Every year billions of birds travel astonishing distances. It is one of nature's great spectacles — but why do they bother? The answer comes down mostly to food.
It's about food, not just cold
The main driver of migration is food. As winter approaches, the insects, fruit and plants many birds rely on become scarce in northern regions, so the birds move to where food is still plentiful. Birds cope with cold surprisingly well if they can eat enough — it is the disappearing food supply that pushes them to leave.
Breeding in the best place
Migration is also about raising young. Many birds fly north in spring to breed, taking advantage of long summer days, an explosion of insects, and more space and nesting sites. Then they return south for winter — in effect getting the best of both worlds by moving between them.
How birds know when and where to go
Birds time their journeys mainly by the changing length of daylight, which triggers hormonal changes that prepare them to travel. They navigate using the sun, the stars, landmarks and even the Earth's magnetic field, and young birds often make their first journey on instinct alone.