Bats may believe that wind turbines are trees, and head toward the energy-generating devices, presenting a danger to the flying mammals.

The U.S. Geological Survey (USGS) captured video of tree bats living around wind turbines, and found that the animals responded to the man-made towers in much the same way that they would a tree. The airborne mammals rarely encounter man-made objects. However, tens of thousands of dead bats each year are now being found under or near industrial wind turbines across the United States. Without a good idea of how many of the animals live in the wild, it is difficult to gauge the damage. Most of these deaths occur during the late summer months of the year, and into autumn, although the reasons for this pattern remain unclear.

Near-infrared cameras were used in the study, along with radar, allowing investigators to film the bats at night. Microphones also recorded the shrill calls of the flying mammals, as part of the study.

"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. 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," Marcos Gorresen of the University of Hawaii at Hilo, co-author of the study, said.

Bats were found to approach stationary or slowly moving turbines, and flew close to the nacells, which hold the blades and the pole.

Researchers are uncertain why bats approach the turbine, but numerous theories are being offered. One idea is that the mammals might expect swarms of insects near the structures, or are looking for a place to set down during flight.

Bats are one of the main predators of flying insects, saving American farmers billions of dollars each year in costs related to pest control. Although they breed slowly, the flying creatures have significant life spans, allowing them consistent control over populations of moths, beetles and other insects.

Birds were also seen approaching the industrial wind turbines during the investigation, although they kept a greater average distance from the devices than did the flying mammals.

Wind turbines are often designed so that they are not triggered until a certain minimum wind speed is achieved, in an effort to reduce wildlife deaths. This new study could provide data needed to better refine those triggers.

Study of how bats react to wind turbines and the dangers presented to the animals was detailed in the journal Proceedings of the National Academy of Sciences.

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