At a wind farm in Alberta, researchers noticed a disturbing trend – the turbines harvesting the wind sweeping across the Canadian prairie were also killing hundreds of migratory bats.
Dead silver-haired and hoary bats piled up beneath the turbines. Necropsies indicated bleeding in the lungs.
Since they were first reported in the late 1990s, bat deaths at wind farms have baffled researchers. At some East Coast projects, turbines kill thousands of bats each year. The death toll is lower at Western wind farms, though no one knows why they occur or why death rates vary by region, said Paul Cryan, a research biologist with the U.S. Geological Survey in Fort Collins, Colo.
In Canada and the United States, wind turbines are believed to kill an estimated 450,000 bats each year. While bird kills at wind farms get more publicity, bat deaths appear to outpace them.
“Prior to their carcasses showing up underneath wind turbines, we never saw this many dead on the ground before,” said Cryan, who has been studying bats for more than 20 years. “We’re still kind of in a state of shock.”
Unlike bird deaths at wind farms, which encompass a range of avian species, most of the bats killed by wind turbines are hoary, silver-haired or eastern red bats.
All three species are migratory bats that roost in trees. The majority of the wind farm deaths occur during a July-through-September migration period for the bats, which coincides with their mating season.
One hypothesis is that the bats mistake the turbines for supersized trees, Cryan said. Bats are social animals, congregating around larger trees, which provide more hollows for roosting and potentially more mates.
“It brings up the disturbing possibility that if you put something up in a grassland that looks like a tree, it might be an inviting structure for bats,” he said.
Some thermal-imaging footage from nighttime video shows bats chasing wind turbine blades. Cryan said researchers don’t know how bats perceive the blades, whose tips move at speeds of up to 180 mph.
A bat’s eyesight is geared toward detecting objects on the horizon. Echolocation – the high-pitched sound waves that allow bats to detect objects – works within 50 to 100 feet. The blades bearing down on them might appear as points of sound to the bats, Cryan said.
The carcasses show injuries consistent with being struck by a blade. A number also had bleeding in the lungs, which scientists believe is caused by rapid changes in air pressure around the blade, Cryan said.
Bats provide natural pest control, collectively consuming millions of tons of insects destructive to agricultural crops and native forests. Researchers don’t know yet how bat deaths from turbines impact overall bat populations.
“It would be a tragedy to lose any of them,” Cryan said, “given their importance to the biodiversity of North America.”