How do birds know how to migrate? Do they have a sixth sense? Even for us humans, it’s hard to put a finger on how we know when the seasons are changing, even when the temperatures still feel like summer. Like a Moon one night past the full, there’s a sliver missing, a little nibbling at the edges of the season.
Even as the apples hang heavy and the air sharpens on the smell of wild grapes, summer’s riot is pared away. 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 the goldenrod, the calico asters? Or a new nameless taste in the September air, as if our senses were privy to some intelligence they would just as soon keep to themselves?
The Annual Migration
If you live within sight of a patch of mud, either riverbank or tidal flat or county fairground after the tents have been folded up, you may have noticed a few odd visitors in 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 in route to impossibly distant climes, often in the Southern Hemisphere, reminding us, too, of a new season in the offing.
Bar-tailed Godwit—Photo by Tony Whitehead
Which Bird Migrates the Furthest?
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 eight straight days to its “wintering” (or second summering) grounds in New Zealand, a trip of some 7,000 miles without a single pit-stop along the way.
In preparation for such a grueling journey out over the open 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.
Migration Route of the Bar-tailed Godwit—Map courtesy of Te Ara, The Encyclopedia of New Zealand
How Birds Know 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.
- As with many birds, the changing angle of sunlight, lower and lower in the sky, triggers these changes and cues the preparation for migration.
- 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 sense the Earth’s magnetic field and are able to correct their course accordingly, as humans would 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.
Purple Sandpiper—Photo by Len Medlock
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 continually astonishing 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.”
Red Knot—Photo by Len Medlock
Many species of birds experience a restlessness, called Zugunruhe, in the weeks before their departure. They seem hyperactive, 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. But nonetheless they feel it coming and ready themselves.
American Avocet—Photo by Len Medlock
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 perceptible, if not always interpretable.
Like the dowser (“dowitcher”) who cuts a branch of apple wood to search for underground springs, we use any means at our disposal, not just eyes and ears. A restlessness in the night; a sun just a smidge lower at high noon; a sudden urge to stack wood and lay in a store of canned tomatoes—each a clue, not quite rising to consciousness.
Where oh where is the mainspring of that clock, vast, elaborate, of which we are ourselves the dial?