Migratory Songbird Mystery Solved

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Until now, ornithologists had been stymied by the details of songbird migration, a source of concern since many of these birds are declining in population and no one knows exactly why.

Songbirds—about 46 percent of Earth's bird species—are too dainty to lug around the most accurate tracking devices, satellite tags that relay their locations immediately to computers. So scientists have mainly relied on "snapshots" of songbird travels, by observing large flocks on weather radar screens or marking birds and trying to re-capture them in transit.

One thrush was tracked with a radio transmitter for an unrivaled 940 miles in 1973.

Thanks to miniaturized tracking technology, Stutchbury's team has blown this record away.

Their breakthrough was made possible with dime-sized geolocators, battery-powered devices weighing a fraction of an ounce that record light and store data on sunrise and sunset times, which vary with latitude and longitude. Stutchbury rigged the birds with mini-versions of devices that were first developed by the British Antarctic Survey to track albatrosses.

Conservation for Songbirds in Decline

In addition to showing the birds flew faster than previously known, the geolocators also revealed that some birds engaged in leisurely stopovers during their fall migration—three to four weeks in Mexico's Yucatan for the purple martins.

Stutchbury learned as well that wood thrushes stuck closely together on their winter grounds in Honduras and Nicaragua. Both sets of information may be useful for future conservation efforts.

Until this study, "We didn't know where—or if—they stopped along their spring or fall migrations," said Duke University conservation ecologist Stuart Pimm, who did not participate in Stutchbury's work.

"This new study reveals where those stopovers are, and that they are important feeding spots."

The Honduras-Nicaragua region is a crucial area to protect, Stutchbury said, because wood thrushes have declined in number by about 30 percent over the past four decades, possibly from threats like deforestation and habitat loss in this region.

"Many species of migratory songbirds over the past 40 years have been in a tailspin. The magic of geolocators is that they will help us direct conservation for individual species. The problems might be different for a thrush, a bobolink, or a shrike."

Overall, it's important to protect songbirds, Stutchbury emphasized, because many control insects or help maintain forests by dispersing seeds. "I like to think of migratory songbirds as nature's blue-collar workers," she said. "They do important jobs."

Many small songbirds like warblers and vireos are still too puny to track with geolocators. But someday, that, too, may be possible—along with other advances, like tracking songbirds with temperature-sensing geolocation.

Russell Greenberg, director of the Smithsonian Migratory Bird Center in Washington, DC, which has also started using geolocators on songbirds, hopes someday to use such devices on birds that winter in the temperate zone, where changing weather may strongly influence migrations.

"There's a lot of research that shows that birds are being affected by climate change," Greenberg said. "If the temperature warms up in January, do they start moving north again?"

Now that Stutchbury and her team have made the initial breakthrough with geolocators, Greenberg said, questions like this may finally be answered.

"It opens up an incredible door to things people didn't think they could do with songbirds."

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