Image: Program for the Conservation of Mexican Bats via Wikimedia

Wind may be the clean energy source of the future, but even renewables take an environmental toll. Wind turbines, for instance, are bat death machines, and a new study offers clues as to why: Bats may be flocking to the whirling steel towers to catch a meal.

The finding is preliminary, focusing on a single wind farm in the Southern Great Plains. But if the study’s results are borne out elsewhere, they could help planners figure out ways to design wind farms that aren’t also death traps.

It’s well-established that bats and turbines aren’t friends. Researchers have estimated that hundreds of thousands of bats die at the blades of North America’s wind industry each year. Many more are killed by the drop in air pressure around turbines, which can cause their lungs to rupture.

As demand for wind energy grows, it’s important for developers to figure out how to site and operate turbines with this wildlife impact in mind. But first, they need to know why bats fly toward turbines.

Some researchers have proposed that bats simply find turbines interesting—perhaps they’re attracted to the red aviation lights located atop a turbine’s nacelle. It’s possible bats misperceive turbines to be trees, or even bodies of water.

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There’s also some evidence that bats are actively foraging around wind turbines. The new study, which took place at the Wolf Ridge Wind Farm in northern Texas, is one of the first to directly test that idea. Several species of bats, including Eastern red bats, hoary bats, and silver-haired bats, forage in the area.

Over the course of seven years beginning in 2010, the researchers at Texas Christian University in Fort Worth used acoustic monitoring to determine which species of bats were hanging around turbines, and to characterize their activity. They also conducted insect surveys at the wind farm in 2012, 2013, and 2015, and examined stomach contents of Eastern red and hoary bat carcasses. DNA in bat stomachs and fecal pellets were analyzed to figure out how the bats’ last meals compared with the insects found at the plant.

Overall, the researchers say their results provide “strong support for the hypothesis that bats are using wind turbines as a foraging resource,” according the study published recently in the journal Peer J.

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All six bat species the researchers looked at exhibited foraging behavior at the turbines, as demonstrated by acoustic signals including “feeding buzzes.” The majority of dead bats had full or partially full stomachs, indicating they probably died while noshing on bugs. And the most popular stomach contents, including partially-digested field crickets and fall army worm moths, were also documented buzzing around turbines.

Paul Cryan, a bat biologist with the US Geological Survey, was impressed by the study, telling Earther it “represents a substantial effort of unprecedented scope.”

“What surprised me most was just how many lines of evidence they gathered that were consistent with the possibility that bats actively feed in close proximity to turbines,” Cryan said. “By showing that the bats struck down by turbine blades were often stuffed with the very kinds of insects the researchers detected flying around the turbines, these new results provide the most comprehensive picture yet of how hunger might be influencing risks that insect-eating bats take around wind turbines.”

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The study has some important limitations. It only surveyed a single wind farm in a single ecosystem, and it didn’t compare foraging behavior at the wind farm with foraging in surrounding ecosystems. That means we don’t know if bats are being drawn to wind farms to forage, or if they’re simply feeding opportunistically while passing through the neighborhood.

We also don’t know if insects are present in higher numbers around turbines, and if so, why.

Still, the study does offer evidence that at least in some cases, bats are taking advantage of turbines to eat. That’s an important insight.

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“Whether insects are attracted to and, therefore, found in higher densities in the vicinity of wind turbines relative to other surrounding habitats will [now] be important to understand,” Ryan Zimmerling, a bat researcher at the Canadian Wildlife Service who was not involved with the study, told Earther.

If turbines are attracting bugs which are attracting bats, wind developers may also want to focus on technological innovations that deter bats from approaching. Cryan says he’s working with engineers to try and change the way bats see turbines from afar.

“If bats visually mistake turbines for the kinds of trees where they consistently find insects, roosts, and/or other bats (multiple resources), then trying to change the way that bats perceive wind turbines from afar might be an effective way to keep them from approaching,” he said.

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Operational changes, such as turning turbines off at low wind speeds when bats are most active, could also help reduce fatalities. In 2015, the American Wind Energy Association introduced voluntary operational guidelines aimed at reducing bat deaths, but their effectiveness remains disputed.

Mainly, the study suggests we need to be paying more attention to why bats are altering their behavior around turbines. Doing so now, while the wind industry is still in its early days, could save bats a world of harm.