Why do birds migrate?

Every year, millions of birds set off on astonishing journeys: they cross continents and oceans, cover thousands of kilometers, and then return to their starting point. Bird migration is one of nature’s most incredible spectacles. But why do they do it? And how do they know where to go without maps or GPS? Let’s find out.

What migration is

Migration is the seasonal, regular movement of animals from one region to another and their subsequent return. In the case of birds, many species travel twice a year: once toward their breeding grounds and once back to their wintering grounds.

Not all birds migrate; many are sedentary. But migratory ones make journeys that test the limits of animal endurance.

The main reason: food and climate

Why do they bother traveling so much? The answer comes down to two words: survival and resources. Birds migrate, above all, to:

• Find food: when winter reaches cold regions, insects vanish and food becomes scarce. By migrating to warmer areas, they find food year-round.
• Escape extreme cold and its harsh conditions.
• Reproduce in places with abundant food and good conditions for raising their chicks.

That is: they follow the food and the good weather. As the Cornell Lab notes, migration is a balance between the risks of the journey and the benefits of reaching a better place.

How do they know when to leave?

Birds don’t have calendars, but they do have finely tuned internal “clocks.” The main signal to start migrating is usually the change in day length (the hours of light), which alerts them to the change of season. Temperature and food availability also play a part. When the moment comes, an instinctive drive prepares them: they overeat to build up fat as fuel for the long journey.

The great mystery: how they navigate

Here’s the most fascinating part. How do they find their way across thousands of kilometers, sometimes for the first time in their lives? Birds use a combination of natural “compasses”:

• The Sun by day and the stars by night as reference.
• Earth’s magnetic field, which many birds seem to “sense” thanks to mechanisms still under study, like an internal compass.
• Landmarks of the landscape: coastlines, rivers, mountains.
• Smell and memory, in some species.

Combining these cues, they manage to navigate with astonishing precision, often returning to the same nest year after year.

Journeys that defy imagination

Some migrations are records of nature. The Arctic tern, for example, travels from the Arctic to Antarctica and back each year, covering tens of thousands of kilometers: the longest known migration. Other birds fly nonstop for days over the ocean. These journeys are exhausting and dangerous: many birds don’t survive, which makes the instinct that drives them all the more admirable.

An instinct that connects the planet

Bird migration is a reminder of how interconnected the natural world is: a swallow you see in summer may have spent the winter thousands of kilometers away. Understanding why they migrate —to survive, eat, and breed— and how they do it —with compasses of sun, stars, and magnetism— also helps us protect them, since they depend on healthy habitats existing all along their route. Each journey is a feat of instinct, endurance, and a sense of direction that science is still unraveling.