Researchers filmed bats flying at night and found they interact with wind turbines in predictable ways.
Bats act as a natural pest control for farmers by gobbling up tons of insects, but many of these flying mammals are found dead beneath wind turbines during the spring and summer, the U.S. Geological Survey reported.
"If we can understand why bats approach wind turbines, we may be able to turn them away," said Paul Cryan, a USGS research scientist and the study's lead author. "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."
Researchers used thermal cameras to monitor bats around three wind turbines in Indiana. They also employed near-infrared security cameras, radar and machines that record the ultrasonic calls of bats to detect over 900 sightings over the several month period.
The team found bats typically approach the turbines multiple times, rather than just flying past, and sometimes flew very close to the monopoles, nacelles (machinery boxes at top of monopoles) or even the blades if they were stationary or slow moving. They determined the bats most often approached the turbines from the downwind side when the wind was blowing. They also approached the turbines more often on brightly moonlit nights. These findings suggest that since bats fly along the airflow paths around tree-like structures and use visual cues at night, they may not be able to distinguish trees from turbines with stationary or slow-moving blades.
"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, an author of the study and scientist with the University of Hawaii at Hilo. "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."
Researchers believe bats may mistake the turbines if they give off a similar airflow pattern as trees, but if the wind rapidly increases it could cause the blades to suddenly turn at a speed to quick for the bats to outmaneuver them.
A common method for reducing bat fatalities by increasing the wind speed threshold at which turbine blades start spinning faster.
"It might be possible to efficiently further reduce fatalities with this method by accounting for sporadic gusts of wind during low-wind periods when bats might be hanging around turbines," said Cris Hein, an author of the study and scientist with Bat Conservation International.
More information on the study is available at USGSFort Collins Science Center, Bat Conservation International and Bats and Wind Energy Cooperative.
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