For several decades, scientists studying the population dynamics of migratory birds have debated the relative importance of events in summer breeding areas versus tropical wintering grounds. Now, in a Dartmouth study reported in the Dec. 4 issue of the journal Science, Peter Marra and colleagues provide evidence that what happens in the tropical forests where migrant warblers spend their winters and what happens in the Northern woods where they spend the next summer are inextricably linked.

Research on the population dynamics of migratory birds has been hampered by the difficulty of tracking individual birds throughout their annual cycle across thousands of miles. By using carbon isotopes as a habitat signature, the investigators in this study were able to link American redstarts at Hubbard Brook Experimental Forest in the White Mountains of New Hampshire with the habitats they occupied the preceding winter. "We've found that the quality of the winter habitat affects not only when birds fly north to the breeding grounds but what condition they're in upon arrival. Both are crucial factors in how well the birds do on their summer breeding grounds," says Marra, who is now Postdoctoral Fellow at the Smithsonian Migratory Bird Center in Washington, D.C. "Clearly, natural selection is acting on these birds year-round."

Marra conducted this research as a graduate student with Richard Holmes, Harris Professor of Environmental Biology at Dartmouth, and their colleague, Keith Hobson, Research Scientist at the Prairie and Northern Wildlife Research Center in Saskatoon, Canada.

Previous studies have found that males in many migratory species arrive at the North American breeding grounds before females and that males in the best condition arrive earlier. These earlier arriving individuals have a competitive edge in reproducing: first choice of prime nesting sites and mates and an opportunity to replace eggs
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Contact: Nancy Serrell
nancy.serrell@dartmouth.edu
603-646-2117
Dartmouth College
3-Dec-1998