Why do birds migrate?

Guided by stars and driven by genes, birds embark on epic annual journeys

Birds don’t migrate because of the cold. A little snow won’t actually kill them, but a scarce food supply will. Birds migrate to follow the bugs, worms or baby rabbits, but the “why” of migration isn’t nearly as fascinating as the “how”.

There are two skills that all migratory birds must possess: orientation and navigation. Orientation is the ability to determine the direction in which you’re travelling. Birds don’t have compasses or GPS devices, but they can orient themselves using the position of the Sun by day and the stars at night. Some birds, like pigeons, can orient themselves against the Earth’s magnetic field.

As for navigation, there are several theories, but no definitive answers. Some birds are believed to ‘pilot’ from one point to the next using large landmarks like coastlines, mountain ranges and even four-lane highways. Other birds learn the migration route by following older birds that have already made the trip. But the most remarkable birds are those who are born with a mental map imprinted into their DNA, able to make difficult long-distance journeys without any outside assistance.

Soaring birds like hawks and pelicans do most of their flying during the day to capitalise on rising thermal drafts. Smaller birds that rely on powered flight mostly fly at night when the atmosphere is more stable.

Migration routes

There are nearly as many migration routes as there are species of migratory birds. Almost all routes have a north- south orientation, since most birds are escaping their northern breeding grounds in the late fall to settle into their wintering territory further south.

In North America, there are four main ‘flyways’ guiding birds from Northern Canada down to Mexico and South America. In Europe, many species breed near the arctic and follow dozens of routes to winter in the African plains.

Other routes aren’t so easy. The short-tailed shearwater draws a looping figure of eight across the Pacific Rim. California gulls breed in Yellowstone National Park and fly west before turning south to their Southern
California breeding ground (the direct route would take them over inhospitable desert.) But no migration route can top that of the tiny arctic tern, which travels from Greenland to Antarctica along the African and South American coasts, often covering more than 81,000km each year.

This article was originally published in How It Works issue 8

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