Feathered Clues Clear Up Migration Mystery

By Cat Lazaroff

WASHINGTON, DC, February 8, 2002 (ENS) - Using chemical clues extracted from feathers, biologists have tracked down the wintering grounds of tiny songbirds that migrate throughout the Western Hemisphere. The findings will help conservationists identify and protect areas crucial to the survival of migrating warblers, now threatened by deforestation and other habitat changes.

The study links the summer breeding grounds of the black-throated blue warbler in eastern North America with its wintering sites more than 1,000 miles away in the Caribbean, and helps to identify where these migratory birds are most vulnerable.


Black-throated blue warblers travel more than 1,000 miles between breeding grounds in the U.S. and wintering grounds throughout the Caribbean (Photo by Bill Dyer, courtesy Cornell Lab of Ornithology)
Black-throated blue warblers belong to a group known as neotropical migrant songbirds, many of which have shown dramatic decreases in population in recent years.

Researchers from Stanford University, Dartmouth College and the Smithsonian Institution's National Museum of Natural History analyzed minute chemical traces of carbon and hydrogen in the feathers of black-throated blue warblers.

"Numerous migratory species that breed in the United States and Canada are in decline," said C. Page Chamberlain, a Stanford professor of geological and environmental sciences and coauthor of an article appearing in today's issue of the journal "Science."

"It is unclear, however, whether these loses are due to problems in their breeding grounds in the north or their wintering habitat in the Caribbean," Chamberlain added.

Central America, which plays winter host to as many as one-third of all North American migrants, lost about 2.3 million acres of forest cover per year between 1990 and 1995, according to the National Audubon Society. Haiti and other Caribbean islands also have suffered widespread deforestation, which conservationists suspect is partly responsible for declining bird populations in Appalachia and other parts of North America.

"To develop successful conservation strategies, you need to know where the birds go in winter," Chamberlain explained. "Our study is the first to link southern wintering and northern breeding populations of a migrant bird."

Black-throated blue warblers and other migratory songbirds are inherently difficult to track on a seasonal basis.

"Biologists have been banding warblers in the U.S. and Canada for 20 years, but only one banded bird has ever been found in the Caribbean," said Dustin Rubenstein, a Ph.D. student in ecology at Cornell University and lead author of the study. Rubenstein analyzed feathers and developed the statistical model used in the study while he was an undergraduate at Dartmouth.

Attempts at tracking warbler migrations with radio transmitters or DNA sampling also have failed, Rubenstein added.

"We used the relatively new technique involving measurements of stable isotopes to learn where specific groups of breeding birds spend their winter," he said. "Ultimately, we hoped the results might help us understand why some populations were declining."


C. Page Chamberlain of Stanford University helped pioneer the technique of measuring carbon and hydrogen isotopes in bird feathers while working at Dartmouth College (Photo courtesy Stanford University)
In 1997, Chamberlain and collaborator Richard Holmes, a Dartmouth biologist, helped pioneer a method of measuring carbon and hydrogen isotopes that naturally accumulate in birds' feathers during long migrations. The technique allows scientists in the lab identify breeding sites of individual warblers.

Isotopes are atoms of the same element that contain different numbers of neutrons. In nature, the most abundant carbon isotope is carbon 12, whose nucleus is made up of six protons and six neutrons. A small percentage of carbon atoms carry an extra neutron and therefore are known as carbon 13.

Hydrogen 1 (the most common hydrogen isotope) only has one proton and no neutrons, but a rarer form, deuterium, contains both a proton and neutron.

"The strength of isotope analysis is that you can use it to track whole populations without injuring or killing individual animals," said Jacob Waldbauer, a research associate in Stanford's Department of Geological and Environmental Sciences, and a coauthor of the "Science" article.

When Chamberlain and his coworkers analyzed warbler feathers collected at various U.S. and Canadian breeding grounds in 1997, they discovered that the ratios of carbon 13 to carbon 12 and deuterium to hydrogen 1 gradually decreased as breeding sites moved north. For example, birds nesting in Georgia had higher had higher amounts of carbon 13 and deuterium in their feathers than those from Ontario, Canada, several hundred miles to the north.

Previous studies showed that isotope levels in warbler feathers directly mirrored those found in the environment - that is, the carbon 13 to carbon 12 ratio in plants and insects consumed by warblers decreased in northern latitudes, as did the deuterium to hydrogen 1 ratio in lakes and rivers.

"In most species of birds, feathers are renewed annually, and thus their isotopic composition should reflect that of the foods and water consumed at the time of feather growth," Chamberlain wrote.


A pair of nesting black throated blue warblers bring food to their young chicks. What the birds eat shows up later in their feathers in the form of different carbon and hydrogen isotopes (Photo courtesy Stanford University)
For the current study, researchers collected feathers from almost 700 black-throated blue warblers. In North America, samples were obtained at 10 breeding sites - from Georgia in the south, to Michigan in the northwest, to New Brunswick, Canada, in the northeast.

In the Caribbean, feathers were collected at 11 wintering locations on four islands - Puerto Rico, Jamaica, Cuba and Hispanola, which is divided between Haiti and the Dominican Republic.

The results, wrote the authors, were striking. Isotopic ratios in feathers collected in the Caribbean decreased from Puerto Rico in the east to Cuba in the west, indicating "that more birds from the northern portion of the breeding range [Canada, Michigan, New York and New England] winter on the westerly islands of Cuba and Jamaica, whereas more birds from the southern portion of the breeding range [the Appalachian mountains from Georgia to West Virginia] winter on the easterly islands of Hispanola and Puerto Rico."

Understanding patterns of migration has important implications for conservation of songbirds and other migratory species.

"Breeding bird survey data from the past 30 years indicate declines in black-throated blue warbler abundance in the southern breeding areas - particularly the southernmost extreme [Georgia and Virginia] - and little change, or even increases, in abundance throughout much of the northern breeding range," the authors wrote.

Furthermore, they wrote, the most extensive deforestation in the Caribbean has occurred on the Haitian island of Hispaniola, which may help explain the sharp population declines observed in southern Appalachia.

"Our results suggest a possible connection between this and declines in the southern breeding population of black-throated blue warblers," said Holmes.

The study suggests that bird conservationists in the U.S. may wish to focus their conservation efforts a little further south.

"For the first time, 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," Chamberlain noted.

Isotope analysis is now being used on European warblers that winter in Africa, Chamberlain noted, as well on salmon and other migratory animals, and has even been used to trace the migration of cocaine from Latin America to the United States.