Bats may mistake wind turbines for open sky, causing deadly collisions

Study may help explain why giant blades kill millions of the animals every year

Wind turbines get a bad rap for killing birds—but they actually kill far more bats. Scientists estimate that millions of bats die every year after slamming into the giant blades, making turbines one of the top killers of the animals worldwide. But just what exactly lures bats to turbines in the first place is a mystery.

New laboratory experiments suggest the key is light. Bats use the brightness of the open sky to navigate—a visual cue replicated by light reflecting off turbine blades. Much like a moth drawn to the flame, these reflections create an “ecological trap,” drawing bats into fatal collisions, researchers report this month in Biology Letters.

“It’s one of those studies that doesn’t get done very often,” says Jack Hooker, a bat biologist at the Bat Conservation Trust who was not involved with the work. Unlike many large-scale studies on bat deaths around turbines, he says the new work focuses on a specific possible cause and tests it with rigorous experiments. Understanding why bats gravitate toward these machines could help researchers find better ways to protect them, he says.

Bats seem to be unusually attracted to turbines. They loiter in the air next to the giant energy-generating machines and spend an inordinate amount of time near their masts and blades.

The attraction is unclear, but what is known is that bats have evolved to use the open sky as a visual guide while navigating. Brighter patches in their blurry field of vision indicate the direction of the sky, and they orient toward them. As a result, Kristin Jonasson—an independent physiological ecologist—hypothesized that at dusk and dawn, turbine blades might reflect just enough moonlight to make them look like the bright sky, luring bats toward them.

To test the hypothesis, Jonasson and colleagues at the University of Colorado Colorado Springs, National Renewable Energy Laboratory, and U.S. Forest Service collected hoary (Lasiurus cinereus) and silver-haired (Lasionycteris noctivagans) bats—two of the biggest victims of wind turbines in North America—from the wild. Back in the lab, they released the animals into a dark maze with two exits. One exit was partially blocked by a white turbine blade reflecting artificial moonlight, whereas the other was unobstructed. Nearly three-fourths of the hoary bats and all but one of the 31 silver-haired bats flew toward the white blade.

“Their echolocation would be telling them there’s a completely open exit, and they still went towards the blocked exit,” Jonasson says. The results, she says, suggest the bats were using their vision rather than their echolocation to find a way out. In the wild, the animals could theoretically use echolocation to avoid the blades, but the loud noise of the turbines might interfere, she says.

Case closed? Not quite, says Christian Voigt, a biologist at the Leibniz Institute for Zoo and Wildlife Research who wasn’t involved with the work. Both bat species used in the study roost on trees and haven’t evolved to fly in tunnels, he notes, making the experiment a poor approximation of their behavior in the wild. The animals may also have been stressed by human handling, he says.

Instead of mistaking the glowing blades for open sky, Voigt says, the wind turbulence and noise caused by the giant blades may be messing up their navigation. The animals might also be mistaking the giant masts for large trees where they can roost. Hooker adds, “There’s also evidence that they’re attracted to the insects that aggregate around [turbines].”Understanding just what attracts bats to turbines will be critical to protecting them,” Hooker says. Current efforts to reduce mortality at turbines involve either using ultrasound noise to jam bat echolocation, making them avoid the space around turbines, or turning turbines off during peak bat activity. Jonasson says the former approach has had mixed results, and the wind energy industry isn’t a fan of the latter.

An alternative might be changing the color of the turbine blades so they reflect less moonlight, Jonasson says. Black wouldn’t work, as such blades are less visible to aircraft and absorb more heat, she says, potentially damaging the turbines. “If we can find colors that are less attractive [to the animals], perhaps you could have both wind energy development and safe bats.”