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.