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Wanderer. A breakthrough technique has allowed scientists to track the migration of the black-throated blue warbler.

Wandering Warblers

Scientists have strengthened the link between deforestation in the tropics and the decline of songbirds that migrate to North America. The team was able to decipher migratory patterns--a daunting task--relatively easily with a new technique that tracks natural chemical markers in the feathers. The new technique could be adapted to study the wanderings of other threatened species, such as salmon and monarch butterflies.

In recent decades, many populations of migratory songbirds that breed in North America have declined sharply. But it's unclear whether fewer birds are breeding in North America or more are being killed in their Caribbean wintering grounds. That's partly because the conventional technique of marking birds with bands has failed miserably at clarifying the bird's migration patterns--only one or two out of tens of thousands of banded birds are ever recovered. To circumvent this problem, biologist Dustin Rubenstein, now at Cornell University, and his colleagues used a new technique developed by Page Chamberlain, now at Stanford University.

The method takes advantage of varying levels of carbon and hydrogen isotopes in the soil, water, and vegetation at different latitudes. Birds take up the isotopes in their food and water, and the isotopes get incorporated into the new feathers they grow after each breeding season. Rubenstein analyzed the isotopic signatures of feathers from wintering warblers to reveal the location of their breeding sites. He found distinct subpopulations with unique migration patterns. Warblers from the northern part of the breeding range--which extends from Georgia to Canada--winter in Cuba and Jamaica, whereas birds from the south go to Hispaniola and Puerto Rico, the team reports in the 8 February issue of Science.

The finding could explain the decline in the southern breeding populations, as Hispaniola has suffered severe deforestation, says Rubenstein. He says the findings could encourage more effective conservation strategies that work across national borders to restore breeding populations by preventing habitat loss in the wintering grounds. "We've never been able to make these strong linkages before," says ecologist Keith Hobson of the Canadian Wildlife Service in Saskatoon. "I'm sure this will lead to investigations in a lot of species."

Related sites
Science paper
Dustin Rubenstein's Web page
USGS stable isotope ecological research
USGS document on stable isotope uses