Science

Scientists Reveal ‘Leapfrog’ Migration in Golden Eagles

A little-studied population in the Eastern U.S. shows variation in its breeding and wintering sites.

A bird with the wingspan of an NBA player seems like it’d be pretty hard to miss. Yet the iconic Golden Eagle has proved so elusive in eastern North America that scientists are only now defining its range and coming up with population stats in the region.

Todd Katzner, a research wildlife biologist at the U.S. Geological Survey, says there are a few thousand Golden Eagles that breed in remote portions of Canada—Quebec and Labrador—and winter in the Appalachian Mountains. His latest study, published today in The Condor: Ornithological Applications, shows that within this group, the eagles that breed the farthest north generally spend the winter farthest south, with some migrating all the way to Alabama and Georgia. In doing so, they “leapfrog” over the birds in the middle, which go only as far south as Pennsylvania and New York. (A few don’t even make it out of Canada.)

The study speculates that this leapfrog pattern is the result of a trade-off. Golden Eagles in Pennsylvania and New York, for instance, face harsher winters. But on the other hand, they have an easier migration and might also get the first crack at favorable breeding territories in the spring.

Katzner and his team discovered this pattern by collecting and analyzing body feathers from 47 eastern Golden Eagles, the majority of which they lured to deer carcasses and trapped in nets. By studying the feathers’ stable hydrogen isotope ratios—a complicated process that involves comparing the number of hydrogen atoms with zero neutrons, which are more plentiful at higher latitudes, to those with one neutron—they were able to get an estimate of where each bird was when its feathers formed.

“It’s not terribly precise, but it’s still informative,” says coauthor David Nelson, an associate professor at the University of Maryland Center for Environmental Science, who points out that isotope analysis is occasionally used in missing-persons cases. Since the eagles were also equipped with GPS trackers, the scientists were able to confirm what they found with the feather analysis and obtain more exact locations, too.

Eastern Golden Eagles are believed to be geographically and possibly genetically distinct from the much larger population of Golden Eagles that spans almost all of western North America. Before the 1930s scientists didn’t realize that this species lived in the East at all, and many birding field guides still don’t mention the smaller population.

Although Katzner didn’t start studying the birds until he moved to Pittsburgh in 2005, he quickly understood how these large creatures could avoid detection. “In western North America, Golden Eagles are said to be an open-country bird,” says Katzner, who now lives in Boise, Idaho. “But in the East they spend the winter primarily in dense deciduous forests, usually at higher elevations, like ridgetops and mountaintops. That’s tough country. Humans don’t go there very frequently, and when we get there, we can’t see very far in that habitat.”

In 2010 Katzner helped found the Eastern Golden Eagle Working Group, and since then scientists have been able to fill in some basic details about these birds’ life histories. It turns out most of the eagles winter in the Appalachians, though others head to the Great Lakes region. During breeding season they move up to the northern reaches of eastern Canada. Golden Eagles historically bred in the northeastern United States, too, but no nest has been confirmed there since the ’90s.

The birds face many direct threats, such as leghold traps set for mammals, poaching, lead poisoning from ammunition in scavenged hunting kills, habitat loss, and collisions with power lines and other manmade structures. As the study notes, because eagles from the same breeding area tend to end up in the same wintering area, they might be more vulnerable to environmental disturbances than species that disperse more broadly. Yet unlike their western cohorts, the eastern Golden Eagle population appears to be increasing, Katzner says.

He hopes that by learning more about the eagles’ summer and winter ranges, as well as their migration routes, scientists will be able to protect them better. (It will help, for instance, in siting new wind farms.) “Golden Eagles are charismatic megafauna,” Katzner says. “[Yet] the birds in the East . . . are understudied and poorly known. So there really is a lot of work to do.”

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