By Richard Black
Environment correspondent, BBC News website
The rotating blade produce a pressure drop that can harm bats
Bats are at risk from wind turbines, researchers have found, because the rotating blades produce a change in air pressure that can kill the mammals.
Canadian scientists examined bats found dead at a wind farm, and concluded that most had internal injuries consistent with sudden loss of air pressure.
Bats use echo-location to avoid hitting the blades but cannot detect the sharp pressure changes around the turbine.
The scientists say wind farms are more of an issue for bats than for birds.
"An atmospheric pressure drop at wind turbine blades is an undetectable - and potentially unforseeable - hazard for bats, thus partially explaining the large number of bat fatalities at these specific structures," said Erin Baerwald, who led the research team at the University of Calgary.
Bat deaths around wind farms have been widely documented across Europe and North America.
Two years ago, EU nations formally agreed to make developers aware of the risks, and find ways of monitoring bat migration routes.
Earlier this year, a bid to build a wind farm near Bideford in north Devon was turned down because of the potential impact on the mammals.
Research is underway to find ways of scaring bats from wind farms
But among all this, understanding of how turbines affect bats has been lacking.
The Calgary team collected carcasses of hoary and silver-haired bats killed at a wind farm in south-western Alberta.
Examinations showed that fewer than half had external injuries that could have been caused by collision.
But about 90% had internal haemorrhaging, most notably in the chest cavity, a condition that puts pressure on the lung and can be fatal.
The idea is that the pressure around a rotating turbine blade is lower than in the surrounding air. A bat flying into the low-pressure zone finds its lungs suddenly expanding, bursting capillaries in the surrounding tissue which then becomes flooded with blood.
Birds, which have more rigid and robust lungs, do not undergo the same trauma from a sudden drop in pressure.
"Given that bats are far more susceptible to barotrauma than birds, and that bat fatalities at wind turbines far outnumber bird fatalities at most sites, wildlife fatalities at wind turbines are now a bat issue, not a bird issue," said Ms Baerwald.
Some research groups are investigating ways to keep bats away from wind farms, and a University of Aberdeen group recently suggested radar emissions might act as a "bat-scarer".
The new research is reported in the journal Current Biology.