On a remote mountaintop in the Dominican Republic just before Christmas, a group of Vermont biologists crouched in the undergrowth and played recorded bird calls hoping for an early holiday present. In the dim light came a reply – “Pweer … pweer … pweer.” A bird darted across the forest opening toward the recording only to meet a fine mesh net in mid-flight. The biologists shined the beam of their headlamps on the net to see a small, olive-brown bird with a spotted breast, a rare Bicknell’s thrush.
This wasn’t just any Bicknell’s thrush. It was one familiar to these biologists, for the small metal band on its leg etched with a number told the incredible story. They had captured this same bird just six months previously on Mount Mansfield in Vermont where it was nesting.
Weighing just one ounce, this bird had recently flown nearly 2,000 miles to this mountaintop, a feat it will repeat every fall of its life. Returning north in spring, it covers 4,000 miles a year.
Bicknell’s thrushes aren’t the only long-distance migrants that make the trip back and forth from the Northern Forest to the tropics. Many other species migrate even farther. They travel by day and night across vast stretches of land and sea. They navigate using familiar landscapes, mental star charts, magnetic fields, and angles of ultra-violet light emitted by the setting sun that are invisible to humans. Worshiped as gods by some past cultures, written into poetry, painted on canvas, studied by scientists, and loved by birders, migrating birds are a marvel. There are nearly 10,000 bird species in the world, and about half of them migrate. From warblers to waterfowl, from hummingbirds to hawks, each presents a remarkable story.
Take the blackpoll warbler. Hold two nickels and a dime in your hand, and that’s about its summer weight. This songbird nests in the mountain forests of New England and New York (and across the north woods all the way up to Alaska). With the coming of frost, blackpolls congregate along the Northeast seaboard. This is a bird that only knows one season—summer—so the seacoast is just a place to check-in for rest and relaxation.
Gorging on insects and berries on the coast, blackpolls soon double their weight. “They actually feel soft and pudgy in your hand,” says Trevor Lloyd-Evans, a biologist at the Manomet Center for Conservation Sciences, who has studied bird migration along the Massachusetts coast for over 30 years. They’ll bulge with fat from throat to tail, preparing for the sign to depart, which arrives as a cold front from the north.
With favorable tailwinds, the birds depart over the Atlantic. Radar data has shown migrating blackpolls flying at 2,600 feet within four hours of their departure, rising to 6,500 feet by the time they pass Bermuda. As they approach the Tropic of Cancer, the winds shift to the northeast, allowing for drift toward the Lesser Antilles and South America. There, flight above 13,000 feet helps them avoid the headwinds common at lower altitudes. Some blackpolls have been recorded reaching 20,000 feet over Antigua. Descending as they approach South America over Tobago, their altitude drops to 2,500 feet in the hours before landing. The entire non-stop flight lasts 80 to 90 hours, with an average speed of 25 miles per hour.
This tiny songbird is fueled entirely by stored fat. Scientists have calculated that a blackpoll warbler has to weigh more than seven-tenths of a gram to have enough stored energy to safely complete its flight. For us, the metabolic equivalent would be to run four-minute miles for 80 hours, according to ornithologists Tim and Janet Williams, retired professors from Swarthmore College. The couple also found that if blackpolls were burning gasoline instead of body fat, they would be getting 720,000 miles to the gallon.
While the thrush and the warbler rely on a full tank of gas for migration, another bird, the broad-winged hawk simply relies on the power of the sun. The broad-wing doesn’t gorge itself. It doesn’t have to, because it wastes little energy on flapping its wings. This hawk, like many hawks, is a glider. Using the lift from rising columns of hot air, thousands of slowly circle upward on thermals and then ride them all the way to Central America and sometimes South America.
This fall, on some sunny day, climb a hill and watch the hawks glide southward. Or just before bedtime, go outside and listen toward the sky. You just might hear the waves of songbirds overhead calling to each other as they head south again.