Birds of a feather – songbirds – Brief Article
Songbirds are small and tough to track when they head south for the winter. Now biologists are learning to trace birds by studying the chemicals in their feathers.
Every autumn millions of small songbirds leave their North American breeding grounds to winter in Central or South America. But exactly which birds go where is anyone’s guess. Because songbirds are small and fly such long distances, they are notoriously difficult to track. As expanding suburbs, deforestation, and other encroachments diminish songbird territory, bird populations have declined sharply over the past ten years. Researchers are thus desperate to figure out songbird migration patterns.
Keith Hobson, a biologist at the Canadian Wildlife Service in Saskatchewan, knows firsthand how frustrating it is to trace the diminutive birds. Songbirds are too small to carry radio transmitters. And because they’re not hunted, banding them is fruitless–after traveling long distances, banded songbirds are rarely recovered. “I’ve personally banded thousands of birds,” says Hobson, “and we’ve got virtually no returns after thirty, forty, fifty thousand birds banded. We know where geese and ducks go,” be adds, largely because of hunters who return bands, “but we haven’t a clue where the warblers and the orioles and the flycatchers and all those wonderful forest birds go.”
A new technique, called isotope tracing, could change this. Developed by Hobson and his colleague Leonard Wassenaar, and simultaneously by Page Chamberlain of Dartmouth, it takes advantage of the peculiar physics of rainfall. Water molecules sometimes contain a heavy form of hydrogen called deuterium. In a rain cloud, water molecules carrying this heavier hydrogen isotope tend to precipitate out first. Distance from the ocean, elevation, and temperature all affect the ratio of heavy-water versus normal-water rainfall in a region. Meteorologists have known this for decades–they’ve even made maps of the continent showing the pattern of isotope ratios in rainfall. In general, the amount of deuterium in rainfall decreases as you go from the Gulf of Mexico northwest to Alaska.
Hobson and Chamberlain both realized that the heavy hydrogen in rainfall would also manifest itself in the food web. Plants incorporate hydrogen from rainwater in their leaves and bear the signature ratio of the region in which they grow. The insects that eat those plants use hydrogen to build their wings and shells, and the birds that eat the insects use it in their growing feathers, Conveniently, while at their northern breeding grounds, songbirds usually grow a new set of feathers before heading south. Hobson and Chamberlain figured that the level of deuterium in a bird’s feathers should match the isotope signature from rainfall at its breeding ground.
To test the idea, Hobson looked at feathers from 140 songbirds of 6 species. taken from 14 breeding locations across North America. The amount of deuterium in their feathers did indeed correlate with what the rainfall maps predicted. As a further test he looked at several species that winter in Guatemala–including hooded warblers, wood thrushes, and gray catbirds–and was able to match them to their known breeding ranges in the eastern United States and Canada. The wide range in the amount of deuterium from birds at a single wintering site showed that they came from a number of different breeding sites.
Best of all, the technique doesn’t apply just to birds. “Pretty much anything that migrates will observe the same rules,” Hobson says. He’s now tracking monarch butterflies by the same method. With the help of schoolchildren across eastern North America, who raised monarchs on local milkweed and allowed Hobson to measure the level of deuterium in their wings, he has created an isotope map of the monarch’s breeding range. Now he’s hoping to trace monarchs from all 13 of their known roost sites in Mexico back to their breeding grounds. “If they’re coining from discrete areas in North America,” he explains, “then each roost site is very important to conserve, because you could be losing a whole chunk of the North American population.” Hobson thinks isotope tracing will be fundamental to understanding migration in small animals. “It’s one more ace up our sleeve in trying to figure out the puzzle.”
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