A study co-authored by University of Hawaiʻi at Hilo scientist Marcos Gorresen shows tree-roosting bats, or “tree bats” may mistake wind turbines for trees and approach the spinning blades at their peril. The study, which took place in Indiana and was led by U.S. Geological Survey scientist Paul Cryan, was the first to use video surveillance cameras to watch bats for several months flying at night near wind turbines manipulated to watch the effects. The study, titled “Behavior of bats at wind turbines,” was published in the Proceedings of the National Academy of Sciences.
Gorresen is with the Hawaiʻi Cooperative Studies Unit, a research program between the Pacific Island Ecosystems Research Center of the U.S. Geological Survey Biological Resources Discipline and UH Hilo.
“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,” says Gorresen of the study’s findings. “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-lived, slow-breeding mammals that serve as the main predators of night flying insects, such as moths and beetles. Insect-eating bats are estimated to save farmers billions of dollars each year in the United States by providing natural pest control.
Historically, fatal collisions of bats and tall, human-made structures were rarely observed, but something changed with the construction of large, industrial wind turbines.
It is now estimated that tens to hundreds of thousands of bats die each year after interacting with the moving blades of wind turbines. Most tree bats are found dead beneath turbines in late summer and autumn. Reasons for this seasonal susceptibility remain a mystery—unknown behaviors of bats may play a role.
Understanding bat behavior
“If we can understand why bats approach wind turbines, we may be able to turn them away,” says lead author 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.”
As well as using “thermal” cameras to study the bats, the researchers also used near-infrared security cameras, radar and machines that record the ultrasonic calls of bats, to catch over 900 bat sightings.
The findings show bats typically approach turbines one or more times rather than just flying past, and sometimes fly very close to the machinery boxes at the top of the monopoles. They also approach stationary or slow-moving blades.
The bats also appeared at turbines more often during brightly moonlit nights.
The authors concluded from these patterns that bats might follow airflow paths around tree-like structures and use visual cues at night, but may not be able to tell a tree from a wind turbine with slow or stopped blades.
The authors say their study shows bats may be more likely to approach turbines when turbines have airflow patterns resembling trees, but then can’t outmaneuver quickly enough if the wind speed rapidly increases and pushes turbine blades to faster speeds.
A current method of reducing bat fatalities at wind turbines is to increase the wind speed threshold at which turbine blades begin operating and spinning fast.
“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,” says Cris Hein, an author of the study and scientist with Bat Conservation International.
The findings also suggest that pointing monitoring or deterrent devices into the downwind airspace of a turbine might have better chances of detecting or keeping bats away than if they are pointed elsewhere.
The authors conclude that increasing understanding of the ways that bats perceive and approach wind turbines helps in the search for solutions to reduce the effects of this important energy source on bat populations.
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