Bird migration has been a great mystery for centuries. Ever since primitive humans in northern latitudes first wondered where all the birds went at the end of summer, fanciful legends have thrived. Aristotle thought birds spent the winter sleeping in hollow trees, caves or beneath the mud in marshes. Others were convinced they flew to the moon to hibernate. Though scientists are still trying to figure out some of the mysteries of migration, we now know a great deal about the phenomenon, thanks to bird-banding records, radiotelemetry, radar and laboratory research. Here are just a few things we’ve learned over the years.
We know that many millions of birds nesting in the temperate zones of North America must migrate south every year to winter in the tropical and semitropical regions of the Caribbean and Central and South America. We also know that the stimulus to migrate is the waning of daylight as the seasons change.
One of the first revelations waterfowl managers learned from their early banding efforts was that these birds follow distinct, traditional migration corridors, or flyways, in their annual trips between breeding grounds in the North and wintering areas in the South. Since 1948, waterfowl have been managed by U.S. Fish and Wildlife Service along four administrative flyways that are based on those migration paths: the Atlantic, Mississippi, Central, and Pacific flyways. Though these flyways chart the travels of ducks and geese, the same routes are flown by millions of songbirds, raptors, shorebirds and waterbirds, heading south in the autumn and north in the spring.
Not all birds follow the crowd, though. A few species ignore the four North American flyways altogether. Tundra swans, for example, travel the continent from the Northwest to the Southeast to winter in Chesapeake Bay. In spring, a half-million sandhill cranes gather from across the American South to rest and feed for a couple of weeks along a few miles of the Platte River in Nebraska. When it’s time to nest, they fan out across the northern part of the globe, all the way from Siberia in the west to Baffin Island in the east.
Some birds fly even farther. The champion globe-trotter is the arctic tern, which covers 25,000 miles each year flying to and from the Antarctic. Because these flights north and south on the flyways require a degree of exertion unmatched by any other vertebrate, energy consumption is critical to bird migration. But a little goes a very long way. According to researchers Timothy and Janet Williams, if a blackpoll warbler burned gasoline instead of fat during its nonstop 85-hour, 1,740-mile flight between Nova Scotia and the Caribbean island of Antigua, it would get 720,000 miles to the gallon.
Like every other aspect of bird behavior, migration is a matter of survival. And luckily for bird-watchers, it provides a great show in spring and fall.