Wind Turbines Give Bats the "Bends," Study Finds

Brian Handwerk
for National Geographic News
August 25, 2008
Wind turbines can kill bats without touching them by causing a bends-like condition due to rapidly dropping air pressure, new research suggests.

Scientists aren't sure why, but bats are attracted to the turbines, which often stand 300 feet (90 meters) high and sport 200-foot (60-meter) blades.

The mammals' curiosity can result in lethal blows by the rotors, which spin at a rate of about 160 miles (260 kilometers) per hour.

But scientist Erin Baerwald and colleagues report that only about half of the bat corpses they found near Alberta, Canada, turbine bases showed any physical evidence of being hit by a blade.

A surprising 90 percent showed signs of internal hemorrhaging—evidence of a drop in air pressure near the blades that causes fatal damage to the bats' lungs with a condition called barotrauma.

In humans, the condition is related to the bends and can affect divers and airplane passengers during ascents and descents.

(Related story: "Military Sonar May Give Whales the Bends, Study Says" [October 1, 2003])

The "Bends"

"As a turbine blade goes around, it creates lift—like an airplane's wings—and there is a small zone of [dropping] pressure, maybe a meter or so in diameter, on the tips of the blades," explained Baerwald, a doctoral candidate at the University of Calgary, in Alberta.

"Bats fly through this area, and their lungs expand, and the fine capillaries around the edges of the lungs burst."

The bats' lungs subsequently fill with fluid, and the animals essentially drown.

"We compare it to divers—they are pretty much dying of the bends," Baerwald said.

Bats have no natural defense against the unnaturally dramatic pressure changes.

"Bats can actually detect pressure changes, but we're talking large-scale, relatively slow changes, like the coming of a storm front," said Baerwald. "This is something entirely different."

Most bats that fall victim to turbines are migrating species, such as hoary bats, eastern red bats, and silver-haired bats.

There are not enough data to determine how wind turbine fatalities might be affecting populations of these slow-reproducing mammals.

Birds are also killed by blows from wind turbine rotors (see a related story), but their rigid, tubelike lungs can better withstand air pressure changes.

The study appears this week in the journal Current Biology.

Curiosity Killed the Bat

"They are the first to have done a large scale look at this barotrauma," Bat Conservation International (BCI) biologist Ed Arnett said of the researchers.

"It's fascinating information," said Arnett, who is not involved with the study.

"But ultimately it might not matter so much how [the bats] die but what is attracting them to the turbines in the first place."

Preventing the bat deaths has challenged experts for years.

"We've partnered with industry and federal agencies to raise and spend about two million dollars looking for a solution," said BCI founder and president Merlin Tuttle.

Laurie Jodziewicz, of the American Wind Energy Association in Washington, D.C., said where the turbines are placed may be the key.

"Bats are not being [killed] at all the wind projects all over the country—it is happening in some places and not others," she said.

"We're trying to determine before construction what areas might be risky."

Turbines create drops in pressure drop during normal operations, so the problem could possibly be addressed by changing when the turbines run, according to BCI's Tuttle.

"A large portion of the kills occur at the lowest wind speeds," he said, "and at those low speeds [the turbines] are not generating appreciable electricity anyway."

Bats also are at particular risk during migration periods in late summer and early fall, when many turbine related fatalities occur.

Arnett, Baerwald, and others are currently conducting tests to see if raising the "cut-in" wind speed at which rotors begin to turn will save bats—particularly during peak migration periods.

"It won't eliminate the problem, but it's a good step in the right direction," Tuttle said.

NOTE: About 90 percent of the bats studied suffered from barotrauma. The name of the ailment was restored to this article for clarification purposes after initial publication.

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