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Interviewee: Erin Baerwald, University of Calgary
Bats Get the Bends
With their amazing flying and hunting abilities, bats are major predators of insect pests. But surprising numbers of bats are being killed at wind energy sites. Biologist Erin Baerwald just couldn’t believe that these adept, radar-equipped flyers were simply flying into the blades. Now her research has proven her right.
“We always correct that way of thinking, saying, ‘No, no, the turbines are colliding with the bats,’” Baerwald says. “But this has really changed the way we think about it, in that the bats aren’t colliding with the turbines, the turbines aren’t colliding with the bats. It’s actually an undetectable hazard.”
Baerwald, a PhD student at the University of Calgary in Alberta, Canada, working in the lab of Robert Barclay, discovered that undetectable hazard by collecting freshly killed bats around wind turbines over periods of several weeks. She and her colleagues were surprised that most had no external injuries.
“They didn’t look like anything had happened to them. They were laying on the ground with no broken wings, no other sorts of injuries you would expect if they were struck by these large blades,” she says.
But bat autopsies revealed that more than 90 percent had lung hemorrhages consistent with “the bends.” Baerwald also sent bat carcasses to a veterinary pathologist, Genevieve D’Amours. “She examined bat lungs under the microscope and she found the same things we had seen,” says Baerwald. “The blood vessels in the lungs had actually burst and the lungs were filling full of fluid, filling full of blood. And all those are injuries consistent with barotraumas, or with flying through an area where the pressure drops dramatically. What happens then is, it makes the lungs overexpand and causes breaks in the small blood vessels around the lungs, and essentially bats are dying from drowning.”
“This is very similar to what happens to deep sea divers who come up too quickly,” she says.
As the researchers wrote in the journal Current Biology, air pressure drops around turbine blades are small but sudden, and research on small mammals with lungs similar to bats have shown it’s enough to be fatal.
Baerwald explains that while there had been some speculation that bats were getting the bends, previous studies of bat deaths at wind energy sites “didn’t have the large sample that we did and didn’t do daily carcass searches… they weren’t out there every day finding the really fresh bats so they could then look at the damage to the lungs.”
Bat biologist Sharon Swartz of Brown University is impressed by the finding. “The bat fatalities at wind farms
have been so difficult to understand, and this makes sense of it in a very straightforward way,” she wrote in an email. “Wonderful scientific problem-solving!”
Swartz, who with aeronautics expert Kenny Breuer studies bat flight mechanics using special wind tunnels, explained that the bats in their experiments don’t experience pressure changes like those at the end of the blades of wind turbines. “In a wind tunnel, we recreate conditions that mimic what happens when bats fly–we just shift the movement of the bat relative to the air… the relative speed of air to bat stays the same. At the fast-moving ends of the turbine blades, the wind speeds are much, much higher.”
Birds and Bats
Baerwald explains that “bird lungs are very different from bat lungs,” which she compares to balloons. “Bird lungs are more rigid and more tube like and they have continual air flow. That makes them less susceptible to their lungs over expanding,” she says.
Much more is also known about migratory bird populations, and the various impacts of hazards to birds. While wind energy has emerged as a comparably small threat to birds, Baerwald points out that researchers have been able to identify measures the wind industry can take to protect birds, especially important endangered birds like large raptors.
“The older turbines with the lattice towers, or the ladder-like towers, the big birds of prey like bald eagles and golden eagles like to perch on them and they make great perching habitat and nesting habitats for birds, and then they were struck by the blades,” says Baerwald. That tower design has been replaced, she says. ”So [now] it’s a solid tower, like a mast and there’s no more places for these birds to nest or perch. So they kind of fixed that.”
But Baerwald says there was no way for wind energy companies to anticipate this hazard to bats.
“This was a complete surprise,” she says. “I don’t think there’s anything anyone could have done. And there’s nothing we can really do on the ground, either. If you find a bat that has been injired but not fatallly killed by the decompression, they die very shortly after, even with lots of tender loving care,”
But now that the mystery is solved, she hopes the industry will work on ways to make this form of green energy even greener.
Bats prefer lower wind speeds because it’s easier for them to fly, Baerwald says. “We know that there are more bats active in low wind speeds, and we know that wind turbines don’t create that much electricity in low wind speeds. So what we have started to do is experiment with shutting turbines off in low wind speeds to see if that effects the number of bats killed. All of the initial studies are very promising that if you shut turbines off in low wind speeds during fall migration when the majority of these fatalities occur, you can actually reduce bat fatalities.”
“We’re still experimenting with which wind speeds are optimal and how many fatalities” can be prevented, she says. She also hopes wind engineers can figure out ways to reduce the pressure drop while still operating efficiently.
Baerwald says she is a big fan of wind energy, as well as a passionate bat aficionado. She’s now doing much-needed research on bat populations that can help address other unknowns, like what ecological impact the wind farm fatalities are having.
“We don’t know what the population sizes of bats are. They’re migratory bats, just a few species that are being killed, and not at every wind energy site,” she says. “So we’re having a hard time getting a handle on what the impact is.”
But she says saving bats, which face other threats such as emerging diseases, is a worthy global cause because of their important role in ecosystems. “They’re one of the only animals that fly at night and eat flying insects like moths. Some of the moths are really important agricultural pests like gypsy moths and spruce budworm that bats eat large numbers of. Because these bats are migratory, and they migrate from Canada potentially to Mexico, what happens to bats in Canada could have implications for ecosytems in Mexico or the southern United States,” she says. “So it really is a bigger issue than some bats dying at turbines in Alberta.”
This research was published in Current Biology, August 25, 2008, and funded by: Natural Sciences and Engineering Research Council of Canada, the University of Calgary, TransAlta Wind, Enmax, Suncor, Alberta Wind Energy Corp., Shell Canada, Bat Conservation International, the North American Bat Conservation Partnership, and the Alberta Conservation Association.
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