A forensic investigation into the etiology of bat mortality at a wind farm: barotrauma or traumatic injury?

Migrating bats have increased mortality near moving turbine blades at wind farms. The authors evaluated competing hypotheses of barotrauma and traumatic injury to determine the cause. They first examined the utility of lungs from salvaged bat carcasses for histopathologic diagnosis of barotrauma and studied laboratory mice as a model system. Postmortem time, environmental temperature, and freezing of carcasses all affected the development of vascular congestion, hemorrhage, and edema. These common tissue artifacts mimicked the diagnostic criteria of pulmonary barotrauma; therefore, lung tissues from salvaged bats should not be used for barotrauma diagnosis. The authors next compared wind farm (WF) bats to building collision (BC) bats collected near downtown Chicago buildings. WF bats had an increased incidence in fracture cases and specific bone fractures and had more external lacerations than BC bats. WF bats had additional features of traumatic injury, including diaphragmatic hernia, subcutaneous hemorrhage, and bone marrow emboli. In summary, 73% (190 of 262) of WF bats had lesions consistent with traumatic injury. The authors then examined for ruptured tympana, a sensitive marker of barotrauma in humans. BC bats had only 1 case (2%, 1 of 42), but this was attributed to concurrent cranial fractures, whereas WF bats had a 20% (16 of 81) incidence. When cases with concurrent traumatic injury were excluded, this yielded a small fraction (6%, 5 of 81) of WF bats with lesions possibly consistent with barotrauma etiology. Forensic pathology examination of the data strongly suggests that traumatic injury is the major cause of bat mortality at wind farms and, at best, barotrauma is a minor etiology.