On a remote mountaintop in the Dominican Republic, a group of biologists crouched in the undergrowth and played pre-recorded bird calls. 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 from the Vermont Center for Ecostudies shined the beam of their headlamps on the net, and there was 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, the small metal band on its leg etched with a number telling the incredible story. They had captured this same bird just six months previously on Mount Mansfield, its breeding grounds.
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. Then, in the spring, it will fly another 2,000 miles back north.
Bicknell’s thrushes aren’t the only long-distance migrants that frequent the Northern Forest – many other species migrate even farther. They travel by day and night across vast stretches of land and sea. They navigate using mental star charts, magnetic fields, angles of ultraviolet light emitted by the setting sun and invisible to us, familiar landscapes, and perhaps other seemingly magical methods. Worshiped as gods, 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, hummingbirds to hawks – each is a remarkable story.
Take the blackpoll warbler. Hold two nickels and a dime in your hand; that is about how much a blackpoll weighs while it is raising its young each summer. This black-capped songbird nests in the mountain forests of New England and New York (and across the North Woods to Alaska.) With the coming of frost, blackpolls move southward to congregate along the eastern seaboard. Those from Alaska will have already flown more than 3,000 miles, but the migration doesn’t end here. This is a bird that only knows one season, summer, so the seacoast is just the place to check in for rest and relaxation.
Gorging on insects and berries, blackpolls soon double their weight. “They actually feel soft and pudgy in your hand,” said 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 the 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 an altitude of 2,600 feet within four hours of their departure, rising to 6,500 feet by the time they pass over 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. “If a blackpoll warbler were burning gasoline instead of its reserves of body fat, it could boast of getting 720,000 miles to the gallon.”
Equally impressive in the long-distance flying department is the diminutive ruby-throated hummingbird (remove the two nickels, leaving just a dime, and you’ll have its weight in your hand.) Despite their tiny size, these birds fly non-stop across the Gulf of Mexico each spring and fall in just 24 hours. Large numbers have been observed flying low over the waves, trying to make landfall in North America in the spring. Like the blackpolls, rubythroats pack on fat to fuel the journey, doubling their weight in preparation.
While the thrush, the warbler, and the hummingbird all rely on a full tank of gas for migration, the broad-winged hawk simply relies on the power of the sun. The broadwing doesn’t gorge itself. It doesn’t have to, because it wastes little energy on flapping its wings at all. This hawk, like many hawks, is a glider. Using the lift from rising columns of hot air, hundreds, thousands, or even tens of thousands of hawks congregate in flocks, or “kettles,” slowly circling upward on thermals and then gliding southward and downward to catch the next free ride. They only migrate during the heat of the day, and they rarely, if ever, sail over open water, where few thermals exist. They follow the land as it becomes narrower and narrower through Central America, until some of them make it all the way into South America after 40 days of migration.
These champions of migration have superb adaptations for the rigors of long distance flight: air-filled bones to lighten the load, large and powerful hearts that are proportionally six times larger than a human heart, enhanced oxygen-carrying capacity in their blood, a supercharged respiratory system using air sacs that allow fresh air to constantly bathe the lungs, feathers for flight and insulation, and specialized pectoral muscles.
This fall, climb a hill and watch the hawks glide southward one sunny day. Or just before bedtime, in a quiet place, go out and listen toward the sky. If you listen closely, you might hear the waves of songbirds overhead calling to each other as they head south again for the winter.