How researchers in a race with wind-power developers are shedding light on the habits, pathways, and winter haunts of a little-understood eastern migrant — the Golden Eagle
Published: December 22, 2011
|The snow had been falling on and off throughout this frigid, silent winter day, leaving deep drifts in the woods on the mountainside.|
Inside a cluttered, overheated office attached to a state vehicle-maintenance shed, Jeff Cooper leans back in his chair and fiddles with a two-way radio that has emitted nothing but static for hours.
A wildlife biologist with the Virginia Department of Game and Inland Fisheries, he tries the radio one last time, attempting to reach an assistant hiding in a blind a few miles farther up the mountain.
Cooper shrugs. Fieldwork entails a lot of waiting — and all the more so when you’re in remote southwestern Virginia, it’s a frigid day in early January, and you’re looking for Golden Eagles.
West of the 100th Meridian, and especially in New Mexico, Colorado, and Wyoming, his task would be easier, since the big bird nests from Alaska south to central Mexico. But in eastern North America, the Golden Eagle is so uncommon and little known that its estimated population size, somewhere between 1,000 and 2,000, is hardly more than a wild guess.
Single Golden Eagle nests have been reported in Maine, and pairs of reintroduced birds are nesting in Tennessee and Georgia. Otherwise, breeding in the East occurs mostly in Canada: south and east of Hudson Bay, in Labrador, and across Quebec to Anticosti Island (see map). Much of the population appears to migrate south in the fall along Appalachian ridgelines to wintering grounds as far south as Alabama, but remote, high-elevation forests in Virginia and West Virginia are thought to make up the eastern birds’ core nonbreeding habitat.
Clinch Mountain Wildlife Management Area, near Blacksburg in Virginia’s narrow western tip, is as good as any place to find them during the winter. In early 2010, Cooper and his colleagues captured four Golden Eagles here and outfitted them with GPS transmitters. All four headed north by mid-April and then spent the breeding season on the Ungava Peninsula in northern Quebec.
The transmitters record an eagle’s location every 30 seconds and send the data back through cellular towers — an inexpensive method that yields far more detailed data than satellite tracking, as long as the eagles pass within range of a tower every so often.
Today, years into a study led by Research Assistant Professor Todd Katzner of West Virginia University, investigators in half a dozen eastern states and Quebec have fitted GPS transmitters on about 40 Golden Eagles and collected good data from about 30 of them. The project has significantly increased our understanding of Golden Eagle behavior throughout the year and heightened concerns that wind-energy development along Appalachian ridges may represent a threat to the population.
In central Pennsylvania, the eagles appear to migrate through a narrow bottleneck along the Allegheny Front and Appalachian ridges to the east, an area of prime wind-energy potential. Katzner and colleagues hope a detailed understanding of the birds’ movements will inform careful development of wind farms planned for the region.
As dusk approaches, Cooper drives his ATV through drifting snow to higher ground, within radio range of his assistant, who has been waiting since sunrise to trigger the rocket-propelled net the researchers use to capture eagles. Cooper finally gets a signal, but the news is discouraging: Nothing yet.
“The logistics are tough,” he says, ready to call it a day. There’s plenty of winter left, though, and cameras at other stations along western Virginia’s mountain ridges have shown regular visits by Golden Eagles. They’re out there somewhere, and tomorrow, he will try again.
Although much remains unclear about eastern Golden Eagles, including such basic aspects of their life history as diet, range, and reproduction, the impact of unfortunately sited or poorly planned wind developments on raptor populations has been demonstrated elsewhere in gruesome fashion. Turbines at Altamont Pass in California are estimated to have killed around 1,000 raptors, including dozens of Golden Eagles, each year. And on Norway’s Smøla islands, a wind farm built a decade ago has dramatically affected the area’s breeding population of White-tailed Eagles. “What we don’t want is to have a site like that in the eastern U.S.,” says Katzner.
So far, no Golden Eagles are known to have been killed by turbines in the East, although a Bald Eagle was killed at an Ontario wind farm in 2009. But new wind farms continue to appear along Appalachian ridges that are likely used by Golden Eagles and other raptors during migration. According to industry groups in the United States and Canada, more than 2,100 turbines had been installed between Virginia and Quebec as of early 2011. Several hundred more are under construction, and numerous projects are in the development or permitting stage.
“We want to protect these birds,” says Charles Maisonneuve, a biologist with the Quebec Ministry of Natural Resources and Wildlife who is working with Katzner on the project. “This [study] helps us try to pinpoint some avenues in order to avoid the collision risk.”
Tracking yields model
By early spring in Virginia, Cooper’s luck had improved considerably. After two successful captures at Clinch Mountain in mid-January, he and his assistants caught five more Golden Eagles in remote, sparsely populated Highland County, Virginia, and several others at sites scattered along the Allegheny and Blue Ridge mountains. In total, he fitted 12 eagles with GPS transmitters in 2011, the second year that he had participated in the study with Katzner, and he hopes to capture 10 more in early 2012.
With major funding from the U.S. Department of Energy and state wildlife agencies in Virginia and Pennsylvania, Katzner and colleagues have begun studying the eagles’ behavior throughout their life cycles by analyzing the reams of data produced by the tracking devices.
Trish Miller, another scientist working with Katzner, has examined Golden Eagle migration through Pennsylvania as part of her doctoral research at Penn State University. Using data from about 20 tagged eagles, Miller has created a three-dimensional model to predict where the eagles are likely to fly under a variety of conditions. Completed in late 2011, her model should allow wind-energy developers and industry regulators to evaluate the level of risk posed by specific turbines and wind farms.
So far, Miller says, it appears that migrating Golden Eagles make significant use of two types of lift: thermal and orographic. Thermal lift is created by rising streams of air produced when the ground below is heated by sunlight. Orographic lift is provided by updrafts caused when air currents are forced upward by mountain ridges.
GPS, eagles, and turbines
West Virginia University’s Todd Katzner isn’t the only field researcher looking to GPS tracking data to help weigh the costs of wind-power development on Golden Eagles.
Carol McIntyre, a wildlife biologist in Alaska’s Denali National Park and Preserve, used satellite telemetry to track eagles that breed in the park and winter from central Alberta to Mexico. “These data can certainly give some idea if a big wind farm on the plains of Montana might affect Golden Eagles in Alaska,” she says.
McIntyre says her data have already been used to plan for wind farms in Alaska and British Columbia. She is now working on a long-term study of Golden Eagle nesting territory and reproductive success in Denali.
Jeff Smith, former science director at HawkWatch International, said that data he’s gathered on Golden Eagle movements in the West have been of value for wind-energy planning, too.
Migration corridors aren’t nearly as constricted in the West as in the Appalachians, but Smith says both he and McIntyre can point to areas where migrating birds tend to concentrate. Also, he says, a proliferation of GPS tracking studies are measuring how proposed wind- and solar-energy developments in California might affect year-round-resident Golden Eagles.
And a study by Audubon Minnesota and state wildlife agencies in Wisconsin and Minnesota is using GPS data to track three Golden Eagles that winter in the Driftless Area of those states. Mark Martell, director of bird conservation for Audubon Minnesota, says the data have been considered in the development of wind-energy projects in the region.
While thermal lift allows eagles to soar more than 1,000 meters above ground level, far beyond the reach of wind-turbine blades, updraft currents have a much lower ceiling, about 200 meters above ground level. “When thermal lifts stop, [the eagles] tend to fly at lower altitudes and concentrate on ridge tops and the Allegheny Front,” says Miller.
This suggests that Golden Eagles may be more vulnerable to wind-turbine collisions in the morning and evenings, and on days when weather conditions inhibit thermals. Additionally, the migratory behavior of adult birds, which appear to head north earlier in the spring and return south later in the fall than juveniles, may put them at additional risk.
Adults migrate north as early as March, and one tagged Golden Eagle was observed migrating south as late as January. Cold temperatures at these times could make the birds rely on low-altitude updrafts more than thermals. And, Miller adds, in a long-lived species with a low reproductive rate like the Golden Eagle, significant adult mortality could seriously harm a population’s stability.
At the same time, Katzner says the eagles’ migratory behavior appears to be fairly uniform in response to various conditions. “That’s a really good thing, because it means that when [they] come down a ridge, we can expect most eagles to behave in the same way.”
The emerging picture of Golden Eagles’ migratory behavior suggests that reasonably confident predictions about flight patterns can be made based on data collected from tagged individuals. At the same time, the study has begun to demonstrate that eastern Golden Eagles routinely make use of the same wind currents along Appalachian ridgelines that attract wind-energy developers — meaning that wind turbines could indeed pose a threat to the iconic birds.
The data have also dramatically expanded scientific understanding of other aspects of eastern Golden Eagle behavior. One fascinating example is apparent leapfrog migration, a phenomenon well documented in other birds of prey. According to Katzner, the southernmost breeding eagles — those that spend their summers on Quebec’s Gaspé Peninsula — migrate short distances to winter territories in the northeastern United States, while eagles that breed in northern Quebec and Labrador seem to migrate much farther, more than 1,500 miles to Virginia or farther south.
According to Cooper, the monitoring data collected during the winter also show that eastern Golden Eagles have relatively small, forested winter ranges, a distinct difference from the huge, open territories preferred by western eagles. (Maisonneuve, however, has found that Golden Eagles on the Gaspé Peninsula prefer open habitat.)
While the overall size and stability of the population remains uncertain, annual hawk watches have recorded increasing numbers of migrating Golden Eagles in the East. Katzner says it is unclear whether that reflects an actual increase, or whether changing hawk-watch parameters, particularly extending counts later into the fall and early winter, mean that previously unnoticed birds are now being counted. Katzner also notes that eagles forage on roadkill deer during the winter. While no evidence explicitly links the two, he speculates that increasing deer populations could support a growing population of eastern Golden Eagles.
The study has also highlighted trapping as another concern. In December 2010, Eagle No. 41, a bird that researchers had tracked for four years, was killed in a trap in Quebec. Earlier in life, No. 41 had also been injured in a trap in West Virginia but was rehabilitated and released back into the wild. A second eagle outfitted with a GPS device was also killed in a wolf snare in Quebec, and birds brought to rehab centers often have been injured in traps and snares.
Katzner hopes his project raises scientific and public awareness of the eastern Golden Eagle, the necessary first step toward safeguarding the population. “A big part of this is just finding out about a species that nobody knew anything about,” he says. “If people don’t know about a species, it’s very difficult to protect it.”
Cooper says the bird had been studied so little that, until recently, neither he nor the Virginia Department of Game and Inland Fisheries paid it any attention. And while the Golden Eagle does not now appear on Virginia’s list of species of conservation concern, he expects that to change, in Virginia and other states across the region, as the importance of the wintering habitat in the central Appalachians becomes known.
Looking forward, Katzner and colleagues plan to continue fitting more eagles with tracking devices, adding even more data to their already-large collection and enabling others to plan for inevitable growth of wind power in the region. For now, Katzner is confident that, with good science and thoughtful planning, Golden Eagles and wind turbines will be able to coexist in the eastern United States.
“If wind-energy development is done right, it could be done without having substantial impact on eagles,” he says. “[But] there are sites that need to be developed carefully, and we certainly hope the information we gather is going to allow people to do that.”
Andrew Jenner is a freelance journalist. He contributed an article about the U.S. Nightjar Survey Network to our June 2010 issue. He lives in Harrisonburg, Virginia.