Survival tip: don't hang around machines that have giant spinning blades. It's a lesson bats have been slow to learn, judging by the large numbers of their corpses found beneath wind turbines. New video footage suggests some bats are attracted to wind farms because they can't tell turbines apart from trees. If it's true, this might help us find ways to keep them safer. "I wish we knew for sure" how big a problem wind farms pose to bats, says USGS research biologist Paul Cryan. Other researchers have estimated that tens of thousands—or even hundreds of thousands—of bats are killed every year by wind turbines in the U.S. and Canada. Without a good idea of the population sizes of these bats, it's hard to put those numbers in perspective. But we do know that bats have long lives and reproduce slowly, which makes them vulnerable. "Bat populations do not respond quickly to rapid losses," Cryan says. And some species of bats seem to die at turbines more often than others, so the danger may not be evenly spread out. The safest solution, Cryan says, is to find ways of stopping bat deaths as soon as possible: "We might have the luxury of time and we might not." The bats that seem most likely to die in wind farm run-ins are "tree bats," species that roost in trees rather than caves. To find out how these bats are getting themselves killed, Cryan and his coauthors monitored turbines at an Indiana wind facility for three months in the summer and fall. They used thermal surveillance cameras and infrared video to spot bats in the dark. Acoustic detectors listened for bat calls, while radar swept the area to find out how many animals of all kinds were passing through. The area around and above the turbines was heavily trafficked—radar detected a few million animals in all, including birds and insects. The video cameras captured footage of just under a thousand bats. Researchers scrutinized this video evidence for clues in the bats' behavior about why wind turbines were killing them. They saw that bats didn't try to avoid the turbines, and in fact seemed drawn to them. While radar picked up lots of birds in the area, the birds rarely came near the turbines. But bats closely approached the turbine poles, nacelles, and even the blades themselves. They dove and swooped around the turbines and chased each other through them. Some would spend several minutes approaching and re-approaching a turbine, as if they couldn't stay away. Most of the species seemed to be tree bats (based on their calls, as well as the handful of dead bats researchers found below the turbines). These bats were more likely to approach the turbines on some nights than on others. When the moon was visible and more than half full, there was more bat activity—maybe something about the light reflecting off the blades attracted their attention. Wind was important too: bats were most active when there was a gentle wind, and they tended to stay on the downwind side of the turbines. All this evidence suggests that certain bats are attracted to wind turbines because they resemble trees. They look vaguely like trees, for one thing, with a trunk-like pole and branch-ish blades. In addition, bats can likely sense air currents. The air flow downwind of a turbine may feel to them like the currents on the sheltered sides of cliffs or trees—places where bats gather in nature. They may like hanging out in these spots because there's extra prey (insects get blown downwind) and fewer predators, and flying is easier. If tree bats can't tell turbines apart from trees, they may swoop and gather around them, as the videos showed, because they're looking for prey. They may also be seeking mates, or just a place to roost for the night. The bats can find some of what they're looking for at wind farms—the authors note that bats sometimes turn up dead beneath turbines with their stomachs full of bugs. But they won't have much luck trying to roost in a metal nacelle. These findings aren't universal, Cryan stresses. The group only studied a few turbines at one Indiana facility. But they do provide hints about how wind farms might stop offing so many bats. For one thing, bats in the study were less attracted to turbines when the wind was blowing hard. This may be because the turbulent air currents from fast-moving blades didn't feel as familiar. Cryan says some wind facilities are already raising their "cut-in" speed—the wind speed at which blades start turning—to try and kill fewer bats. If turbines could also avoid turning on suddenly in strong gusts of wind, they might avoid killing bats that had been hanging around and enjoying the earlier breeze. There are also ways to scare off bats using loud noises or other stimuli, Cryan says. If wind farms targeted these efforts at the space downwind of the turbines, they might have more success. "We hope that our work inspires others to look into this mystery, gather evidence, and see if science can triumph," Cryan says. So far the wind turbines have been triumphing over the bats, but that doesn't have to be the end of the story.

Image: by SCA Svenska Cellulosa Aktiebolaget (via Flickr)

Paul. M. Cryan, P. Marcos Gorresen, Cris D. Hein, Michael R. Schirmacher, Robert H. Diehl, Manuela M. Huso, David T. S. Hayman, Paul D. Fricker, Frank J. Bonaccorso, Douglas H. Johnson, Kevin Heist, & David C. Dalton (2014). Behavior of bats at wind turbines PNAS : 10.1073/pnas.1406672111