Ecology student Erin Baerwald holds a hoary bat she found injured in Pincher Creek's turbine fields. ((CBC))

Hundreds of bats found dead each year around wind turbines have suffered internal trauma from a sudden drop in air pressure at the turbine blades, according to new research to be published this week.

A University of Calgary research team looking for reasons behind the fatalities at the Summerview Wind Farm in Pincher Creek, Alta., said their findings, to be published Tuesday in the journal Current Biology, may finally answer the question of why bats appear particularly vulnerable to turbines.

University of Calgary graduate student Erin Baerwald said in a statement that while bats can detect turbines through their sonar-like echolocation ability, the same ability offers no protection from pressure drops.

"An atmospheric-pressure drop at wind-turbine blades is an undetectable — and potentially unforeseeable — hazard for bats, thus partially explaining the large number of bat fatalities at these specific structures," she said.

The condition, known as barotrauma, affects bats more than birds because bat lungs are balloon-like and can over-expand, bursting surrounding capillaries. Bird lungs are more rigid and tube-like and better able to withstand sudden changes in air pressure.

The spinning of a wind turbine's blade tends to increase air pressure as the wind comes to the blades, and then lower it dramatically in the blade's wake. Modern wind turbines can turn at speeds of 55 to 80 metres per second, resulting in a pressure drop in the range of five to 10 kilopascals, the researchers note.

The group looked at 188 bats killed at turbines in southwestern Alberta and found 87 had no external injuries.

Of the 75 recently killed bats on which they were able to conduct more detailed post-mortem tests, 69 had internal hemorrhaging but only 32 had external injuries. Detailed dissections of some of the bats revealed a host of internal traumas, from air-filled bubbles visible on the lung surface to lesions on the lungs consistent with barotrauma.

The study was funded by industry, government and conservation organizations to address the question of why bats in particular were dying, which has been one of the most perplexing issues around the establishment of wind farms across North America.

The Canadian Wind Energy Association says Canada's current installed capacity is 1,876 megawatts, although wind farms tend to run at about 32 per cent of listed capacity. Alberta leads all provinces with an installed capacity of 523.97 MW in 2008.