Investigating the causes of death for wind turbine-associated bat fatalities

Wind turbine-associated bat mortality is occurring at unanticipated rates, yet our understanding of the causes of these fatalities is limited. The prominent proximate causes of bat deaths at wind turbines are direct collision (i.e., blunt-force trauma) and barotrauma. The objectives of this study were to use veterinary diagnostic procedures to determine the lesions associated with bats killed by wind turbines and investigate relationships between patterns of injuries and proximate causes of death. A majority of the bats (74%; 29 of 39) examined by radiology had bone fractures; most of these fractures were in the wings and none was in the hind limbs. Visual inspection resulted in 33% fewer detected bone fractures when compared with radiology results. Bats dropped from a turbine nacelle (91.44 m) to determine extent and type(s) of bone fracture did not show signs of significant bone damage. Approximately one-half (52%; 12 of 23) of bats whose ears were examined had mild to severe hemorrhaging in the middle or inner ears (or both). None of the bats found during this study had any pre-existing disease. It is difficult to attribute individual fatalities exclusively to either direct collision or barotrauma. Gross necropsy, histopathology, and radiology complement each others' deficiencies and together give the best insight into cause of death. Delayed lethal effects after nonlethal contact with wind turbines are poorly understood and difficult to quantify by mortality searches alone but can result in underestimating bat mortality caused by wind energy facilities.