The mystery that had biologists crawling under wind turbines in southern Alberta was a simple contradiction: dead, unmarked bats. A bird found under a turbine often looks like what happened to it. Broken wings, blunt force trauma, the signature of a collision with the tip of a blade moving at highway speeds or faster. But at wind farms across North America, as bat deaths during fall migration piled up, searchers continued to find animals that seemed to have simply fallen from the sky. This was doubly strange because bats, unlike birds, carry sonar. An echolocating bat detects moving objects better than stationary ones. Of all the animals, the one that should never hit a rotating blade was the one that died under them in the greatest numbers. Opening up the evidence A team at the University of Calgary led by Erin Baerwald decided to stop guessing and start dissecting. At a wind farm in the prairies of southwestern Alberta, on a major bat migration route, teams collected 188 dead bats overnight, mostly hoary and silver-haired bats on their fall journey south. Of those 188, 87 did not have any external injuries that could explain their death. No broken wings or lacerations. Almost half of the victims of a giant spinning machine were left unharmed. The team then performed necropsies on 75 of the freshest corpses in the camp, and the pattern reversed. Only 32 of the 75 had obvious external injuries. But 69 of them, nine out of ten, had bleeding inside the chest or abdomen: blood that had leaked out of the vessels and pooled in the body cavity. Under the microscope, lung tissue told the same story: Each of the 17 bats examined histologically showed lung lesions. Most of these animals had not been hit. Something had broken them from within. Killed by air The answer, published in Current Biology in 2008, is called barotrauma, and its versions are well known to divers and pilots. It is tissue damage caused by a sudden change in pressure, and the lungs, being air bags, take the brunt of it. The blade of a wind turbine is an aerodynamic profile, like a wing, and when it cuts through the air it drags a much lower pressure zone behind it. The team calculated drops of 5 to 10 kilopascals in the leaf trail, a range known, based on lab work, to be lethal to small mammals. A bat flying into that invisible pocket experiences the air in its lungs abruptly expanding faster than it can exhale. The delicate capillaries around the air sacs rupture, blood floods the lungs and chest, and the animal dies without even touching the machine that killed it. Birds do better in the same air for an anatomical reason: Their lungs are rigid, reinforced structures, while a bat’s are large, elastic and balloon-like, excellent for powered flight and catastrophically vulnerable to decompression. As Baerwald points out, a pressure drop in the blades is an undetectable danger. Bat sonar, so good at mapping solid objects, has nothing to say about a region of air. The fine print, honestly said, science has argued about proportions ever since, as it should. Later studies, including work re-examining corpses with more forensic methods, concluded that direct blade strikes cause more deaths than the 2008 paper implied, as an indirect impact can also cause internal bleeding, and some researchers now consider collision to be the primary cause of death and barotrauma to be a contributing or secondary factor. Computational studies of blade wakes have found that the strongest pressure drops are limited to small regions near the blade tips, suggesting that a bat must pass nearby to be at risk. What no one disputes is the main discovery: a large proportion of bats killed by turbines die with fatal internal injuries and no external injuries, the pressure field around a moving blade is dangerous in itself, and the kill zone of a turbine is larger than the machine. It is not necessary for the blade to touch you. He just needs to miss you closely. Why it matters beyond the necropsy table Wind turbines now kill hundreds of thousands of bats a year in North America, most of them migratory species already under pressure, and bats reproduce slowly, typically one brood per year, so the losses are compounded. Alberta’s work reshaped how the industry responds. If bats die near the blades and not just on them, the solution is not to mulch but to avoid them, and the most effective tool found so far is almost embarrassingly simple: increasing the wind speed at which the turbines begin to spin during migration nights. Bats prefer to fly in light winds, and studies, including one by the same Calgary group, found that idling blades in light winds reduces bat deaths by half or more, sacrificing a small portion of the annual generation. The image that persists in the study is that of the silentest type of victim. A bat soaring through the dark prairie sky, sonar reading clear air ahead, passing behind a blade it successfully avoided, into a pocket of nothingness. Standards Space Daily articles are edited and fact-checked before publication. We use artificial intelligence tools in the newsroom. See our editorial standards and masthead.
