Large numbers of migratory bats are killed by wind turbines across North America and there are increasing concerns that this level of anthropogenic mortality may threaten populations. Although the ultimate causes of bat fatalities at wind farms are not well understood, several lines of evidence suggest that bats are attracted to wind turbines. Some of the attraction hypotheses suggest that the wind turbines provide one or more resources for bats (e.g., foraging or roosting opportunities) or that bats misperceive the wind turbines to provide a resource (e.g., echolocating bats misidentify the smooth tower surfaces to be water). These various attractors are not mutually exclusive and their relative importance likely varies by species, time of year, and geographic location. A growing body of evidence, including observations of bats at wind turbines, analyses of bat carcasses and their stomach contents, and acoustic surveys at wind turbines, has revealed new insights into bat behavior at wind turbines. For example, infrared and night vision imagery show bats exploring and contacting the monopoles, nacelles, and blades of wind turbines, especially at low wind speeds. These flight patterns are suggestive of aerial pursuits and gleaning prey items, roost investigation, mating behavior, and drinking behavior by bats.
Given the increasing demand for wind energy and evidence that bats are attracted to wind turbines, the need to develop cost-effective and practical mitigation strategies is urgent. Current mitigation strategies include siting restrictions and operational minimization. Siting restrictions limit where wind facilities can be developed or delineate setbacks from important bat habitats such as commuting routes and maternity roosts. Operational minimization restricts wind turbine blade rotation on low wind speed nights and has been shown to reduce bat fatality rates by at least 50% where implemented. Although these strategies can avoid or minimize bat fatalities, they also limit wind energy production. Because of concerns about power loss and the economic feasibility of implementing operational minimization in low wind regions, recent research efforts are focused on technological solutions that allow wind turbines to operate normally. One such solution is to broadcast ultrasonic sound from wind turbines to deter echolocating bats from approaching and entering the rotor swept zone. Preliminary results are promising and several studies are underway to improve this technology. Other possible solutions include using ultra-violet light as a deterrent and the development of turbine surface materials that reduce the attractiveness of wind turbine towers to bats.