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Every year, hundreds of thousands of bats are killed by flying into the giant blades of wind turbines. It is one of the leading causes of bat mortality in North America and Europe, according to Bat Conservation International. However, the reasons for these fatal collisions are largely unknown. One possibility examined by a new study in Biology Letters is that they may be attracted by light reflected off the blades, much like when moths make a beeline for a flame.

This idea is based on how bats navigate. The flying mammals use the open sky to help them find their way while flying. Kristin Jonasson, an independent physiological ecologist, believes that at dawn and dusk, the giant turbines may reflect just enough light to appear as a clear opening in the sky. This could trick the bats into flying directly toward the spinning blades.

To test this hypothesis, Jonasson and colleagues at the University of Colorado Colorado Springs, National Renewable Energy Laboratory and the U.S. Forestry Service flew 242 hoary bats (Lasiurus cinereus) and 154 silver-haired bats (Lasionycteris noctivagans) through a dark Y-shaped maze with two different exit options.

At the end of one arm, the researchers placed a white section of a real wind turbine blade reflecting artificial moonlight and in the other arm, a less reflective black blade. Bats were twice as likely to fly toward the white blade as the black one. However, when the alternative was an unobstructed exit, almost three-quarters of hoary bats and nearly all of the silver-haired bats flew toward the white turbine blade.

"Our work identifies a link between a sensory pollutant—reflective turbine surfaces—and a risky behavior: approaching these surfaces," wrote the researchers in their paper.

What's next?

The research team acknowledges that more study is needed to fully understand why wind farms are the cause of so many bat fatalities. The experiment was conducted with only two species and in a specially designed maze, rather than in real-world conditions where their behavior might be different. It may even have nothing to do with light. Some scientists believe that the wind turbulence and noise created by the blades could disrupt bat navigation.

Understanding why bats are attracted to turbines is important. As the world is increasingly turning to wind power as a clean and renewable energy source, insights from studies like this could inform future turbine designs or lead to modifications of current structures to protect bat populations.