Why Do Birds Migrate? How They Know When to Go

Why do birds migrate? How do they know when it’s time to go? Do they have a sixth sense? Learn about the mysterious migrations of fall!

For humans, it’s hard to put a finger on how we know when the seasons are changing from summer to fall. Even as the apples hang heavy and the air sharpens with the smell of wild grapes, summer’s riot is pared away. Like a Moon one night past the Full Moon, there’s a sliver missing, a little nibbling at the edges of the season.

How do birds know when it’s time to migrate? Is it a rattle in the oak leaves, frayed and leathery, when the wind blows? Or a dryness in the roadside flowers, the thirsty look of goldenrod or calico asters? Is there something in the September air that they would just as soon keep to themselves?

Which Birds Migrate?

If you live within sight of a patch of mud—along a riverbank, on a tidal flat, or at a county fairground after the tents have been folded up—you may have noticed a few odd visitors in recent weeks. Shorebirds such as sandpipers, plovers, dowitchers, and godwits are among the early fall migrants. Long- and short-legged, long- and short-billed, gray and brown and white, they stop to forage in the mud along the way.

Long before ice and snow threaten to lock them in, many have left their breeding grounds, some as far north as the Arctic Circle, and are headed for distant places, often in the Southern Hemisphere. They remind us, too, of a new season in the offing.

Of course, not all birds migrate. Those that do may travel hundreds or even thousands of miles.

Why Do Birds Migrate?

It’s often a matter of resources. During spring and summer, food and nesting sites are plentiful in many birds’ breeding grounds. As the seasons change, food can become scarce, and birds may travel to places where it is easier to find enough to eat.

Migration isn’t simply a search for warmer weather. Birds move between places that provide what they need at different times of year. When spring comes around, many return to their breeding grounds as food sources—such as fresh buds, leaves, fruit, and insects—become available again.

How Do Birds Know When to Migrate?

Birds rely on more than one cue. For many species, the changing length of daylight helps set the timing of migration. Food availability, weather, and temperature can also influence when birds prepare to leave. Some species can tolerate cold temperatures if they can still find enough food.

Many migratory birds also experience seasonal restlessness in fall and spring. This behavior, called Zugunruhe, is especially noticeable in birds kept in captivity: they may become active at night during the season when they would normally migrate. Day length is one factor that helps regulate this behavior. 

How do birds find their way over such long distances? Depending on the species, they can use cues from the Sun, stars, landmarks, and Earth’s magnetic field. Researchers have found evidence that birds can use a magnetic compass, though exactly how they sense magnetic information is still being studied.

There’s evidence that some birds use stars to orient themselves on clear nights. Others use the Sun as a compass. Birds may combine these cues, adjusting as the sky changes and they move across the landscape. See more about how birds navigate at night. 

Which Bird Migrates the Farthest?

One remarkable example is a young bar-tailed godwit known as B6. In 2022, it flew about 8,425 miles from Alaska to Tasmania, Australia, without stopping—a journey that took 11 days. Before such a long flight, godwits build up fat reserves to fuel their trip.

Bird Sense: How Shorebirds Find Food

Even when shorebirds finally flutter down to a beach or tidal flat, their senses are at work. Red knots probe the mud or sand for worms and mollusks. Researchers have proposed that pressure-sensitive receptors in a red knot’s bill help it detect buried prey by sensing small changes in the soft sediment. 

As Tim Birkhead writes in his book Bird Sense, “Rapid and repeated probing, so typical of these wading birds, is thought to allow them to build up a composite three-dimensional image of food items hidden in the sand.”

Sensing the Seasons

Even without magnets in our heads, barometric gauges in our noses, or wings on our backs, we feel the gears of the seasons click forward—slowly, so slowly—at least perceptibly, if not always interpretably.

Like a dowser with a forked branch—or a dowitcher probing the mud—we use whatever clues are at hand. A restless night, the Sun just a smidge lower at high noon, or a sudden urge to stack wood: each is a clue, even if we don’t quite know what it means.

Do you feel that restlessness in the fall and spring, like our feathered friends?

Read more about hawks and the fall hawk watch.