Isotopes From Feathers Reveal Bird Migration

By analyzing stable isotopes of hydrogen, carbon, and nitrogen in bird feathers, Duxbury has been able to trace Burrowing Owls wintering grounds in southern Texas and central Mexico, as well as migrating Peregrine Falcons caught on the gulf coast of Texas, back to their breeding grounds in Canada.

The principle behind the work is simple: birds are what they eat. And what birds eat while growing feathers on the breeding grounds contains isotopes of hydrogen, carbon, and nitrogen. These vary in predictable patterns across North America.

Duxbury will be presenting a paper on his work on Wednesday, November 5, at the annual meeting of the Geological Society of America in Seattle, WA. Scientists there are exploring the evolving interface between isotope geochemistry and ecology.

Hydrogen and its heavier version, the isotope deuterium, are both naturally found in molecules of rain water. But as the cycle of evaporation and precipitation repeats across North America and over mountainous regions, the heavier deuterium isotopes get left behind. That creates well-mapped hydrogen/deuterium trends across the continent, Duxbury explains.

"There is a well known gradient of depleting deuterium/hydrogen ratios from the Gulf of Mexico and the Atlantic Ocean across the eastern part of North America," said Duxbury. As you get near mountains there is also a noticeable elevation effect that reflects how changes in elevation also cause precipitation cycles.

The hydrogen isotope signature of animals is essentially the isotope signature found in the water and food they eat. Furthermore, the isotope signature found at the bottom of the food chains can be passed up to the top of food chains. The result is that isotopic signatures in the feathers of the top predators reflect the area where the food was consumed while the feathers were grown.

Carbon isotopes, also found in feathers, vary with latitude due to different growing conditions for plants across the continent. Even nitrogen isotopes can help track birds, though nitrogen isotopes variations are not found in predictable patterns. The application of nitrogen-rich fertilizers in agricultural areas can also alter nitrogen isotope ratios, Duxbury explains.

To collect the feathers for analysis, Duxbury and his colleagues rely on other researchers across North America. "Since 1995 I've had other researchers who were banding birds gather feathers all across North America," Duxbury said.

In order to get a local isotope baseline for a bird population the researchers first gather feathers from nestlings at their nest sites. Then they gather feathers from birds on migration or on their wintering grounds to trace them back to the isotope baseline based on the nestlings.

In the case of Burrowing Owls, the stable isotope technique has traced unbanded owls wintering in central Mexico back to Canadian breeding populations, said Duxbury. Subsequent analyses have also revealed that Burrowing Owls disperse more widely between breeding seasons than previously thought. That discovery, in turn, can be applied to population models used in the conservation of Burrowing Owls.

This relatively new technique will not replace banding, says Duxbury, since it cannot trace a bird to an exact location. However, the recovery of a banded bird is very rare event, and so it takes decades to accumulate data. Stable-isotope analysis is providing similar dispersal and migration data, but at a far greater rate. In essence, every bird that is captured for a feather sample is equivalent to a band recovery, Duxbury says.

"Essentially, it's not as good as getting a band return, which gives you A to B," says Duxbury. "You can't say exactly where a bird's origin was, but you can narrow it down to a region. For instance, with an isotope signature we can get it back to southern Alberta, whereas a band can get it to an exact nest location."

Satellite telemetry is by far the most accurate method of tracking birds. However, it comes with a hefty price. In addition, technology has not developed satellite transmitters small enough for Burrowing Owls, says Duxbury.