Why Do Birds Migrate?

Short answer

• Birds migrate mainly to follow food. They travel north in spring to breed where insects, plants, and nesting space are abundant, and they travel south in fall when northern food supplies collapse. Breeding grounds, long daylight hours, and safe nesting sites are the other large rewards.
• Cold alone is not the main reason. Many birds, even hummingbirds, can survive freezing temperatures as long as they can find enough food. What cold does is shut off the food supply: insects die or disappear, water freezes over, and seeds get buried under snow.
• Day length is the clock that tells birds when to go. Shortening days in fall and lengthening days in spring trigger hormonal changes that prepare the body for travel, build up fat, and produce a restless urge to fly.
• Not all birds migrate. Roughly half of the world's nearly 10,000 known bird species migrate, according to National Geographic, while other estimates put the share of fully migratory species near 20 percent. The figures differ because researchers define migration differently, and because many species are partial migrants, in which some populations travel while others stay.
• Birds navigate with the sun, the stars, landmarks, and the Earth's magnetic field, and many travel at night along established flyways. The Arctic tern makes the longest journey, covering about 44,000 miles in a year on its pole to pole route, and the bar-tailed godwit holds the nonstop record, flying more than 7,500 miles from Alaska to New Zealand in 11 days without landing.
• Migration is the most dangerous part of a bird's year. It is the highest cause of death in both young and adult birds, and human hazards such as buildings, habitat loss, and shifting seasons add to the risk.

What Migration Means for a Bird

Migration is a word people use loosely, so it helps to start with the strict meaning. In ethology, the study of animal behavior, migration is the regular, usually seasonal, movement of all or part of an animal population to and from a given area. That definition comes from Britannica, and it carries three tests. The movement repeats on a cycle, usually once a year. It goes to a different area and then comes back. And it involves a population, or a large part of one, rather than a single wandering bird.

The return journey matters. A bird that leaves home for good has dispersed or emigrated. It has not migrated. Young birds of many species leave the place where they were born and settle in a new territory. That one way journey is dispersal, and it happens once in a lifetime. A bird that flies south for the winter and returns to the same breeding ground in spring has migrated, because the round trip is the point. Our guide to what animals migrate explains how this definition applies across birds, mammals, fish, and insects, and why distance alone does not make a migration.

Two lookalikes cause most of the confusion. Nomadism is movement without a fixed route or a regular return: birds follow food wherever it appears, rather than traveling between two seasonal homes. Irruptions are a related case, in which large numbers of birds appear somewhere at irregular times and then disappear, often after a food crop fails. Britannica contrasts both with migration, because neither one is a regular round trip. A bird can be nomadic in one year and migratory in another, and some species mix the two strategies, but the distinction is real: migration is a timetable, nomadism is a search.

Nor is migration always long, or always north to south. Some tropical birds make short trips up and down mountains as the seasons change. Some species migrate east to west. A yellow-eyed junco in Arizona may move only about a mile downslope during a severe snowy winter, according to National Geographic, and that short move still counts, because it is a regular seasonal shift between two areas. The question in the title is therefore not only about marathon fliers. It is about any bird that keeps two homes and travels between them on a schedule.

The Short Answer in One Sentence: Food

If this article had to give one reason, it would be food. The Cornell Lab of Ornithology puts it plainly in its guide to bird migration: migration is almost always about finding food. Birds migrate to move from areas where resources are low or decreasing to areas where resources are high or increasing. The Cornell Lab’s ornithologists make the same point in their public question sessions: birds that feed on flying insects have nothing to eat in a Canadian winter, so they have to go somewhere else, while chickadees that can find insect eggs hidden in bark can stay all year.

That is why the cold weather explanation, though common, is misleading. Cold is a problem, but it is usually a secondhand one. The Cornell Lab notes that escaping the cold is a motivating factor, but many species, including hummingbirds, can withstand freezing temperatures as long as an adequate supply of food is available. A bird is a warm blooded animal with an excellent coat of feathers, and it can keep itself warm in bitter weather if it can eat enough to fuel the effort. What it cannot do is eat insects that do not exist in January, drink from a lake sealed under ice, or dig seeds out from under deep snow. Cold matters because it removes the food, not because the bird cannot stand the temperature.

Food also explains the timing in a way that temperature alone cannot. Insects, nectar, fruit, seeds, and small aquatic animals all follow a seasonal curve, and birds follow the curve. In spring, the north offers an explosion of insects and budding plants. In fall, that food collapses, and the birds move south to where it still exists. A warbler that eats caterpillars cannot survive a northern winter no matter how mild the weather is, because the caterpillars are gone. A duck that feeds in open water cannot stay on a frozen lake. The bird is not fleeing the cold. It is following its dinner.

Breeding is the second large reason, and it is closely tied to food. Birds do not only need to eat. They need to raise young, and raising young takes far more food than keeping one adult alive. The northern summer offers the richest nursery on the planet, and birds travel thousands of miles to use it for a few short months, then leave when the nursery closes.

Why Breed So Far North

The journey north in spring looks wasteful until you see what the destination offers. The vast boreal forests and open tundra of the Arctic provide huge expanses for shorebirds and songbirds to nest in, compared with the crowded forests of the tropics. That is the Cornell Lab’s description, and the British Trust for Ornithology, the BTO, makes the same point from the European side: many birds that winter in Britain or Africa breed in the far north, where the tundra and boreal forest give them room that the crowded south cannot.

Space is only the start. The far north also offers fewer predators for ground nesting birds. The BTO notes that ground nesters in Britain must contend with foxes, weasels, rats, polecats, and hedgehogs, while in the far north the main mammalian nest predators are Arctic foxes, wolverines, and the occasional polar bear. Fewer predators mean more chicks survive, which is the whole point of the trip.

Then there is the food. The northern summer produces an insect boom on a scale the tropics cannot match at that season. Daylight promotes rapid growth of fresh vegetation, which feeds herbivorous birds such as geese directly, and provides habitat for the insects that feed insect eating birds such as waders and warblers. The BTO puts the payoff in plain terms: constant daylight allows nonstop foraging, which improves the body condition of the birds, and that translates into more eggs and healthier babies. A bird that raises its young in the Arctic summer is not being sentimental about the landscape. It is cashing in a seasonal surplus.

The return leg has a different logic. The breeding ground is superb for raising chicks and terrible for surviving winter. Once the young can fly, the adults have no reason to stay. They move south to places where food is still available, rest and molt, and wait for the cycle to start again. Migration lets one bird use two places, each at the season when it is at its best, instead of enduring one place all year round.

Daylight: The Extra Hours That Make Breeding Pay

Daylight deserves its own section, because it is both a reason to migrate and the signal that sets the journey in motion. Near the equator, daytime lasts about 12 hours all year. Near the poles in summer, it lasts far longer, and in the high Arctic it barely ends at all. The Cornell Lab lists daylight as one of the three resources birds gain by migrating, alongside food and breeding sites: by traveling toward the poles, birds get much longer days just when they need them most for raising their young.

Long days matter in two ways. First, they give parents more hours to hunt and feed nestlings. A parent that can forage for 18 or 20 hours a day can deliver far more food than one limited to 12 hours, and chicks grow faster when food arrives steadily. Near the Arctic, near continuous light in June means feeding can continue almost around the clock, which speeds development and shortens the window when chicks are most vulnerable to predators and bad weather. Second, the extra light drives the plant and insect growth that creates the food in the first place. The long days and the food boom are the same event, seen from two sides.

Daylight also helps explain why birds that stay in the tropics all year tend to raise fewer young at a time. A tropical bird has about 12 hours of foraging time every day of the year, and its food supply varies little with the season. A migratory bird accepts a dangerous journey in exchange for a short season of near unlimited daylight and food, and uses it to raise a larger family than a tropical bird could manage. The trade is real, and for hundreds of species it pays.

What Triggers Migration: The Clock, the Hormones, and the Weather

Knowing why birds migrate is different from knowing how they know when to go, and the two questions have different answers. The reasons are food, breeding, and daylight. The trigger is mainly day length, with weather deciding the exact night of departure.

Birds are exquisitely sensitive to changes in day length. The BTO explains that arrival and departure dates are often remarkably consistent from year to year, because birds use environmental cues to decide when to move, and one of the most prominent is day length. In spring, longer days and shorter nights signal the birds to prepare their bodies for migration. In fall, shortening daylight tells them it is time to head south. During these periods birds are described as photo-sensitive, meaning they respond to changing daylight hours.

The mechanism runs through hormones. As days shorten in late summer and fall, changes in light alter the birds’ biochemistry: melatonin and other hormones are released, fat storage begins, and the birds become restless. The Cornell Lab describes the same sequence: changes in the length of the day stimulate hormones that tell the birds it is time to fly, and internal biological clocks detect the shift of the seasons, using cues such as changes in light and possibly air temperature. Ornithologists call the restlessness migratory restlessness, from the German word Zugunruhe, and caged birds show it clearly, becoming fidgety and pointing their bodies in the direction they want to fly as migration time approaches.

Preparation is physical as well as behavioral. Before departure, birds enter a feeding frenzy. The Cornell Lab’s migration scientist Lucy Hawkes describes it simply: the birds know they have to migrate soon and get massive. The BTO gives the numbers: migratory birds deposit large stores of fat before departure, and songbirds can be composed of up to 50 percent body fat immediately before their first major migratory flight. Fat is the most energy dense fuel in the body, and it powers flights that can last for days without rest or refueling. At the same time, the major flight muscles change, and the metabolic pathways for flight are strengthened. Remarkably, the birds do not train for this. The BTO notes that the change to the flight muscles occurs without birds having to increase flight activity: they do not train for migrations through increasing exercise, they simply take off one day, ready to go.

Weather then decides the departure night. The Cornell Lab notes that local weather, rain, wind, and air temperature can influence when migratory birds take to the skies, and National Geographic makes the same point: favorable winds behind a passing cold front can launch huge numbers of birds at once, while headwinds and storms keep them grounded. Day length sets the season, hormones prepare the body, and a good tailwind picks the night. That is why migration often arrives in waves rather than a steady trickle: thousands of birds that have been ready for days all leave on the same good night.

There is one more trigger worth knowing, because it sends birds north in late spring instead of south in fall. Waterfowl such as ducks and geese replace all their flight feathers at once after breeding, a process called simultaneous molt, which leaves them flightless for about a month. Many drakes, non-breeding birds, and unsuccessful females then undertake a molt migration, traveling north to large, safe wetlands in Canada’s boreal and Arctic regions, where they find the food and cover they need to regrow their feathers. Ducks Unlimited Canada reports that these birds may travel hundreds of kilometers, sometimes as far as 1,200 kilometers, to reach molting grounds, and that they stay there for a few weeks before flying south again. Molt is therefore a genuine reason to migrate, separate from food and breeding: the bird travels to a safe place to renew the feathers that make the next migration possible.

Why Some Birds Stay: Residents and Partial Migrants

If migration pays so well, why do so many birds not bother? The answer is that migration is expensive and dangerous, and a bird that can find food all year has no reason to pay the price. Permanent residents are birds that can find adequate supplies of food year round. Chickadees pick insect eggs out of bark in winter. Woodpeckers find grubs under bark. Cardinals, blue jays, and many finches eat seeds that last through the cold months. Some birds also switch diet with the season, eating insects in summer and seeds or berries in winter, which lets them stay in one place while the food around them changes.

Human help matters too. Bird feeders, gardens, and city parks now let some birds survive winters that would once have starved them, and some hummingbirds choose not to migrate if they are fed through the winter. That does not make feeders a substitute for migration, and it does not mean every bird can stay. It means the line between migrant and resident is set by food, and food can change.

Most migratory species are not all or nothing. National Geographic reports that 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 remain near their breeding grounds across the seasons while others travel south and return north. In many species, the populations at higher latitudes migrate while the populations further south stay, a pattern called leapfrog migration: the northern birds fly over the resident southern birds to winter even further south. Age and sex matter as well. Young birds often winter further south than adults, and in some species males winter further north than females, probably because of competition for winter food. The pattern varies from species to species, and even from year to year, depending on how hard the winter is and how much food is available.

This is also why published figures for how many bird species migrate disagree. National Geographic states that roughly half of the world’s nearly 10,000 known bird species migrate. The BTO, in a podcast discussion with its senior research ecologists, puts the figure at approximately 20 percent of roughly 11,000 species. Both figures are honest. They differ because they count different things: the higher figure includes partial migrants and short distance movers, while the lower figure counts mainly species in which migration is the standard behavior for the whole population. A reader who sees two different numbers is not looking at an error. They are looking at two definitions. The useful statement is the range, and the reason for it.

Where They Go and Why: Flyways, Destinations, and Shortcuts

Birds do not scatter at random. Most follow established routes called flyways, generally north to south corridors that offer the best opportunities for rest and refueling along the way. In North America, the U.S. Fish and Wildlife Service manages migratory birds around four flyways: the Pacific, Central, Mississippi, and Atlantic. These are administrative divisions based on the routes birds actually follow between nesting and wintering areas, and hundreds of millions of birds travel them each fall between September and November. Other parts of the world have their own flyway systems, such as the East Atlantic Flyway and the East Asia Australasia Flyway, which carry birds between Europe, Africa, Asia, and Australia.

The destinations are set by the same logic as the journey: food and safety. Warblers that breed in Canadian forests winter in Mexico, Central America, and the Caribbean. Shorebirds that nest on the Arctic tundra winter on mudflats as far south as Patagonia. Swallows that breed in Britain spend the winter in southern Africa, and the BTO notes that in both places the arrival of the swallow is celebrated as a sign of spring. Waterfowl move to open wetlands and lakes that stay unfrozen. Seabirds such as Manx shearwaters travel between British breeding grounds and Patagonia, stopping to refuel at sea for up to two weeks at a time.

Not every migration follows the north to south pattern. Some tropical birds move up and down mountains with the seasons, breeding at high elevations and wintering in the foothills. Some species migrate east to west. Some, such as the bar-tailed godwit, cross entire oceans. And some birds make loop migrations, taking one route south and a different route north to catch favorable winds in each direction. The routes are shaped by geography, wind, and the bird’s own body. The BTO explains that wing shape strongly dictates which routes a bird can take: large broad winged birds such as storks, vultures, and eagles rely on soaring over land on rising columns of warm air, and avoid long sea crossings because thermals do not form over water, which funnels their routes around narrow crossing points such as the Strait of Gibraltar. Smaller birds, which flap rather than soar, cross the sea more freely and take more varied routes.

Stopover sites are as important as the endpoints. A migrating bird is an athlete running on stored fat, and it must refuel along the way. About 500,000 sandhill cranes stop each spring on Nebraska’s Platte River to rest and feed on their way north, according to the Cornell Lab. Coastal wetlands, river valleys, and mountain passes serve the same role for millions of other birds. Lose the stopover, and the migration fails, even if the breeding and wintering grounds are intact. That is why conservation of migratory birds is not only about protecting where they nest or where they spend the winter. It is about protecting the whole route.

How They Find Their Way

The navigation question is the most remarkable part of migration, and the part scientists understand least completely. A young bird on its first migration can find a wintering ground it has never seen. A cuckoo displaced nearly 1,500 miles from its breeding ground before migration can steer back to its normal route, according to experiments reported by National Geographic. Young cuckoos displaced the same way found their way back to roughly the same flight path, which suggests that much of the ability is inherited rather than learned, though researchers still debate how much is innate and how much is learned from other birds along the way.

Birds use several tools at once, switching between them as conditions change. The sun is a compass, corrected by an internal clock that accounts for the sun’s movement through the day. At night, star patterns serve the same purpose, and most migratory birds travel at night, when wind currents are smoother and the moon and stars guide their way. Landmarks such as coastlines, mountain ranges, rivers, and vegetation zones give reference points along the route, and seabirds such as albatrosses, fulmars, and shearwaters also use their sense of smell to navigate over the open ocean, according to the BTO. Some birds listen, too: nocturnal flight calls from other migrating birds may guide inexperienced birds, even those of other species.

The most surprising tool is the magnetic field. National Geographic reports that adult birds use a magnetic compass in addition to celestial cues, describing it as an internal positioning system that can prevent them from getting lost even when no landmarks are visible. The exact sense organ involved is still a subject of research, and honest articles say so rather than describing the mechanism as settled. What is settled is the result: birds return to the same breeding and wintering areas year after year with astonishing precision, using a combination of tools no single one of which explains the whole journey. Anyone who wants to know more about the people who study this should read our guide to what an ornithologist is, since ornithologists are the researchers who tag, track, and decode these journeys.

The Record Holders

A few species show what migration can demand at its extreme, and they are worth meeting individually, because each one illustrates a different part of the story.

The Arctic tern holds the distance record. National Geographic states that Arctic terns hold the record for the longest migration of any animal in the world, traveling each year from the Arctic to the Antarctic. The figures look contradictory at first, and the reason is worth understanding because it applies to every long migration. Britannica gives about 17,600 kilometers, or 11,000 miles, from the breeding range to the wintering grounds, which is the straight line distance between the two endpoints. Tracking studies tell a different story: researchers who fitted tiny geolocators to terns breeding in Greenland found the birds following zigzagging routes across the Atlantic, covering about 44,000 miles, or 71,000 kilometers, in a year. National Geographic’s 2021 account gives an even higher figure, more than 60,000 miles, for the pole to pole round trip. None of these is wrong. They measure different things: the straight line distance, the actual winding route flown, and the route of birds tracked on the longest routes. The tracked figure of about 44,000 miles a year is the best supported typical total.

The endurance record belongs to the bar-tailed godwit. In 2020, scientists recorded a godwit flying more than 7,500 miles from Alaska to New Zealand across the Pacific Ocean for 11 days straight, without stopping to feed or rest. National Geographic calls it the longest known nonstop migratory flight. The bird crossed an ocean with no land in between, powered entirely by fat stored before departure. It is a reminder that migration is not only about distance. It is also about doing without food, water, and rest for days at a time.

Smaller birds make journeys that are just as striking for their scale. Calliope hummingbirds, the smallest bird in North America, make 5,600 mile round trips between the northern Rockies and the pine oak forests of Mexico, according to National Geographic. Great snipes fly more than 4,200 miles nonstop between Europe and sub-Saharan Africa at speeds of up to 60 miles per hour. Even flightless birds migrate: the Adelie penguin makes a nearly 8,000 mile trek through frigid Antarctica. Size predicts almost nothing about whether a bird migrates, or how far. The various kinds of birds found in our bird guide show how wide the range is, and many of the best known examples belong to the birds of North America, which include some of the longest distance travelers of all.

What Can Go Wrong: The Costs and the Risks

Migration is dangerous by design. Birds travel through unfamiliar territory, often in huge numbers, at exactly the season when food is scarcest. The BTO states plainly that migrations are generally the highest cause of mortality in both juvenile and adult birds, and that the energetic costs of migration are high, typically representing the largest sustained period of energy expenditure in a bird’s annual cycle. Flight is the most energetically demanding form of movement, and during bouts of flight a bird’s heart rate can increase by 400 percent. The fat stored before departure fuels this effort, but the flight muscles work constantly, and the bird has a new problem to deal with: overheating. The core body temperature of birds in flight has been recorded as high as 44.4 degrees Celsius, about 112 degrees Fahrenheit, and overheating can force a bird to stop flying, which is dangerous if the terrain below is sea or desert.

Predators learn the routes. Along flyways, hawks and falcons hunt exhausted songbirds. At stopover sites, the concentration of tired birds draws predators and spreads parasites. Weather can kill as surely as any predator. A storm at sea can scatter a flock, and a cold snap can freeze the insects a bird depends on for fuel. Birds that arrive at a stopover to find the food gone, because of drought or a late spring, may not have enough fat left to continue.

Human activity adds dangers that evolution never prepared migrants for. Buildings are among the worst. A study published in the journal The Condor in 2014, by scientists from the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service, estimated that between 365 million and 988 million birds are killed each year in the United States by collisions with buildings, based on more than 92,000 fatality records from 23 studies. Light pollution makes the problem worse: it disorients nocturnal migrants, according to Britannica, and leads to collisions and population declines. Habitat loss removes the stopover wetlands and forests that migrants depend on, and roads, fences, and other structures can block routes that birds have used for generations.

Climate change is already shifting the timing of migration. National Geographic reports that bird migrations are starting a little earlier in the spring. Black-throated blue warblers are migrating almost five days earlier on average than they did in the 1960s. American robins bound for Canada are arriving 12 days earlier in the spring than they did in 1994. Whooping cranes are showing up nearly 22 days earlier at their stopover site in Nebraska in spring, and leaving almost 21 days later in fall, than they did in the 1940s. These shifts may help birds if the plants and insects at their breeding grounds shift by the same amount, but not all species can adjust at the same rate. A bird that arrives before its food is ready, or after the peak has passed, raises fewer young. The BTO adds a reason for caution: birds that depend on day length, which does not change, may be less able to alter their migration timing than birds that depend on temperature, which is rising. The timing of migration is tuned to the seasons, and the seasons are moving.

Watching Migration, and Why It Matters

Migration is not only a scientific subject. It is one of the most visible natural events a person can watch, and it happens overhead twice a year in most of North America and Europe. The best time to watch is during the waves: a few days after a cold front passes in fall, or on warm south winds in spring, when huge numbers of birds move on the same night. Dawn after a big night of migration is often the best time to see them, as birds drop out of the sky into parks, gardens, and wetlands to rest and feed. Many of the birds that fill the night sky are silent travelers, but some call as they fly, and listening after dark during peak migration can reveal birds that are invisible by day. Our guide to birds that sing at night covers the species whose voices carry after dark, including some that are heard on migration.

Watching also helps the birds. Keeping house lights low during peak migration nights reduces collisions, and keeping feeders clean and filled helps tired migrants refuel. The birds that pass over a garden in a single night may have flown hundreds of miles without stopping, and the fat they carry is all they have. A safe place to rest and a little food can be the difference between completing the journey and not.

The larger reason migration matters is that it connects places. A warbler that nests in Canada, winters in Costa Rica, and stops in Texas in between depends on all three countries, and on every stopover in between. Protecting a breeding forest in the north means little if the wintering forest in the south is cleared, and protecting both means little if the wetlands in the middle are drained. Migration is a reminder that the natural world is not divided into separate places. It is one system, and the birds are the evidence, flying through it twice a year.

Frequently Asked Questions

Why do birds migrate south in winter?

They migrate south to follow food, not simply to escape cold. Insects, nectar, and open water disappear from northern breeding grounds in winter, and birds move to places where food is still available. Many birds can survive freezing temperatures as long as they can eat enough, but they cannot survive a winter with no food at all.

What triggers birds to start migrating?

Day length is the main trigger. Shortening days in fall and lengthening days in spring cause hormonal changes that prepare the body for travel, build up fat stores, and produce migratory restlessness. Local weather then decides the exact night: a favorable tailwind behind a passing front can launch thousands of birds at once, while storms and headwinds keep them grounded.

Do all birds migrate?

No. Roughly half of the world’s nearly 10,000 known bird species migrate, according to National Geographic, while other estimates count only fully migratory species and put the share near 20 percent. Many species are partial migrants, in which some populations travel while others stay. American robins are a familiar example: some remain near their breeding grounds all year while others travel south and return north.

How do birds know where to go?

They use a combination of tools: the sun and stars as a compass, landmarks such as coastlines and mountain ranges, the Earth’s magnetic field, and in some seabirds, smell. Much of the ability appears to be inherited, since young birds on their first migration can find wintering grounds they have never seen, but birds also learn from experience and from other birds along the way. The exact mechanism of the magnetic sense is still being researched.

How far do birds migrate?

It varies enormously. A yellow-eyed junco may move only about a mile downslope for the winter, while an Arctic tern covers about 44,000 miles in a year on its pole to pole route. The bar-tailed godwit holds the nonstop record, flying more than 7,500 miles from Alaska to New Zealand in 11 days without landing. Most migratory birds travel far less than the record holders, but even a few hundred miles is a major undertaking for a bird weighing less than an ounce.

Why do some birds migrate at night?

Most migratory birds travel at night, when wind currents are smoother, the air is cooler, and the moon and stars can guide them. Flying at night also lets them avoid daytime predators such as hawks, and leaves the daylight hours free for feeding at stopover sites. They fly all night, feed all day, and repeat the pattern until they arrive.

What is the difference between migration and nomadism?

Migration is a regular, seasonal round trip between two areas, such as a breeding ground and a wintering ground, repeated on a predictable schedule. Nomadism is movement without a fixed route or a regular return, following food or water wherever it appears. A nomadic bird may travel further in total than a migrant, but its movements have no fixed seasonal endpoints, so they are not migration in the strict sense.

Do birds migrate for the same reasons as other animals?

Largely yes. Across birds, mammals, fish, and insects, migration is driven by food, breeding, and seasonal change: animals move to be where resources are richest at each stage of their lives. What makes birds different is flight, which lets them travel further and faster than most other animals, and lets them use breeding grounds that would be unreachable on foot.

Sources & references

• Cornell Lab of Ornithology, All About Birds, "The Basics of Bird Migration: How, Why, When, and Where", https://www.allaboutbirds.org/news/the-basics-how-why-when-and-where-of-bird-migration/ (food as the main driver, cold survivable with food, daylight and breeding sites, migration types, triggers, sandhill crane staging; direct page returned 403 on 11 Nov 2026, claims cross-checked against the Cornell Lab Bird Academy migration sessions and the BTO masterclass below)
• Cornell Lab of Ornithology, Bird Academy, "Exploring Bird Migration: Your Questions Answered", https://academy.allaboutbirds.org/live-event/exploring-bird-migration-your-questions-answered/ (migration almost always about food, chickadees staying through winter, day length as the trigger; read via published session text 11 Nov 2026)
• Cornell Lab of Ornithology, Bird Academy, "Ask an Ornithologist: Mysteries of Migration", https://academy.allaboutbirds.org/live-event/ask-an-ornithologist-mysteries-of-migration/ (migration as movement for food and breeding, innate navigation, learning on the way; read via published session text 11 Nov 2026)
• British Trust for Ornithology, "Bird migration: a masterclass", by Steve Portugal, https://www.bto.org/learn/about-birds/articles/bird-migration-masterclass (reasons to migrate, day length and hormones, fat up to 50 percent of body weight, heart rate and body temperature in flight, migration as the highest cause of mortality, navigation by smell in seabirds, wing shape and routes, swallow and osprey routes; opened and read 11 Nov 2026)
• British Trust for Ornithology, "Beyond the Bird Table" podcast, migration episode, https://podscan.fm/podcasts/beyond-the-bird-table (about 20 percent of roughly 11,000 bird species migrate, resources as the driving factor, altitudinal migration; read via published episode text 11 Nov 2026)
• National Geographic, "Bird Migration Is One of Nature's Great Wonders. Here's How They Do It.", 5 May 2021, https://www.nationalgeographic.com/animals/article/bird-migration-one-of-natures-wonders-heres-how-they-do-it (roughly half of nearly 10,000 species migrate, godwit and great snipe records, calliope hummingbird, Adelie penguin, partial migration, navigation experiments, triggers, climate timing shifts; opened and read 11 Nov 2026)
• Britannica, "Migration | Definition, Animals, and Facts", https://www.britannica.com/science/migration-animal (definition of migration, contrast with emigration, invasion, and range expansion, altitudinal migration, navigation by landmarks, celestial cues, and magnetic fields, light pollution; opened and read 11 Nov 2026)
• U.S. Fish and Wildlife Service, "Migratory Bird Program Administrative Flyways", https://www.fws.gov/partner/migratory-bird-program-administrative-flyways?page=18 (four North American flyways: Atlantic, Mississippi, Central, Pacific; read via published page text 11 Nov 2026)
• U.S. Fish and Wildlife Service, "Fall Flights: Migration Hot Spots Supported by Industry Excise Tax", 28 Oct 2026, https://www.fws.gov/story/2026-10/fall-flights-migration-hot-spots-supported-industry-excise-tax (hundreds of millions of birds moving September to November along the four flyways; read via published page text 11 Nov 2026)
• Ducks Unlimited Canada, "Why are the birds flying north, again?", https://www.ducks.ca/stories/waterfowl/why-are-the-birds-flying-north-again/ (molt migration, simultaneous wing molt, flightless for about a month, travel to northern wetlands for safety and nutrients; read via published page text 11 Nov 2026)
• Loss, S. R., Will, T., Loss, S. S., and Marra, P. P., "Bird-building collisions in the United States: Estimates of annual mortality and species vulnerability", The Condor: Ornithological Applications, 2014, http://www.bioone.org/doi/abs/10.1650/CONDOR-13-090.1 (365 to 988 million birds killed annually by building collisions in the United States, based on 92,869 records from 23 studies; read via published abstract and study summary 11 Nov 2026)