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>  News Releases >   2002 >   February

Researchers link migratory birds' winter, summer ranges

Posted 02/07/02

Researchers from Dartmouth College, Stanford University and the Smithsonian Institution's National Museum of Natural History are now one step closer to understanding the elusive migratory patterns of a songbird species called the black-throated blue warbler. Their findings in a new study link this species' summer breeding grounds in eastern North America with its wintering sites in the Caribbean, and help to identify where these migratory birds are most vulnerable.

Black-throated blue warblers belong to a group known as Neotropical migrant songbirds, some of which have been declining in abundance in recent years.

In a paper published in the Feb. 8, 2002, issue of Science, the researchers linked the summer and winter ranges of the black-throated blue warbler by analyzing stable isotopes in feathers. The research team determined that these songbirds segregate in their Caribbean wintering grounds according to where they live and breed during the summer months in North America. The findings will be useful in developing more effective conservation initiatives for Neotropical migrants, which are a significant part of bird diversity in North America, as well as in the Neotropics, an area that encompasses Mexico, parts of South America and the Caribbean.

Because these animals are distributed widely and travel long distances each year, it has been difficult to track and study their migratory movements with conventional techniques, such as bird banding or radiotelemetry," said Dustin Rubenstein, senior author of the paper, a 1999 Dartmouth graduate and currently a Ph.D. student at Cornell University. "We used the relatively new technique involving measurements of stable isotopes to learn where specific groups of breeding birds spend their winter. Ultimately, we hoped the results might help us understand why some populations were declining."

The researchers now have much more insight into these migratory patterns. The study shows that black-throated blue warblers breeding during the summer months in the northern part of their range (New Brunswick to northern Michigan, New York and New England) spend the winter mainly in the western Caribbean islands of Cuba and Jamaica. In contrast, warblers breeding in the southern portion of their range (in the Appalachian Mountains from West Virginia to Georgia) winter mostly in the eastern region of the wintering grounds (Puerto Rico and Hispaniola).

These locales were determined by analyzing isotopes contained in the birds' feathers. For these warblers, feathers grown during or just after the breeding season contain carbon and hydrogen isotope signatures that reflect a bird's diet and that of the local (breeding) environment. Because the isotopes vary naturally among geographic areas in North America and because the birds only molt once a year (in the breeding areas), analyses of isotopes in feathers collected from wintering birds can reveal the location of their breeding sites.

"This is the most comprehensive analysis to date of the connectivity between breeding and wintering ranges of a songbird species," said C. Page Chamberlain, Rubenstein's undergraduate co-advisor and former professor in Dartmouth's Department of Earth Sciences, now at Stanford University. "It builds on an isotopic study we originally developed at Dartmouth, and provides detailed data and statistical analysis that allows for linking summer and winter bird populations."

"With this research, we might now be able to link changes in the black-throated blue warbler abundances to habitat change in both its wintering and breeding areas," said Richard Holmes, Dartmouth Professor of Biological Sciences, Rubenstein's other co-advisor on the project. "For instance, some of the most severe deforestation in the western hemisphere has occurred in Haiti on the island of Hispaniola, and our results suggest a possible connection between this and declines in the southern breeding population of black-throated blue warblers."

The project initially developed as a collaborative effort between Holmes, an expert on migratory songbirds, and Chamberlain, a geochemist specializing in isotopes. "However, the project really gained momentum in 1997 when Dustin took on this project, as a senior honors student in Biology and Earth Sciences," said Holmes. "It was exciting to watch him develop as a researcher. This is one great example of Dartmouth's hands-on approach to teaching and its commitment to undergraduate, as well as graduate, research."

Rubenstein, Chamberlain, and Holmes were joined by Matthew Ayres, Associate Professor of Biological Sciences at Dartmouth; Jacob Waldbauer, 2001 Dartmouth graduate, currently a research technician at Stanford; and Drs. Gary Graves and Noreen Tuross, both of the Smithsonian Institution. This study represents how novel techniques, along with interdisciplinary collaborations, can help resolve an issue in biology and ecology that has, until now, remained poorly understood.

-Susan Knapp

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