Autopsies of 188 bats, killed at a Canadian wind farm, reveal more than 90 per cent had no external injuries from blade collisions but died from ruptured blood vessels in their lungs.

University of Calgary ecologist Erin Baerwald said, ''What we found is a lot of internal hemorrhaging.''

Dr Baerwald's examination of hoary and silver-haired bats at the Summerview wind farm in south-western Alberta discovered four times as many bats were killed than birds.

This initially baffled the research team, because bats routinely use sonar navigation to detect, and avoid, fast-moving objects in their flight path.

University of Melbourne bat expert Belinda Appleton said the Canadian study confirmed long-standing concerns about the lack of scientific monitoring of bat kills at wind farms across Australia.

''Bat deaths at wind farms go unreported and no research is being done on numbers and frequency, so we don't have any data to give a clear idea of the extent of the problem,'' she said.

Evidence suggests wind farms in two areas of Victoria have contributed to a decline in colonies of southern bent-wing bats. The bats were previously listed as threatened, but their status was recently upgraded to critically endangered after a sharp decline in numbers.

In a paper published online, Dr Baerwald outlines evidence that bats are killed by decompression, or pulmonary barotrauma, caused by rapid air pressure reduction near the moving turbine blades.

As turbine blades cut through the air, they produce zones of low air pressure around the tips of the blades. These pressure drops range from five to 10 kilopascals (a unit measuring pressure), which is higher than the level shown to kill laboratory rats.

Birds' lungs are more rigid and their capillaries are stronger, allowing them to withstand the impact of a sudden drop in air pressure.

''If the bats have a lungful of air as they fly through the air pressure change, there's nowhere for the air to go. The small blood vessels around the lungs burst and fill the lungs with fluid and blood,'' Dr Baerwald said.

A spokeswoman for the Clean Energy Council an umbrella group that includes the Australian Wind Energy Association said most wind farms had introduced best practice guidelines to protect native wildlife.

''I cannot imagine them riding roughshod over birds, bats or people when it comes to protecting the environment,'' she said.