IMAGINE A SINGLE BIRD DURING A SPRING dusk moments before flight. Maybe an Upland Sandpiper standing on the vast Argentine pampas, ready to launch itself on an 8,000-mile trek to its breeding grounds on Alaska’s upland tundra. Or a Blackpoll Warbler in an orange grove in Colombia, set to make a 6,000-mile journey to Canada’s boreal forest. Or a Yellow-billed Cuckoo leaving the Bolivian plains, bound for the deciduous woodlands of Illinois, some 4,000 miles away. Or a Bobolink, a Scarlet Tanager, a Wilson’s Warbler … Tonight, conditions seem right. The way ingrained inside. The script passed down for millennia. The birds will flow through the sky, a living river rolling through darkness, skirting storms and artificial light and a minefield of inhospitable landscapes below. Zoom in: That single bird spreads its wings, joining thousands upon thousands of birds spread across the night sky. Pull back: The continent becomes a beating heart, the birds its blood pulsing back and forth, a mystery we’re just beginning to understand.
Billions of birds travel north in spring and south in fall, hundreds of species keeping a cycle of movement, and they do so primarily at night. While some large birds like hawks, cranes, and waterfowl are daytime travelers, most migrants—including the vast majority of songbirds—are on the wing in the dark. The reasons include a calmer atmosphere, guidance from the stars, and safety from predators. Meanwhile we humans by and large have no inkling of the tremendous journeys made while we sleep. Even for those who study migration, the story is unfolding in ways never before possible. “The more you can see what’s going on, the more fascinating it becomes,” says Jeffrey Buler, a University of Delaware wildlife ecologist. “And we’re making new discoveries all the time.”
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