phenotype could be exploited to uncover the molecular pathways underlying bat migration and orientation. Initiatives to sequence the genome of all bat species are underway (BAT 1K; http://www.bat1k.com ). With blueprints in hand, applying cutting-edge genomic and epigenomic tools in these species (from blood samples, wing punches or even brain tissues in the case of the abundant T. brasiliensis ) should take off.

The cyclic, mostly seasonal, nature of many species migrations up in the sky, on land and in the oceans has fascinated scientists and the public alike for decades. Its broad occurrence across the animal kingdom begs the question of whether common mechanisms or different molecular toolkits underlie migratory strategies. The involvement of genes associated with clock function in different taxa suggests that mechanisms timing the migration may use a similar molecular pathway. In contrast, the key genomic regions and molecular underpinnings responsible for variation in migratory orientation remain largely unknown as the regions identified in different species so far lack consistency and have not been validated functionally. The answers are out there somewhere in the DNA sequences, and systematically implementing integrative approaches in a diverse array of species should accelerate the pace at which we learn where to look to unveil some of the molecular secrets behind migratory behaviour.

We thank Gernot Segelbacher, Krista Nichols, MonarchWatch, Ajay Narendra, Jens Rydell, and Melinda Baerwald for allowing use of their photos in Figs 1 , 2 and 4 ; Andrew Clarke for generating the map in Fig. 4 B; and Aldrin Lugena for comments on the manuscript.

Author contributions

C.M. and M.L. contributed equally to the writing of this manuscript.

C.M. was supported by grants from the National Science Foundation (IOS-1456985 and IOS-1754725) and the Esther A. and Joseph Klingenstein Fund; M.L. was supported by the Max Planck Society (Max-Planck-Gesellschaft) through a Max Planck Research Group grant.

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World Migratory Bird Day 2024: Protect Insects, Protect Birds

Shorebirds are probably not the first group of birds that come to mind when thinking of ‘insectivorous birds’. Just like the more recognizable aerial insectivores—flycatchers, swallows, and swifts—many shorebirds also rely on insects to provide their basic energy needs. This is especially true while they complete their impressive long-distance migrations and when chicks are growing rapidly, transitioning from vulnerable flightless young to winged acrobats of the air. 

Rick Lanctot and Sarah Saalfeld , two shorebird biologists with the Alaska Migratory Bird office in Anchorage, have been studying arctic-breeding shorebirds for years at their main field site in Utqiagvik, Alaska , which is located within the current and ancestral homelands of the Iñupiat peoples. The focus of their study is to understand what factors limit the size of breeding shorebird populations. Environmental factors like timing of snow melt, seasonal temperatures, amount of predators, and insect abundance can impact the survival of adult birds, how many nests hatch, and how well chicks grow and survive within a population.

The Semipalmated Sandpiper , a 2024 World Migratory Bird Day featured species, breeds throughout the Alaskan and Canadian Arctic. It's listed as ‘Near Threatened’ by the Global IUCN Red List , and its central and eastern populations are listed as U.S. Fish and Wildlife Birds of Conservation Concern , a designation representing the service’s highest bird conservation priorities outside of species listed by the Endangered Species Act . 

The Semipalmated Sandpiper is also a focal species of Lanctot and Saalfeld’s Arctic shorebird research. Countless hours of fieldwork looking for and monitoring nests, capturing adults and chicks, while simultaneously monitoring ecological conditions, like insect emergence, have led to important discoveries about the species’ ability, or rather inability, to adapt to a warming arctic. 

“There appears to be a long-term trend, with lots of interannual variation, toward earlier, warmer, and longer summers in Utqiaġvik”, says Lanctot. 

This variability in the summer breeding season has made it difficult for long-distant migratory shorebirds like the Semipalmated Sandpiper. The start of these birds’ northward migrations is relatively fixed, based mainly on the amount of daylight, limiting when birds can reach their arctic breeding grounds. Meanwhile, spring air temperatures and snow melt in the Arctic are generally getting earlier and by association so too is the timing of insect emergence.

“In years when spring arrives exceptionally early, shorebirds arrive late to the party, and a mismatch happens between peak insect emergence and shorebird nests hatching. Our research has shown that chicks hatching after peak insect emergence have lower growth and survival”, Saalfeld explains.

Adult arctic-breeding shorebirds rely on insects to replenish their energy reserves after a long migration and to lay eggs. Newly hatched chicks are even more dependent on insects as they feed themselves, growing from small puff balls to winged adults in about 2-4 weeks. Lanctot and Saalfeld’s long-term data in Utqiaġvik points to some species able to nest slightly earlier, but no species has shown the ability to keep pace with advancing snowmelt, further revealing how long-term climatic changes will impact shorebirds.

It’s not just the breeding grounds where insects play an important role in Semipalmated Sandpiper survival. A large-scale, collaborative study that tracked Semipalmated Sandpiper movements using very small tracking devices, revealed important areas these birds rely on during migration and winter. 

The prairie pothole region , an expanse of shallow lakes and marshes stretching from Iowa to northwest Alberta, is famous for its importance to ducks, but it is also extremely important to many migratory shorebird species, like the Semipalmated Sandpiper. They utilize this region during both fall and spring migration, staying for about a week, as they refuel to continue their impressive journeys south to central and northern South America and north back to the Arctic. 

More than half of the prairie pothole region has been drained for farming. Agrochemical contamination is a real concern in the remaining wetlands, with widespread detection of neonicotinoid insecticides throughout the region. Neonicotinoids are one of the most widely used insecticides. Applied as coatings on crop seeds, they are incorporated into the plant as they grow. Studies have found that the chemicals can stay in the soil, leeching into adjacent wetlands. 

New research on neonicotinoid contamination in shorebirds in the prairie pothole region found that Semipalmated Sandpipers had some of the highest levels, along with Killdeer and Lesser Yellowlegs . It’s not yet known if these levels affect birds’ health, but studies have shown neonicotinoids can suppress birds’ appetites, which is critical during these refueling stopovers. The chemicals may also reduce the amount of insect prey available to shorebirds, yet another potential threat to this already at-risk group of birds.

The decline and disturbance in insect populations across the flyways compounds the threat to birds’ existence and overall welfare. To preserve the delicate balance between birds and insects, it is crucial to take proactive and effective conservation measures. A range of strategies can safeguard these vital components of our ecosystems.

How You Can Help

This World Migratory Bird Day, each of us can play a role in safeguarding our insects for the benefit of birds. Browse this list of actions and learn how you can “Protect Insects, Protect Birds.”

Leave the leaves. Create a thriving ecosystem for insects and birds by leaving leaves in your garden or yard. The leaf litter acts as a natural shelter, food source, and breeding ground for various insect species. The decaying leaves also attract insects that are essential for insectivorous birds’ diets, promoting biodiversity and ecological balance. By refraining from raking leaves, you contribute to a healthier and more sustainable environment for both insects and birds.

Pesticide Alternatives . Stop using pesticides and herbicides, which kill or harm insects - including pollinators like butterflies, birds, and the plants that many insects and other wildlife rely on for food. Alternatively, opt for organic products, targeted application, or integrated pest management systems that minimize negative impacts to insect populations and the birds and other species that depend on them.

Convert your lawn or part of your lawn. (which has limited value for insect production) to a native plant garden. Aiming for 70% native plants is a good rule of thumb to benefit pollinators and other beneficial insects. Less lawn means less mowing with both time and money saved. A win-win for you and for wildlife!

Supports pollinator-friendly initiatives . Advocate for and support local policies and initiatives that protect insects and their habitats.

Maintain and/or restore grassland ecosystems. Shorebirds migrating through the central U.S. rely on flooded fields for fuel and rest. If you or your family farms, encourage them to diversity crops, practice crop rotation, and employ organic farming techniques to reduce pesticide and herbicide treatments. 

Maintain and/or restore water bodies . Clean water is essential to protect insectivorous aquatic communities and is also important for many waterbird species including shorebirds.

Choose Bird Friendly certified coffee and chocolate . Over 40% of the world’s coffee is farmed in full sun, despite coffee’s ability to thrive under a forest canopy. Bird Friendly certified coffee and cocoa ensures these plants are grown with a mix of foliage cover, tree height, and biodiversity creating quality habitat for birds and other wildlife. Bird Friendly coffee and cocoa is also organic! Look for the official "Bird Friendly" certification logo to know you’re getting coffee and chocolate that adheres to research-backed standards that conserve habitat and protect migratory birds. 

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August 31, 2023

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Tracking the migration adventures of Black-winged Monarchs

by Andrea Wild, CSIRO

Black-winged Monarchs (Monarcha frater) are songbirds that live in the rainforests of New Guinea and northern Australia. They feed on insects and belong to the same bird family, Monarchidae, as flycatchers and magpie-larks.

One of the four subspecies of Black-winged Monarch, Monarcha frater canescens (or M. f. canescens for short), migrates to Australia to breed from November to March in the tropical forests of Far North Queensland.

When cooler temperatures arrive, M. f. canescens vanishes. Do these birds spend winter in New Guinea where the other subspecies live? The whereabouts of M. f. canescens during the non-breeding season is a mystery that scientists have pondered for more than 100 years.

A migration mystery

Dr. Leo Joseph, Director of our Australian National Wildlife Collection, studied specimens of Black-winged Monarch held in natural history collections to pinpoint the whereabouts of M. f. canescens during the non-breeding season.

The paper, "The Black-winged Monarch (Monarcha frater): Geographic variation, taxonomy, a 'new' population, and an enduring mystery in migration," by authors from CSIRO, American Museum of Natural History and Sicklebill Safaris, was published in Avian Research .

He said specimens held in collections reveal much more than the plumage and features of the species. "Specimens reveal the morphology of individuals, but they also record dates and location data, showing us exactly when an individual bird was present in a particular place ," Leo said.

"For this study, we used 194 specimens of Black-winged Monarchs, which we think is the total number of this species held in the world's museums and collections."

As well as studying these 194 specimens, Leo and his collaborators used journal papers, photos published online and in books and recordings of bird song.

Despite their extensive study , they were unable to solve the mystery of where M. f. canescens migrate to when they leave their breeding range in Far North Queensland each March.

"We can be sure they go to New Guinea, but exactly where remains unknown," Leo said.

But their research did reveal a surprise among the four subspecies.

One species or many?

While looking at the 194 Black-winged Monarch specimens, the researchers had to consider the physical characteristics used to assign each bird to its subspecies. This would let them match M. f. canescens specimens from Queensland to any specimens from New Guinea.

They noticed the specimens don't fit well into four subspecies.

"Based on differences in the patterns of the feathers on their heads, we think there may actually be three species of Black-winged Monarchs, not one species divided into four subspecies," Leo said.

The team suggested names for the three kinds of Black-winged Monarchs. They are:

• Arfak Monarch (M. frater) for the birds living in the northwest of the island of New Guinea
• Masked Monarch (M. periophthalmicus) for the birds elsewhere in New Guinea
• Pearly Monarch for the birds that breed in Queensland and disappear somewhere into New Guinea when not breeding.

"Whether there truly are three separate species of Black-winged Monarchs is something that DNA analysis could resolve," Leo said.

An enduring mystery

Bird specimens in museums and collections are known as "study skins." Their bones and soft tissues have been removed and specimens stuffed with cotton wool in a process similar to taxidermy.

Only four of the 194 of the Black-winged Monarch specimens used in this study have frozen tissue stored for DNA analysis. But with recently developed DNA sequencing techniques, researchers can now extract DNA from the toe pad skin of bird specimens. Leo's team will use this technique to test their hypothesis that Black-winged Monarchs are actually three separate species.

"Bird taxonomy is complex. Genomic analysis is really the best way to resolve intricate evolutionary relationships in cases like these. But the mystery of where the Pearly Monarch goes when it leaves Queensland endures," Leo said.

Leo noted that GPS trackers would not be useful, in part because they would require capturing, releasing and recapturing the birds. Instead, field observers should pay careful attention to the head pattern of any individuals in southern central New Guinea.

"Knowing which birds live where is vital for conservation. To misquote Oscar Wilde, 'to lose a species is tragic; to not know where it is for half of every year is careless,'" he said.

Provided by CSIRO

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