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Beware the Bends

While white-nose syndrome poses a risk to many species of cave-dwelling bats in the Northeast, migratory tree bats across the nation face a different type of threat: wind turbines.

photo of a modern wind turbine

Unlike their hibernating cousins, migratory tree bats – which include the hoary bat, the silver-haired bat, and the eastern red bat – do not inhabit caves. Instead they roost in trees and seasonally travel long distances following warmer weather, much like birds. Over the past several years, migratory bats have been found dead by the hundreds and even the thousands under wind turbines during their peak mating and migrating seasons. According to the US Geological Survey, migratory tree bats comprise approximately three-quarters of the bat fatalities observed at wind turbine sites.

“Bats cannot sustain these impacts without it affecting their ability to recover,” says Ed Arnett, the director of Science and Policy at Bat Conservation International.

However, many of these mortalities are not due to direct impact with a turning blade. Rather, it appears that flying in close proximity to wind turbines can, under certain wind conditions, cause the bats to succumb to “barotrauma.”

Also known as “the bends,” barotrauma occurs when a sudden drop in air pressure near moving turbine blades causes lung hemorrhage in bats, similar to what is experienced by scuba divers who ascend or descend too quickly during deep-sea exploration. Scientists became aware of barotrauma’s role in bat fatalities in 2008, when necropsies showed telltale damage to the lungs of bats found dead beneath turbines.

Risk of bat fatality due to barotrauma is especially high during low-wind conditions. This has led scientists to hypothesize that raising the “cut-in speed” of the turbines – that is, the wind speed at which the turbines turn on – would help lower the risk to bats. Studies testing this theory have yielded promising results.

Arnett’s team conducted studies at Iberdrola Renewables’ Casselman Wind Farm in Somerset County, Pennsylvania, in which the turbines were shut down at the site during low wind conditions. The experiment resulted in an annual reduction of bat fatalities by a range of 44 to 93 percent, while causing a negligible power loss of less than 1 percent.

In addition to shutting off wind turbines during low wind condition or times of peak bat activity (summer and early fall), Arnett has also been experimenting with the use of ultrasound devices. When mounted on turbines, these devices discourage bats from approaching them, resulting in an 18 to 60 percent reduction in mortalities. The technology is still in the testing stage and is unavailable for commercial use.

Numbers Game

They’re reclusive by day and highly mobile by night. So how do we go about counting bats, let alone assessing how many have died due to a mysterious disease?

It’s not easy, says Carthage College professor Deanne Byrnes, recently named chair of the board of directors of the North American Society for Bat Research. To identify bats in the wild, scientists rely on two primary methods: acoustic monitoring, and trapping and marking. Both suffer from shortcomings that leave serious gaps in information; one is too broad in scope, the other too narrow. In the end, Byrnes said, we have no accurate numbers on how many bats exist in the country. “Our estimates have huge error bars around them.… It’s a problem. It’s a big problem.”

The problem isn’t unique to bats. When it comes to surveying wildlife populations, biologists are often making no more than educated guesses. And the implications of those guesses can be immense, influencing important conservation and development decisions.

There are no regulations mandating the use of mitigation measures at wind energy sites in the United States to reduce impacts on bat populations. “Unfortunately, the use of [mitigation] measures at most wind energy sites is still voluntary and it seems like a majority of wind farms are still apt not to use them,” Arnett says.

Arnett expects that as bat populations continue to plummet from WNS, more species will receive federal protection under the Endangered Species Act, and that the protection-measures that come from the listing will trickle down to non-listed bat species.

One example of this is Beach Ridge Wind Farm in Greenbrier County, West Virginia, owned by Invenergy Renewables, which shuts down its turbines every evening from sunset to sunrise from the first of April until mid-November as the result of a settlement agreement brought by environmental groups to enforce greater protections of the federally endangered Indiana bat.

The federal government is considering endangered status for three cave-dwelling bat species that have suffered drastic population declines from WNS: the northern myotis, the eastern small-footed myotis, and the little brown bat.

“I do think that if the listings for these [cave-dwelling bat species] are approved by the US Fish and Wildlife Service, it will definitely affect … wind energy projects,” says Mollie Matheson, a Conservation Advocate with the Center for Biological Diversity.

Since tree bats are migratory and travel under the cover of night, it has been difficult to take a reliable census of their populations or migration routes. However, Arnett believes that as bat biologists refine their methods to better track migratory tree bats, there will be an improved ability to identify their critical habitat and migration corridors. And that might help wind developers in better siting future projects to avoid impacting bats altogether.


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