Large numbers of bats get killed because of wind turbines. The species of bats that perch in trees, in particular, were found to be more vulnerable to these fatal accidents. Findings of a new study finally unraveled why wind turbines draw in these flying mammals to their death.

For the study published in the Proceedings of the National Academy of Sciences on Sept. 29, U.S. Paul Cryan, from the United States Geological Survey (USGS) in Fort Collins, and colleagues wanted to better understand the behavior of "tree bats" that make them at high risk of dying at wind turbines so they used thermal cameras, infrared videos and other surveillance equipment to monitor bats at three wind turbines in Indiana for a little over two months.

The researchers used the video footages their equipment has taken to look for clues in the behavior of the bats that could explain why they are susceptible to getting killed by the wind turbines. They observed that the animals were not trying to avoid the turbines unlike the birds that rarely approach the structures. The bats, in fact, seemed to be drawn to the wind turbines closely approaching them and appearing to play around and chase each other around the structure.

Cryan and his colleagues also observed that there are nights when the bats were more likely to come near the turbines such as when the moon was visible indicating that the light reflected on the blades could be drawing the bats' attention. The animals also tend to be more active when there was gentle wind with the nocturnal animals likely to settle on the turbine's downwind side leading the researchers to conclude that the wind turbines attract bats because they look like trees.

"We discovered previously undescribed patterns in the ways bats approach and interact with turbines, suggesting behaviors that evolved at tall trees might be the reason why many bats die at wind turbines," Cryan and his colleagues wrote.

The researchers said that their findings could pave way for the adoption of measures that could reduce the likelihood of bats dying at wind turbines. Cryan said that knowing the reason why bats approach wind turbines could provide clues on how to turn them away.

"The way bats approach turbines suggests they follow air currents and use their dim-adapted vision to find and closely investigate tall things shaped like trees," said study author Marcos Gorresen, from the University of Hawaii at Hilo. "We see these behaviors less often on darker nights and when fast-moving turbine blades are creating chaotic downwind turbulence. This may be because bats are less likely to mistake turbines for trees and approach them in those conditions."

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