Golden eagles love Pennsylvania's ridge-and-valley region. The hunched-up topography, with its long linear corridors running southwest to northeast, makes a perfect thruway for their spring and fall migrations. Sustained updrafts along the ridge crests are a particular boon to these and other large raptors, who rely on lift for soaring long distances.
According to wildlife biologist and Penn State doctoral candidate Trish Miller, those same ridges are perfect for generating wind energy. Unfortunately, birds and wind turbines don't always get along.
In northern California, where wind power is prevalent, the clash is a deadly one: at Altamont Pass, a large wind farm near San Francisco, 65-70 golden eagles are felled every year, Miller noted. A new, taller generation of turbines has bettered the chances for some species but not the aerial hunters, who fly relatively high and focused on their prey.
Miller has been studying golden eagles for more than a decade. Working with Rob Brooks, professor of geography and ecology at Penn State, and Todd Katzner, research assistant professor at West Virginia University, she is a key part of an international effort aimed at lessening the risk these rare birds face as they migrate through areas where wind turbines are increasingly common.
There have been no golden eagles killed yet by turbines in the East, Miller said, but the eastern population of the species is very small.
"Nobody has a clue about how many there really are," she said, "but the best estimate is fewer than 2,000." That's compared to 45,000 birds west of the Great Plains. "If we build an Altamont Pass here, we could have a big problem."
The eastern eagles, Miller explained, spend their summers in eastern Quebec, and the majority of the population winter in the highlands of Virginia and West Virginia. Twice a year they migrate through the ridge-and-valley region, a fairly narrow passage that is home to a nascent but growing wind-power industry.
To get a better fix on their behavior, Miller, who worked previously for the Carnegie Museum of Natural History, follows the birds using a GPS tracking system. Starting in late winter, she and her team set up camera traps baited with deer carcasses. When an eagle lands and takes the bait, it is netted, blood-tested for lead and mercury, and fitted with a small telemetry unit designed by Miller's husband, Michael Lanzone, before being released. The unit fits the bird like a small backpack and collects data on its location every 30 seconds.
That half-minute interval allows for close tracking. "We can see when they're following a ridge, when they're off the ridge, when they cross ridges," Miller said. She can also gauge altitude, which is crucial since the danger zone is below the standard turbine height of 150 meters.
Over the past three springs, Miller and her colleagues have followed 21 birds, collecting 18,000 data points to pinpoint flight paths and habitats. From these, Miller created fine-grained GIS maps of individual migration routes.
"Our current model has a resolution of 30 meters," she said.
Among her findings are that, during spring migration, adult eagles tend to cluster in the ridge-and-valley region, while juveniles fly more to the west, over the broader Allegheny Plateau.
"Adults are moving in the first two weeks of March," she said. "There's not a lot of thermal development yet, so they go to the most likely place for lift. Juveniles are moving in April and May, when it's warmed up."
Overlapping this route data with maps showing planned turbine sites gives Miller a way to assess the risk of collision. Not surprisingly, she has found it to be highest in the ridge-and-valley region. But even within that area, she pointed out, there are good and bad locations for siting turbines.
"With this map we can see the high-risk areas and more importantly the low-risk areas -- area suitable for wind power where there is also low probability of eagle use," Miller said.
Her next step, once the model is refined, is to make it available to wind power developers who might use it to inform future siting decisions. Where turbines already exist, her data may help prompt other solutions, such as shutting down turbines during peak migration periods.
"We're starting to get a better understanding of migration behavior," Miller said. "The ultimate goal is to give wind-power developers a tool to reduce the risk."
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