Tracking the songbirds

The Baltimore Sun

In the fall of 2007, a purple martin carrying a miniature locating device on its back made its annual migration from northwest Pennsylvania to the Brazilian rain forest in just 43 days.

Then, to the astonishment of songbird scientists, it made the 4,300-mile return trip to its spring breeding grounds in less than two weeks, averaging 311 miles per day - three times faster than previous estimates.

That was "really stunning. I don't think anyone had any idea these little songbirds could travel this fast," said biologist Bridget Stutchbury of York University in Toronto. She led the migration study published in today's edition of the journal Science.

The study is the first to map the migration of North American songbirds from start to finish, she said. And it demonstrates the value of newly miniaturized "geo-locators" for efforts to protect songbird populations, which are in steep decline.

"The conservation implications for this are really enormous," Stutchbury said.

Scientists have known that the destruction and fragmentation of forests in North America are among the factors that have contributed to population declines here. "What we don't know so much about is their survival during migration and what threats they face on the wintering grounds" in the tropics, Stutchbury said.

By correlating population declines with more complete data on where they winter, scientists can identify potential causes and solutions. "Geo-locators break completely new ground, and more and more people will use them over the coming years," she said.

Previous efforts to track songbird migrations fared poorly. Banded birds flew off and no one knew where they went or how long the journey took. Radio transmitters were too weak and satellite devices too bulky.

Stutchbury's geo-locators were developed by the British Antarctic Survey for larger birds, then miniaturized for songbirds. Smaller than a postage stamp, they combine a clock on a computer chip with a light detector mounted on a short stalk. Strapped to the bird like a child's backpack, it weighs just 0.05 of an ounce. By recording sunrise and sunset times, the chip provides all the data researchers need to map the bird's daily movements.

But first they have to recapture them.

Stutchbury and her team initially trapped or netted 14 wood thrushes and 20 purple martins in Pennsylvania's Erie and Crawford counties, attached the geo-locators and set them free.

Last spring, they recaptured five of the wood thrushes and two of the purple martins, downloaded the data from their backpacks and mapped their movements.

They found four of the wood thrushes had spent two or three weeks in the southeastern U.S. before crossing the Gulf of Mexico. They flew at night to wintering grounds in a small section of eastern Honduras or Nicaragua. That suggests different thrush populations may winter together in different places, she said. "I imagine [wood thrushes] from Maryland ... would go somewhere else."

The purple martins, flying in daylight, stopped for several weeks in Mexico's Yucatan peninsula before flying on to Manaus, Brazil, in the Amazon forest. One took a side trip south to Bolivia. Their return trip from Brazil in springtime took just 13 days.

Stutchbury said a high-speed return has "huge" survival advantages. Early arrival on the breeding grounds means access to higher-quality territory with higher-quality mates.

Her work continues. Thirty-five wood thrushes were sent off last fall carrying geo-locators. At least 15 are expected back this spring.

The study was funded by the National Geographic Society.

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