Go out in your yard some evening this fall, cock your head towards the sky, and listen. Thrushes, warblers, sparrows, and blackbirds will be swiftly moving across the night sky toward their winter homes, and as they pass over, you can hear them whisper short “chip” notes among themselves. Some nights, birds will constantly stream by, heard but not seen.
Biologists have long been trying to discover exactly where these songbirds are going. The geographic breadth of songbird habitats – from as far south as Argentina to as far north as James Bay in Canada – makes finding the summer and winter home addresses of a migrating songbird a tough prospect. But finding that address is important: habitat loss ranks as one of the biggest threats to songbird populations, so knowing a bird’s home could go a long way toward songbird conservation.
Bird banding has been the traditional tracking system of ornithologists, who have been using bands ever since John James Audubon put silver cords on the legs of eastern phoebe nestlings over 200 years ago. Since 1902, over 65 million birds in North America have been banded. Each aluminum band has a code etched onto it that is like a bird social security number. But we only get to learn the whereabouts of a banded bird if it is later captured or found dead, and only a fraction of a percent of all banded songbirds are ever found again.
Biologists have recently begun working with a new tool that is a great improvement over the aluminum leg band. Chemical signatures locked in feathers can be read much like a canceled stamp on the outside of an envelope, telling you where the letter originated. These signatures come from naturally occurring patterns of stable isotopes in rainwater that vary predictably across North America. They are transferred up the food web from plants to insects to songbird feathers.
For example, hydrogen and its heavier isotope, deuterium, are both found naturally in rainwater. Both combine with oxygen to form water. But since deuterium is heavier than regular hydrogen, water molecules containing deuterium tend to condense into rainwater more readily than regular hydrogen-based water molecules. This means that, as storms move across North America, the amount of deuterium remaining in the clouds diminishes, leaving a pattern of ever-less deuterium as storms move across the continent. Researchers have been able to collect rainwater from across North America to create a “deuterium map.”
Deuterium levels are in turn reflected in birds’ feathers. Since adult songbirds generally grow new feathers once each year in late summer on or near their breeding grounds, the amount of deuterium found in the feathers correlates with the general location of each bird’s summer nesting area. Even when a bird is caught much later at its winter home, the feathers still have the deuterium signature of the summer habitat where they were grown.
Biologists from the Vermont Institute of Natural Science and the Canadian Wildlife Service have begun to use this tool to help locate the winter and summer homes of Bicknell’s thrush, one of the rarest migratory songbirds in North America. This species breeds on mountaintops throughout the Northeast, from the Catskills in New York to the highlands of the Gaspé Peninsula in Quebec, and winters on various mountain ranges in the Caribbean.
Because our local populations of Bicknell’s thrush live on separate mountain ranges, one concern biologists have is that if individuals that have breeding homes in the Green Mountains, for example, in turn all have a winter home on a single mountain range in the Caribbean and that mountain range is affected by intensive deforestation, we might lose all of the Bicknell’s from the Green Mountains at once. Since only 8 percent of the forest is left in the Dominican Republic, and far less in Haiti, this is not an unlikely possibility.
Researchers have used fine mesh nets to catch thrushes on their Northeast breeding grounds, to create a map of isotope values, and on their wintering grounds, to match the isotope signatures back to the breeding ground.
The results have been surprising and favorable to songbird conservation. Individuals from any one particular place on the wintering grounds had very diverse levels of deuterium, indicating a lot of mixing of populations from the summer breeding grounds. If a particular forest is cleared on the wintering grounds, therefore, only a few birds are lost from each area of the breeding range instead of a catastrophic loss of all the birds from a single breeding place.
Scientists continue to refine the techniques to pinpoint locations more accurately and include other migrating wildlife. Recently one researcher analyzed bird-breath isotopes to track what the birds ate on migration the day before. Maybe soon we’ll be offering isotope-signature breath mints at our bird feeders.