Image: COURTESY OF R. T. HOLMES/Dartmouth

Keeping track of migratory birds that travel more than 1,000 miles in search of warmer climes is a monumental task. Banding, in which small metal rings are attached to a bird's leg to allow identification when the bird is caught later on, often has a success rate as low as 2 percent. Now, according to a report published today in Science, a new technique that exploits chemicals present in feathers may make following some birds' complex travel habits much easier.

Dustin Rubenstein, then at Dartmouth College, and colleagues describe their work with the black-throated warbler (see image), a songbird that breeds in the eastern United States and Canada in the spring before flying south to winter in the Caribbean. The team collected feathers from nearly 700 birds and analyzed their carbon and hydrogen content. The most abundant form of carbon, carbon 12, contains six protons and six neutrons while that of hydrogen has only one proton and no neutrons. Each element also has a naturally occurring stable isotope containing an extra neutron, termed C13 and deuterium, respectively. The birds incorporate these heavy isotopes into their feathers, which grow each year during or just after the breeding season, from the insects and plants they eat. Because the isotopic ratios of carbon and hydrogen display geographic variation, the composition of a feather can reveal where the bird was when the feather grew.

The researchers found distinct patterns in the warbler's migration habits. Individuals that bred in the northern part of the species' rangefrom New Brunswick to northern Michigan, New York and New Englandwintered in the western Caribbean islands of Cuba and Jamaica. Those from the southern portion of their range, in contrast, flew to Puerto Rico and Hispaniola, the eastern region of the wintering grounds. The birds "appear to segregate on the wintering grounds with respect to breeding latitude," the authors report.

A better understanding of the warbler's migratory preferences may help the scientists keep its numbers from declining. "For the first time," co-author C. Page Chamberlain of Stanford University notes, "we have evidence that it may be possible to restore breeding populations in the north by preventing habitat loss in the south, allowing us to develop an international conservation strategy for dealing with songbirds."