Just as human breathalyzers measure an individual's blood-alcohol level, David Podlesak says that his bird breathalyzer measures the "carbon signature" of a bird's last meal.
"We measure the ratio of the isotopes of carbon 12 to carbon 13, and this carbon signature in their breath can tell us what the bird ate earlier in the day," said the 36-year-old Wakefield resident who is nearing completion of his doctorate.
Podlesak is the first to adapt and use the bird breathalyzer on small songbirds and to verify that the measurements are accurate. The device was created recently by Kent Hatch at Brigham Young University for use with pigeons.
The birds breathe into a small mask connected to a balloon filled with pure oxygen. As the bird inhales the oxygen, it exhales carbon dioxide, replacing the oxygen in the balloon with carbon dioxide within one minute. The carbon dioxide is then analyzed for its carbon signature.
This carbon signature slowly makes its way from the bird's breath to its blood and eventually into its feathers. Podlesak is the first to use carbon signatures in different tissues from the same bird to create a dietary timeline for migrating birds.
"The signature in the feather tells us what the bird ate on its breeding grounds a month or two ago; the signature in its red blood cells tells what it ate within the last two or three weeks; and the signature in its plasma indicates what it ate two or three days ago," Podlesak said. "If the signature is different between its feathers and its breath, that says that the bird ate something different or changed its diet and is using a different resource to fatten up on migration."
That's an important distinction, according to Podlesak. "If birds are looking for different foods while they're migrating -- maybe something that has more proteins or more fats -- then we need to make sure that those resources are available at popular migratory stopover sites."
Podlesak's research is based on birds caught on Block Island, R.I., one of the major sites on the East Coast where birds stop to feed during their southward migration each fall. He captures yellow-rumped warblers, white-throated sparrows, ruby crowned kinglets, golden crowned kinglets and gray catbirds in nets, and takes breath, blood and feather samples before releasing the birds back into the wild.
His results so far have been insightful and have raised additional questions. Yellow rumped warblers, for instance, are known to primarily eat bayberry on migration. But in each of the last two years, Podlesak caught a number of warblers that ate something very different than was expected. He wonders if the birds stopped somewhere out of the ordinary or if there is an unknown migratory stopover site that provides the birds with an unusual diet.
Podlesak also discovered that some white-throated sparrows switched their diet during migration from insects and berries to corn, suggesting that the birds ate at bird feeders. This raises questions about the importance of feeders to the birds as they migrate. Podlesak hopes to answer questions about both species with additional research. Funding for his research comes from the National Science Foundation, the URI Agriculture Experiment Station, Sigma Xi, and The Nature Conservancy, which is protecting the coastal scrub habitat on Block Island where Podlesak conducts his field work.