Chemical Evidence Proves The Old Adage, "Birds Of A Feather Flock Together"

“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 co-author of the Science paper. “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.”

According to the National Audubon Society, 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. 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,” he added. “Our study is the first to link southern wintering and northern breeding populations of a migrant bird.”

Black-throated warblers

In the their study, the researchers analyzed minute chemical traces of carbon and hydrogen in the feathers of black-throated blue warblers (Dendroica caerulescens) – a species that breeds in the eastern United States and Canada in the spring, then flies south to winter in various Caribbean islands including Cuba, Puerto Rico and Jamaica.

Dustin Rubenstein – a Ph.D. student in ecology at Cornell University and lead author of the Science study – analyzed feathers and developed the statistical model used in the study while he was an undergraduate at Dartmouth working with then faculty co-advisers Chamberlain and Richard T. Holmes, also a co-author.

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,” he said, noting that attempts at tracking warbler migrations with radio transmitters or DNA sampling also have failed.

In 1997, Chamberlain and Holmes helped pioneer a new technique enabling scientists in the lab to identify breeding sites of individual warblers by measuring carbon and hydrogen isotopes that naturally accumulate in the birds’ feathers during long migrations.

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.

When Chamberlain and his co-workers 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.

“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.

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

Conservation implications

In their Science study, researchers collected feathers from nearly 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, say 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, they conclude: “In fact, 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.”

Furthermore, they wrote, the most extensive deforestation in the Caribbean has occurred in Haiti, suggesting a correlation between severe wintering habitat loss on Hispanola Island and sharp population declines in southern Appalachia.

“Now, 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 said.

“The strength of isotope analysis is that you can use it to track whole populations without injuring or killing individual animals,” added co-author Jacob Waldbauer, a research associate in Stanford’s Department of Geological and Environmental Sciences.

“The technique works best with species that have distinct migratory patterns,” he noted.

Chamberlain points out that isotope analysis is being used on European warblers that winter in Africa, 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.

Other Science co-authors are Matthew Ayers of Dartmouth; and Gary Graves and Noreen Tuross of the Smithsonian Institution’s National Museum of Natural History. Research was supported by grants from Dartmouth College, the Howard Hughes Medical Institute, the Andrew W. Mellon Foundation, the National Science Foundation and the Smithsonian Institution.