Why Do Birds Migrate? Food, Daylight and Breeding Explained

The main reason: food and long summer days

Every spring, temperate and Arctic regions explode with life. Insects hatch in huge numbers, plants flower and set seed, and fish and invertebrates become easy to catch. At the same time, days are long: north of the Arctic Circle the sun does not set for weeks in midsummer. Long days give parents more hours to gather food for hungry chicks, and fewer competitors and predators share the bounty than in the crowded tropics.

When autumn comes, that food disappears. Flying insects vanish in frost, lakes freeze and snow covers seeds. A small insect-eater such as the barn swallow could not survive a European or Canadian winter, so it flies to Africa or South America, where insects are still plentiful. In other words, birds do not leave because of the cold itself, but because of what the cold does to their food supply.

Breeding success and safety

Migration is costly and dangerous, so it must pay off in more surviving chicks. Studies of many species show that migrants which breed in the north raise larger broods than related resident birds in the tropics. Seasonal superabundance of food is one reason; another is that northern breeding grounds often have fewer nest predators and parasites per bird.

Some seabirds and shorebirds migrate to reach safe nesting sites rather than food. They breed on remote islands, cliffs or tundra where foxes and rats are scarce, then spend the rest of the year at sea or on distant coasts where feeding is easier.

How did bird migration evolve?

Migration has evolved many times in different bird families, and it can change surprisingly fast. One leading idea, sometimes called the “southern home” theory, is that many migrants descend from tropical ancestors that began moving north in summer to exploit seasonal food, and returned south each winter. Another view holds that some northern birds started moving south to escape harsh winters. Both processes probably happened in different groups.

A striking modern example is the blackcap, a European warbler. Since the 1960s, a growing number of blackcaps from central Europe have started wintering in Britain and Ireland instead of Spain and Portugal. Experiments by German researchers showed that the new direction is inherited: chicks of these birds, raised in captivity, oriented toward the northwest in autumn. Garden bird food and milder winters probably helped the new route succeed.

How do birds know when and where to go?

Birds are not taught a calendar. The main trigger is day length, which changes their hormones and makes them eat more and store fat, sometimes doubling their body weight before a long sea crossing. Caged migrants become restless at night in the migration season, a behavior known by the German word Zugunruhe, meaning “migratory restlessness”.

An inborn annual clock, weather and wind direction then fine-tune the exact departure date. To find their way, birds combine several compasses: the sun, the stars, the Earth’s magnetic field, landmarks and possibly smell. The details are explained in how do birds know where to migrate?

Different kinds of migration

Not all migrants travel between continents. Ornithologists recognize several patterns:

• Long-distance migrants, such as swallows, swifts, cuckoos and most North American warblers, spend the northern winter in the tropics or the Southern Hemisphere.
• Short-distance migrants, such as many American robins and European chaffinches, move a few hundred kilometers to milder areas.
• Partial migrants are species in which some individuals migrate and others stay. In many European robin populations, females and young birds are more likely to leave than adult males.
• Altitudinal migrants move down mountains in winter rather than south, as some mountain birds in the Rockies and the Alps do.
• Irruptive species, such as crossbills, waxwings and snowy owls, move south in large numbers only in years when their food fails.

Roughly one in five of the world’s bird species makes regular long-distance migrations, and in places such as Canada and northern Europe the majority of breeding birds leave for the winter.

The record holders

The Arctic tern makes the longest known migration of any animal. Terns fitted with tiny geolocators flew roughly 70,000–90,000 km (43,000–56,000 mi) in a year between their Arctic breeding grounds and the Antarctic, enjoying two summers and more daylight than any other creature. In 2022, a young bar-tailed godwit flew more than 13,000 km (8,000 mi) nonstop from Alaska to Tasmania in about 11 days, without feeding or resting.

Small birds make astonishing journeys too. The ruby-throated hummingbird, which weighs only about 3 g (0.1 oz), crosses the Gulf of Mexico in one flight of up to about 800 km (500 mi), as described in where do hummingbirds go in winter? More examples are in our list of the longest animal migrations and in which bird has the longest migration?

How climate change is shifting migration

Bird migration is changing as the climate warms. Many European and North American species now arrive several days to weeks earlier in spring than they did in the mid-20th century, and some short-distance migrants stay farther north in winter or stop migrating altogether.

Long-distance migrants face a particular problem. Many decide when to leave Africa or South America by day length, which does not change with the climate, while spring at their breeding grounds now starts earlier. In the Netherlands, pied flycatchers in areas where the caterpillar peak had moved earliest declined much more steeply than in areas with a later peak, because their chicks hatched after the best food had gone. Expanding deserts, drought in the Sahel and the loss of stopover wetlands add further pressure on these travelers.