If it is spring or fall, the great pivot points of the year, then the continents are swarming with billions of traveling birds—a flood so great that even the most ignorant and unobservant notice, if nothing else, the skeins of geese and the flocks of robins.

But the migration’s breadth goes far beyond those obvious watersheds, shifting endlessly across distance and season.  In the middle of July, Hudsonian godwits lift off from the iceberg-choked shores of the Beaufort Sea, heading southeast along the northern rim of Canada to Labrador, then vaulting south in a nonstop flight to Venezuela.  In the snow squalls of December, goshawks and golden eagles fly south along the ridges of the Appalachians, over oak trees that rattle their last stiff, dead leaves in the wind.  Even within the tropics, a land where migration would seem unnecessary, birds move with the seasonal rains and droughts across hundreds of miles, following the blossoming of flowers or the ripening of fruit.

They do not all follow the expected course, nor do they always travel by wing.  With the first winter snows, blue grouse leave the more temperate foothills of the Western mountains and migrate—on foot, no less—into the bitter, wind-drifted high country, searching for conifer needles to eat.

A very few travel even beyond the bounds of the hemisphere.  Tiny song birds from Alaska leap west across the Bering Sea to the Philippines, and others from eastern Canada cross the North Atlantic to Europe and Central Africa.  Short-tailed albatrosses from Japan glide down the coast of Washington in summer on wings as fragile as a whisper; in those same waters the albatrosses pass shearwaters from New Zealand and storm-petrels from Antarctica and the Galapagos.

Even the darkness moves with the passage of birds.  On soft spring midnights, the air is alive with the flight notes of unseen warblers and vireos, thrushes and orioles, sparrows and tanagers, filtering down through the moonlight like the voices of stars.
Bird migration is the one truly unifying natural phenomenon in the world, stitching the continents together in a way that even the great weather systems, which roar out of the poles but fizzle at the equator, fail to do.  It is an enormously complex subject, perhaps the most compelling drama in all of natural history.

That such delicate creatures undertake these epic journeys defies belief.  Only recently have scientists discovered that some shorebirds apparently fly nonstop from the southern tip of South America to the coast of New Jersey, a journey of ten days—240 hours of uninterrupted flight.  Even more remarkable are the four-ounce Arctic terns that leave the northern fringe of the continent each autumn, flying east across the Atlantic to Europe.  They push south along the bulge of Africa, recross the Atlantic to the edge of South America, and spend the winter months moving east off Antarctica.  In spring they reverse course, moving up southern Africa and lancing back to Canada'a figure eight inscribed on half the globe, a track that returns them, often as not, to precisely the same sheltered nook where they nested the summer before.

Even scientists have little grasp of the numbers of birds involved in this seasonal ebb and flow.  In the spring, hordes of warblers, tanagers, vireos, and other tropical migrants cross the Gulf of Mexico each night, arriving on the U.S. coast at a rate that may exceed a hundred thousand songbirds per mile of shoreline, with tens of millions making landfall each day.  On a single autumn night several years ago, radar on Cape Cod indicated that 12 million songbirds passed overhead.  And on the narrow coastal plane of Veracruz, Mexico, biologists discovered only recently one of the greatest raptor migrations in the world, where nearly a million hawks have been counted in a single day. In all, scientists guess, more than 5 billion birds annually weave this incredible tapestry across the hemisphere. 

Because they travel such extraordinary distances, often with different requirements for food and shelter along the way, migratory birds pose one of the stickiest conservation challenges in the world.  In the past, preservation programs focused on saving breeding areas, but experts now realize they must also save wintering grounds and migratory stopovers if this global web isn’t to unravel.