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For the first time, biologists say they've been able to track individual songbirds on a full roundtrip, from their breeding grounds in North America, down to their winter haunts in Latin America and back again.

NPR's Richard Harris explains how they did it.

RICHARD HARRIS: Ornithologist Bridget Stutchbury fell in love with North American songbirds many years ago, and has made a career of trying to understand how they manage their incredible voyages.

Ms. BRIDGET STUTCHBURY (Ornithologist, York University): I've always been frustrated that they leave in the fall and disappear, and then, as if by magic, show up in the spring, and we really have no idea where individuals go.

HARRIS: The biologist at York University in Toronto says it's possible to track really hefty birds.

Ms. STUTCHBURY: Satellite transmitters have been used for larger birds like peregrine falcons, but those devices are way too big. They're almost the size of a songbird. You can't - they wouldn't go anywhere if you put one on their back.

HARRIS: So instead, she turned to a tiny gizmo that was invented by researchers at the British Antarctic Survey called a geolocator. It's half the weight of a dime.

Ms. STUTCHBURY: These devices measure light. So you put them on the bird's back as a little backpack, and off the bird goes on migration. And when it comes back the next spring, you catch the bird, take the backpack off, and download the data onto your computer.

HARRIS: The computer can analyze a daily record of sunrise and sunset these birds experienced during their migration, and get at least some idea of where the birds have been - plus or minus a couple hundred miles. And Stutchbury and her colleagues now report in Science magazine, it really works. They managed to track five wood thrushes from Pennsylvania to Central America and back. They also followed the flight of two purple martins, a type of swallow.

Ms. STUTCHBURY: I was surprised that both of them spent most of the winter in the Amazon Basin.

HARRIS: Up north, martins usually fly over wide-open fields, catching insects on the wing. But in winter, for some reason, they seem to head for the woods.

Even more surprising was how fast one of those martins high-tailed it back from the Amazon.

Ms. STUTCHBURY: It flew back to its breeding colony in the northern U.S. in only 13 days. This is incredible. I had no idea that songbirds could go this fast.

HARRIS: Also surprising was the flight path of one of the wood thrushes she tracked. Ornithologists assumed that these birds fly straight home from Latin America, right across the Gulf. Four of them did, but the fifth bird she followed actually took the long way around, flying up the coast of Mexico.

Ms. STUTCHBURY: Presumably, this bird was in poor condition, and flying across the Gulf of Mexico usually requires a 12- or 14-hour nonstop flight over water. And if the birds run out of steam over the Gulf of Mexico and go in the water, they're doomed. They're going to die.

HARRIS: Now, this first successful experiment with songbird backpacks isn't simply a matter of curiosity. Stutchbury and other ornithologists are trying to figure out why some species of North American songbirds are in distress. Is it trouble here on their breeding grounds, or trouble down south?

Pete Marra, at the Smithsonian Migratory Bird Center, says these tracking devices could help them solve that mystery.

Mr. PETE MARRA (Ecologist, Smithsonian Migratory Bird Center): I actually think that the floodgates are open now, and I think a lot of people are going to be deploying these on as many species as they possibly can. I'm going to put on about 100 on wood thrushes this year across their entire range.

HARRIS: That could provide a clue about why one of the most melodious denizens of the northern woods is slowly fading away.

Richard Harris, NPR News.

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