For years researchers have been puzzled by the number of bats killed by wind turbines. Birds, yes. But bats, in theory, should be able to avoid the towers because of their innate sonar systems that orient them in space. Nonetheless, they die in the thousands, in far greater numbers than birds. Some research found that they died because the enormous changes in pressure as the blades sweep through the air ruptured their delicate ear drums, causing a hemorrhage known as barotrauma. Now, a new study based on bat autopsies from the University of Wisconsin found that the problem is far more complicated.
X-ray of a bat killed by a wind turbine at a southeastern Wisconsin wind energy facility during fall 2009. Note the compound fracture of the right arm and the dislocation of the right shoulder joint.
The scientists found 41 dead bats over three months beneath 29 turbines in Wisconsin. They conducted autopsies, including X-rays, on the animals to determine precisely why they died.
"Half the bats had trauma to the inner ear -- their ear drums blew out," said David Drake, one of the researchers on the study published this week in the journal of Mammology. And three fourths had broken bones, primarily wings, he said.
Why does it matter? Because knowing why they die might help in designing towers and blades that are not quite so lethal, he said. For example, changing the shape of a blade could reduce the pressure gradient enough to prevent trauma to their ear canals, he said. Maybe changing the height of the tower would help avoid fatal collisions.
But the science is in its infancy, Drake said. in addition, not much is known about bats, much less about how they behave around wind towers or why they get close to them at all.
In the Midwest, migrating tree roosting bats are the ones that are killed most often. Drake said one theory is that they look for the tallest thing on the landscape -- often a wind turbine -- in which to roost and mate. There are also studies that correlate bat deaths with low wind, perhaps because that's the best time for bats to feed. "They are so focused on feeding that they don’t pay attention to the blades until it's too late, Drake said. Even so, the bats need 20 meters to sense to a moving object. But the blades of a turbine move so fast they have only a quarter second to move out of the way.
Drake said no one has a good estimate of how many bats are killed by wind turbines. But now it matters more than ever because bats are being decimated by a disease called white nose syndrome that destroys whole colonies. The disease is common in east, and is steadily moving west. So the total impact on bats is worrisome.
"With white nose syndrome coming, all bets are off," he said. "When they are suffering 100 percent mortality from white nose, then added mortality from wind farms, it's hard to tell."