Bats who evolved to roost in trees suffer peak fatalities when it’s less windy during the late summer and in the fall. Using thermal cameras, researchers may have finally figured out what puts these high-flying nocturnal critters at risk: Air currents surrounding the turbines mimic those that surround the tall trees where they like to roost. The findings, published in Proceedings of the National Academy of Sciences this week, offers ideas for adjusting turbines to cut down risks to tree bats.
The blades of industrial wind turbines are estimated to kill up to hundreds of thousands of bats a year. Tree bats found dead beneath turbines suggest that there’s a seasonal susceptibility, but researchers are unsure about what bat behaviors factor into these fatalities. "If we can understand why bats approach wind turbines, we may be able to turn them away," Paul Cryan from the U.S. Geological Survey says in a news release.
Cryan and colleagues investigated whether wind speeds and turbine blade rotation speeds alter bat behavior. They set up thermal surveillance cameras, near-infrared video to monitor nighttime airspace, acoustic detectors to record the ultrasonic calls of bats, and radar to monitor bat behavior at three experimentally manipulated wind turbine sites in Indiana from July to October of 2012. In this time, they observed bats on 993 occasions.
More tree-roosting bats were seen near turbines during periods of low wind than during high-wind conditions. Bats approached turbines from high above the ground and from a downwind direction more frequently with increasing wind speeds -- but only when the blades were turning slower than usual. However, when turbine blades turned freely at full speed, the bats approached less frequently from a downwind direction with increasing wind speeds.
The results suggest that bats orient toward turbines by sensing airflow paths, and they might not be able to perceive the difference between a tree and a wind turbine with slowed or stopped blades. Tree bats, in particular, may be more attracted to turbines because air currents around turbines resemble those around tall trees that harbor clouds of insect prey on their downwind sides or provide sheltered roosting sites.
"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," explains study coauthor Marcos Gorresen from the University of Hawaii, 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."
In this video from a temperature-imaging camera, you can see a bat interacting with a wind turbine at about 3 a.m. on a brightly moonlit night in late August. One way to reduce bat fatalities could be to increase the wind speed threshold where blades begin to operate and spin fast, and to also somehow use this method to account for sporadic gusts of wind during low-wind periods when bats are hanging around.