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Wind Turbines Kill Hundreds of Thousands of Bats, and Now We Know Why

Unfortunately, we don't really know how to stop them from dying (yet).

Wind energy is great: It's clean, it's often cheap, and in many locales, it's an ideal power source. Except for the whole animal-killing thing. Unfortunately, every year, wind kills hundreds of thousands of bats in the United States—maybe as many as 600,000, according to some estimates.

Now, we finally have a good idea why: The bats likely think the turbines are trees, because the air currents around slowly-moving turbines mimic those that move around their wooden counterparts.

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To study the effect, Paul Cryan, a United States Geological Survey researcher, and his team observed roughly 1,000 bats at specially-manipulated wind turbines over the course of two months in 2012. Tree-roosting bats are most commonly found among those dead, and that played out in a study Cryan and company just published in Proceedings of the National Academies of Sciences.

"Our observations suggest that bats may be more likely to closely approach wind turbines when the winds are not blowing hard and when the turbines might feel like trees to bats from a downwind airflow perspective," Cryan told me. "Bats may follow distinct, tree-like air-flow paths when approaching turbines."

I BELIEVE THAT BATS AND WIND TURBINES MIGHT BE COMPATIBLE

In other words, in certain wind conditions, bats may be misled into trying to roost in the wind turbines. When there's a slight gust, well, the bats can be killed. Cryan says that bats are most likely to approach the turbines at night, and are likely to stay away from turbines when there are high-speed winds, because the air currents that a fast-spinning turbine creates aren't at all like those found near trees.

"We suggest that wind gusts during low wind periods may put the bats at risk if they are lingering near the turbines when the blades transition to and from high rotation speeds," he said.

Although we know how the bats are dying, and now even what cues they may be responding to, we don't know exactly why they do it, and why they always seem to approach from downwind when wind speeds pick up to be faster than one meter per second. That's important, because, well, if wind power is going to be successful, turbines can't be killing hundreds of thousands of bats every year, especially considering the incredible decline bats are already having.

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"I believe that bats and wind turbines might be compatible, we just need to figure out why they are approaching and find ways of convincing them to stay away," Cryan said.

But answering the question of how to keep bats away from turbines is much trickier than finding out what the mechanism of their attraction is, unfortunately. And we don't really have a lot of leads at this point.

"We don't know of any obvious ways to keep bats away from turbines yet, but we hope that our findings help find effective methods," Cryan said. "We suggest that pointing [ultrasonic wave-making devices] into the downwind airspace behind a turbine might have a better chance of targeting a bat than pointing them elsewhere around the turbine."

"We also suggest that changing the appearance of a turbine might help visually orienting bats know it is not a tree, but we did not study any approach in particular," he added.

So, that's where we are for now. The problem, beyond, you know, making sure wind turbines don't kill off a very important part of the ecosystem (and often important pollinators) is that we still don't know just how big of a problem this is.

Though we can count how many bats are being killed by wind turbines—although even that is an inexact science—we don't know the overall population numbers for a lot of bat species. We also, unfortunately, don't know exactly why wind turbines are killing birds en masse, either. Cryan says that, during the course of his study, it became obvious that birds are dying for a different reason altogether.

"Bird fatalities are found at wind turbines, but most of those collisions tend to be spread across many different species and we just don't see the predominance of a few species among fatalities like we do with tree bats," he said.

So, yeah. Wind power: It's great, when it's not knocking animals out of the sky.