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The Whole Migration and Nothing But

12 February 2009 (All day)
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(Main) Patrick M Kramer; (Inset) Tim Morton

Tracked. An adult purple martin wearing a geolocator while perched (main) and in flight (inset).

With the help of tiny position-sensing backpacks, for the first time researchers have tracked songbirds throughout their annual migration route. The findings not only pinpoint where the birds spend their time but also reveal that they can travel faster than expected. The information may provide clues to what is causing the decline of some songbird populations.

Weighing in at between 40 and 50 grams, songbirds are too small for satellite tracking often used to monitor larger animals such as wolves, elephants, and whales, and too weak to carry long-term tracking devices. So researchers investigating migration have been limited to techniques such as tracking flocks of songbirds with radar over short distances and studying them in their stopover locations. Based on these studies, researchers were able to figure out when and where songbirds bred, rested, and spent their winters, but they could not nail down where individual birds went or determine their long-term rate of travel.

That's now changed, thanks to a serendipitous find. When Bridget Stutchbury, a biologist at York University in Toronto, Canada, attended an ornithology conference in 2006, she noticed that researchers were tagging a geolocator device to the legs of albatrosses to study their movement. The researchers had a 1.5-gram device that was light enough to strap to songbirds, says Stutchbury, but, because of the size of the birds, it would need to be tied to their backs, not their feet.

Over the winter, Stutchbury worked with the British Antarctic Survey, which had developed the geolocators, to optimize them for songbird use. In August of 2007, she and her team strapped geolocator backpacks to 14 wood thrushes and 20 purple martins. When they recaptured five of the wood thrushes and two of the purple martins the following spring, they were surprised to see that one of the birds made it from the Amazon to Pennsylvania in just 13 days, flying at an average of 577 km per day--about three times faster than estimates for other songbird species. And, as the researchers report today in Science, the return flights are anywhere between two and six times faster than in the fall, allowing the birds to beat their comrades to mating grounds.

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Bird watching. A wood thrush feeds its young while wearing a geolocator.
National Geographic

The new geolocators also showed that the wood thrushes spent between 1 and 4 weeks in the southeastern United States and Yucatan Peninsula before reaching their destination in a relatively small area of Honduras and Nicaragua. Stutchbury says this discovery is important because it identifies a very specific winter furlough location for survival of wood thrushes, which have declined 30% since the 1960s. "If that area undergoes heavy deforestation, we can expect to find really big impacts on the [songbird] populations in that area," says Stutchbury.

The purple martins stopped over in the Yucatan Peninsula for 3 to 4 weeks before heading down to Brazil, where they, unlike the wood thrushes, continued to move around. Purple martin populations have not fallen as much as those of wood thrushes have, and this increased mobility, which gives them the ability to choose from different habitats, might explain why, Stutchbury says.

"It's never been quite clear whether [the declines are] happening in one season or the other," but the new tracking technique "will help to give us a better understanding of what's ultimately leading to those population declines," says behavioral ecologist Dustin Rubenstein of the University of California, Berkeley. This work helps to "catapult us out of ignorance," adds physiological ecologist Martin Wikelski of the Max Planck Institute for Ornithology in Radolfzell, Germany. However, he cautions that the technology provides information on only the songbirds that make it back to their breeding grounds. To find out why some songbird species are declining, we need to find out what is happening to the unsuccessful birds, he says.

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