The bats of America are in dire straits. In the Eastern U.S., hundreds of thousands of hibernating bats have died from the mysterious fungal affliction known as white nose syndrome. To make matters worse, tree bats are getting whacked by wind turbines.
Bats live up to 30 years and have one of the lowest reproductive rates among mammals: sometimes just one pup per year. This means the death toll from wind turbines could be debilitating. It's unlike anything researchers have seen before in terms of a man-made structure killing bats, says Fort Collins USGS bat expert Paul Cryan. "This came at us completely out of the blue."
Concern about wind turbines' "lawnmower effect" used to be focused on birds. But now it looks like bats are suffering the most. One reason is the difference between bat and bird morphology. Bats, like other mammals, have delicate alveoli in their lungs. The thin membranes can't withstand extreme pressure changes, and suffer from "barotrauma," like scuba divers' ears and lungs when they go deep under water without equalizing properly. Ninety percent of the bats that researchers at the University of Calgary found under turbines had died of internal hemorrhaging, not from collision with the blades. In fact, only fifty percent of the bats showed signs of impact.
Birds, in contrast, have evolved tougher structures in their lungs for gas exchange, making them more tolerant of the dramatic pressure changes created by spinning turbines. That's one explanation for why more bats are dying from wind turbines than birds.
But it doesn't explain seasonal peaks in the bat deaths, which spike in late summer and fall. Nor does it shed light on why taller wind turbines seem to cause higher fatalities.
Paul Cryan has been studying the most affected species, the hoary bat, for over 10 years now. He suspects that the fatalities are related to tree bats' roosting and mating behaviors. In late summer and autumn, migratory tree bats tend to seek the highest trees, and male bats prove their prowess by defending their chosen territory. There are other possibilities, though. Bats could be trying to take advantage of the high wind "corridors" the wind turbines were built to harness. They could be finding more to eat near the turbines or be drawn to the noises they make.
Whether the doomed bats are tilting at windmills to win a mate, or getting disoriented and flying into them by accident, there's one clue that could significantly reduce casualties. Most occur on mellow nights when winds are low. According to Cryan, when the turbines are shut off on those nights when the wind is between 4 and 6 meters per second, death rates drop by 50-80%. If this is true, we may be able to give bats a break during their migration periods without losing too much renewable power.
It's hard to say how wind turbine casualties will affect bat populations as a whole, because we don't actually know how many bats are out there. There could be tens of millions, or just tens of thousands, of the secretive creatures roosting in our forests at night. Bats seem familiar and comforting as they wipe out the mosquitoes in our backyards on summer evenings, occasionally getting trapped in our garages and requiring direction with a broom. But in reality we know precious little about them -- where they go when they migrate for the winter, or what we'd do without them if they did not come back.