Resilient as bald eagles are, they are no match for wind turbine blades, whose spinning tips can reach speeds of up to 200 mph.

Intent on keeping the two apart, a team of University of Minnesota researchers with expertise in wildlife behavior, neuroscience and mechanical engineering is continuing its work to find solutions through what bald and golden eagles hear and how it affects their behavior. The findings might help create acoustic deterrents at wind farms that could save the raptors from injury or death as they fly in the airspace near the gargantuan structures.

The group's initial research in 2017-2018, funded by the Department of Energy, included The Raptor Center, St. Anthony Falls Laboratory and experts in hearing sciences. The work was lab-intensive and involved measuring the response of eagles and red-tailed hawks, while sedated, to natural and artificial sounds. The new work will take those findings outdoors to monitor the behavior of bald and golden eagles during flighted tests, among other experiments.

Paid for by a $261,000 grant recommended by the Legislative-Citizens Commission on Minnesota Resources, the next phase is well-timed.

Amid the drive for more renewable energy (wind accounted for 3% of the U.S. energy used in 2020), wind turbines stand to increase in size and number across southwestern Minnesota and the Dakotas. Some will be the equivalent of 60-story structures.

What's unknown and appears difficult to quantify after years of studies are the species most at risk and the number of birds killed by hitting wind turbines. The American Bird Conservancy estimates more than a half-million birds are killed annually, based on three separate studies published in 2013 and 2014. By comparison, it's estimated that about 600,000 birds are killed a year from striking windows.

One of the researchers is Julia Ponder, a professor in the College of Veterinary Population Medicine and former director of The Raptor Center, whose birds were part of the initial research. She acknowledged the challenge of collecting solid data about the mortality of eagles owing to wind farms and the effectiveness of current deterrents but said the search for solutions is important.

The search also is necessary. Although eagles are no longer an endangered species, they are protected by several federal laws, including the Bald and Golden Eagle Protection Act.

Currently, some wind farms use mechanical detection to lessen their impact on wildlife. Some operators employ radar or have workers scan the skies, and shut down the turbines when they anticipate migratory birds or others moving through. Acoustic deterrents already are used in places, such as one called DTBird in parts of Europe and whose effectiveness still is being studied.

Ponder said the work on eagles could ultimately help other species.

"We have to figure out what the actual problems are and then how to mitigate and deal with them," she said.

In the prior research, bald and golden eagles (and red-tailed hawks as surrogates) were sedated and then monitored to measure their hearing ability but also how they responded to natural sounds — ones they'd hear in the wild like eagle grunts and screams or the caws of crows — vs. human-made sounds like white noise.

The birds did react differently to different sounds and responded most to natural sounds. The more the artificial sounds were played, the less impact they had on the raptors.

The testing elicited some other new understanding about eagles' hearing, too.

"What we learned from the test of their sensitivity while they were sedated is that they are sort of average in their hearing," said Peggy Nelson, a language and hearing sciences professor at the University of Minnesota. "They are not remarkable."

Nelson said other species — including humans — are likely more sensitive at more frequencies than eagles, which are sensitive to certain sounds at midranges but not very high- or low-frequency sounds.

"That surprised me," she added. "I thought they'd be really in good in their hearing sensitivity, but they are kind of average."

Now that the researchers know what sounds eagles react to and in what range, they will take the work outdoors to monitor the raptors' behavior. Some tests will involve tethering the birds and observing the eagles' reaction while in flight to different sounds. Maybe, Nelson said, the team will learn of a sound that steers them clear of trouble. But maybe not.

"Do you change your behavior at all when this sound comes on?" Nelson said. "Or are you just ignoring? … We need to find this out."

She said some natural sounds might be a deterrent — or even attractive to eagles.

One area of focus will be the effectiveness of grunts, which the eagles responded to most in the first study. Nelson said that sound that might not deter them, but attract and keep them away from trouble — for example, drawing them away from wind turbines.

"They may not be really afraid or anxious of any sound," Nelson said. "We don't know."

Other approaches

Taber Allison is research director at American Wind Wildlife Institute (AWWI), a nonprofit with stakeholders in renewable energy and conservation. The group is evaluating systems like DTBird and is aware of the research in Minnesota and elsewhere.

Allison said studies to fine-tune possible acoustic deterrents for target species like eagles makes sense, and fits with other "minimization strategies."

"I think it's worth doing," he said.

Allison said DTBird uses a camera designed to detect and track moving birds and estimate their distance, coupled with a warning sound speaker. In one test, DTBird detected 63% of drones used as surrogates for eagles but was limited by sun glare. The deterrence rate for golden eagles in response to warning signals was 52%. The system produced a large number of false-positive detections, too, resulting in unnecessary warning signals. The varied outcomes, like those in other, similar studies of bird collisions with turbines, underscore the challenges of finding solutions.

"The goal is to try to avoid and minimize as much 'take' as is practicable at each project," Allison said.

As a wildlife conservationist, Ponder said she supports clean energy as earth-friendly, but that too often finding answers for the impact of development doesn't occur early enough in the process.

"This is an opportunity to get knowledge … trying to contribute to solving the problem and finding ways that we, humans, can live in harmony and sustainability with Mother Nature and wildlife," Ponder said. "We have only one earth and we have to take care of it."