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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 on the smell of wild grapes, summer’s riot is pared away. Like a Moon one night past the full, 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, either riverbank, tidal flat, or county fairground, after the tents have been folded up, you may have noticed a few odd visitors in the past weeks. Shorebirds—the sandpipers, the plovers, the dowitchers, the godwits, long- and short-legged, long- and short-billed, gray and brown and white—are among the earliest migrants going south for the winter, stopping for a muddy meal along the way.

Long before ice and snow threaten to lock them in, they have left their breeding grounds, many from as far north as the Arctic Circle, and are en route to impossibly distant climes, often in the Southern Hemisphere, reminding us, too, of a new season in the offing.

Of course, not all birds migrate, but those that do are capable of traveling hundreds or even thousands of miles.

Why Do Birds Migrate?

It’s all a matter of resources. During the spring and summer, food sources and nesting sites are plentiful, but when fall and winter come around, these valuable resources nearly disappear, and there’s not enough to sustain the entire bird population.

Unlike most “grounded” animals, birds are capable of greater mobility and make good use of it. As the cold season approaches, they migrate to warmer climates—where food sources are still plentiful —to chase the valuable resources they need to live. Then, when spring comes back around, birds migrate back to where they started as food sources, such as fresh buds, leaves, and fruit, or exploding insect populations rebound and become sustainable again. 

How Birds Know When to Migrate?

In order to navigate between such far-flung places, godwits, like most long-distance migrants, rely on more than one set of clues. 

Changes in the food supply, the changing angle of sunlight, and the lower and lower sky cue the preparation for migration. Lower temperatures can also be a factor, though many species can actually tolerate freezing temperatures if food is available.

But also interestingly, many species of birds experience restlessness every fall and spring, known as Zugunruhe (TSOOG-un-roo-uh), in the weeks before their departure. This “migration anxiety” makes them seem hyperactive and antsy, particularly in the evenings or during the night, moving around or breaking into song for no reason at all. Their sleep patterns are changing. They know they are on the cusp of something new, a change of scene, an adventure. For this year’s fledglings, what that adventure will be is inconceivable. Nonetheless, they feel it coming and ready themselves.

There is evidence that on clear nights, they use stars to orient themselves, by day, the Sun—and are able to use them even as these markers shift position in the sky. There are further indications that birds use the Earth’s magnetic field to migrate, much as humans use a compass. Even more amazingly, they seem to recognize variations in that magnetic field and their particular position within it, as though they possessed not only a compass but a map as well—a kind of topographical or GPS overview of the entire landscape.

How this is accomplished is not known, perhaps by some as yet undescribed chemical process inside the avian eye.

See more about how birds navigate at night. 

Which Bird Migrates the Farthest?

The longest unbroken annual migration is that of the bar-tailed godwit, a large shorebird that leaves its breeding grounds in Alaska and flies for 8 straight days to its “wintering” (or second summering) grounds in New Zealand. This trip is some 7,000 miles without a single pit-stop along the way.

In preparation for such a grueling journey over the Pacific, the godwit stores up an enormous amount of fuel in the form of fat, while many of its internal organs not used for flight, such as the liver and the intestines, atrophy almost to the point of disappearing altogether.

Bird Sense

Even when these shorebirds finally flutter down to land on their home beach, they rely on powers of perception that border on the magical. Probing here and there for worms or mollusks, the beak of the red knot causes a tiny pressure wave in the sand or mud. Where that wave meets an object—a motionless clamshell, for instance—it is disturbed. And the bird’s beak itself, like a bat echolocating in the dark, is actually sensitive to such minute variations in the pressure wave. In other words, shorebirds are not simply stabbing at random in search of a meal but, like precision instruments, are actually testing the mud or sand. 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 or barometric gauges in our noses, even without wings on our backs and 7,000 miles to cover, we feel the gears of the seasons click forward, slowly, so slowly—at least perceptibly, if not always interpretable.

Like the dowser (“dowitcher”) who cuts a branch of apple wood to search for underground springs, humans use any means at our disposal, not just eyes and ears. A restless night, the Sun just a smidge lower at high noon, or a sudden urge to stack wood—each a clue, not quite rising to consciousness.

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

Read more about hawks and the fall hawk watch.