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Wind turbines and birds don’t mix well. Birds can fly in but not out.

Estimates of death by turbine, made by the U.S. Fish and Wildlife Service (USFWS), range from 140,000 to 500,000 birds per year. The number can grow with each new turbine installation.

Efforts continue to be made to lessen the kill. These deaths are a concern to conservationists, birders and the turbine companies themselves.

There is hope that a recent study by the Raptor Center at the University of Minnesota and several partners will contribute to a solution.

Raptors are especially vulnerable to collisions due to their flight behaviors, according to the USFWS.

Turbines can be 600 feet tall, blade tips moving 200 miles per hour in a good wind. Those blades are nothing but a blur to birds approaching the tower. The whir of the blades carries no meaning.

Researchers, including Dr. Julia Ponder, executive director of the Raptor Center, want to learn what frequencies and levels of sound can be heard by the birds, and their reactions to them.

The study involved two bald eagles and a red-tailed hawk from the Raptor Center, and a third eagle from the Wildlife Rehabilitation Center of Minnesota. Testing was done at the University of Minnesota’s Center for Applied and Translational Sensory Science.

Eagles and hawks have hearing on par with songbirds, which is not particularly sensitive. These raptors hunt with their eyes, unlike owls, which depend on sound.

With tiny electric probes in their brains, the research birds, under close observation in soundproof chambers, were given both natural and synthetic sounds.

“Now we know what they hear,” Ponder said in a telephone interview. “The next question is, do they and how do they respond to those sounds?

“We want to evaluate how sounds can be used to alter flight pattern, and will the birds become habituated to the sounds,” she said.

Those and other questions await answers. Ponder and her team are actively seeking funding for more research.

Turbine sites are chosen after much study. Wind certainly is a factor, but so is the presence of birds. Surveys are made, species listed, individuals counted, routines watched. If the bird has a route or a nest, they are noted.

Bob Janssen, author of five books on birds in Minnesota, spent a cold December day on agricultural land near Albert Lea, Minn. He was counting birds as part of turbine placement research. He’s been doing this for a turbine company in all seasons for four years. On that December day he listed crows, blue jays and horned larks.

After records, study and decisions, despite all that caution the raptor shows up to hunt prairie rodents feeding in the turbine array. How can those birds be alerted to the danger?

Large raptors, like bald eagles, are a well-defined concern in turbine placement. The eagles have a long but slow life history. They must survive five or six years as immature birds before they can reproduce.

Ponder pointed to a USFWS report stating that losing as little as 6 to 10% of the U.S. bald eagle population could destabilize present numbers.

“It’s frequently quite surprising to me how low the loss rate needs to be to destabilize a population,” Ponder said.

Concern is higher for golden eagles, which have a smaller North American population.

This is a long-term effort to avoid population crises in the future, according to a study summary.

Results of these tests will be presented in May to the 177th meeting of the Acoustical Society of America.

According to the U.S. Geological Survey, on average 3,000 wind turbines have been built in the U.S. each year since 2005. As of January 2019, the U.S. Wind Turbine Database lists more than 58,000 turbines.

Now for some perspective: 500,000 bird deaths per year by turbine — that’s .0135% of the estimated annual U.S. bird kill by cats.

Lifelong birder Jim Williams can be reached at woodduck38@gmail.com. Join his conversation about birds at startribune.com/wingnut.