Wind turbines mostly get bad press for killing birds, but they might actually have a bigger impact on bats, which appear to be killed in large numbers at wind farms. This might disproportionately affect bat populations because, as the authors of a new paper put it, "Bats are long-lived mammals with low reproductive potential and require high adult survivorship to maintain populations."

A team of researchers decided to find out why bats have so many lethal interactions with the turbines. To do so, they used infrared video cameras that imaged any warm-blooded animals close to the wind farm. They also used radar to follow groups of animals flying around the site, which included flocks of migratory birds. In addition, audio recorders were used to determine which species of bats were present, as well as whether they were hunting. Combined with over 1,300 hours of video footage, the data presented a fairly complete picture of the areas near the wind farm in question.

Eighty percent of the close encounters observed between animal and equipment involved bats. The mammals flew near the blades and the body of the wind turbines, sometimes within two meters. In some cases, they chased each other around the hardware or lingered for several minutes near it. The close approaches were more common under two conditions: the presence of bright moonlight and during periods of low winds. The bats also typically approached from the downwind direction when the wind was slow.

When the wind was higher, the bats generally avoided the turbines; as a result, they made fewer approaches when the blades were moving. But they also made fewer approaches even when the authors stopped the blades while the wind was higher, so there are several things potentially going on here.

What about a wind turbine might attract a bat? The authors note that the species most often killed by wind turbines nests in trees, and the bats might view the turbines as a potential place to rest once daylight arrives. In some cases, they have also been seen to hunt insects on the downwind side of a large wind break, presumably because prey tends to congregate there. There's also some speculation that the aerodynamics of flight may simply be more efficient behind a wind break. The one unifying feature of all these ideas is that the bats are simply viewing the wind turbines as really big trees.

The pattern identified by the authors suggests that it might be possible to manage wind farms to reduce bat fatalities. For instance, since bats are less likely to approach when the wind is higher, the authors suggest that stopping the turbines during periods of low wind might reduce the chances of a fatal run-in between bats and blades. Whether it makes much economic sense to do so, however, isn't addressed.

PNAS, 2014. DOI: 10.1073/pnas.1406672111 (About DOIs).