Tree-roosting bats or tree bats mistake wind turbines for trees and end up losing their lives when they come in contact with the spinning blades, according to a study led by the University of Hawaii.

For the study, the researchers used video surveillance thermal cameras to observe bats for several months flying at night near three experimentally manipulated wind turbines in Indiana. The researchers also used infrared security cameras, radar, and machines to record the ultrasonic calls of bats and catch over 900 bat sightings.

The researchers found that bats approach stationary or slow-moving blades one or more times instead of just flying by. The slow-breeding mammals sometimes fly very close to the machinery boxes at top of the monopoles.

"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," said Marcos Gorresen, a scientist with the University of Hawaii at Hilo and an author of the study, in a statement. "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."

Bats are long-living creatures that serve as the main predators of night flying insects, like moths and beetles. Insect-eating bats save farmers billions of dollars every year by providing natural pest control.

The construction of large, industrial wind turbines has caused several fatal collisions between bats and tall, human-made structures. It is estimated that tens to hundreds of thousands of bats perish each year after approaching the moving blades of wind turbines. Most tree bats are found dead beneath turbines in late summer and autumn.

"If we can understand why bats approach wind turbines, we may be able to turn them away," said lead author and U.S. Geological Survey scientist Paul Cryan. "Advances in technology helped us overcome the difficulties of watching small bats flying in the dark around the 40-story heights of wind turbines. The new behaviors we saw are useful clues in the quest to know how bats perceive wind turbines and why they approach them."

The finding is published in the Proceedings of the National Academy of Sciences.