Dead bats are turning up beneath wind turbines all over the world. Bat fatalities have now been documented at nearly every wind facility in North America where adequate surveys for bats have been conducted, and several of these sites are estimated to cause the deaths of thousands of bats per year. This unanticipated and unprecedented problem for bats has moved to the forefront of conservation and management efforts directed toward this poorly understood group of mammals. The mystery of why bats die at turbine sites remains unsolved. Is it a simple case of flying in the wrong place at the wrong time? Are bats attracted to the spinning turbine blades? Why are so many bats colliding with turbines compared to their infrequent crashes with other tall, human-made structures?
Although these questions remain unanswered, potential clues can be found in the patterns of fatalities. Foremost, the majority of bats killed by wind turbines are species that rely on trees as roosts throughout the year and migrate long distances; we call these species “migratory tree bats.†Currently, migratory tree bats compose more than three quarters of the bat fatalities observed at wind energy sites. The other striking pattern is that the vast majority of bat fatalities at wind turbines occur during late summer and autumn. This seasonal peak in fatalities coincides with periods of both autumn migration and mating behavior of tree bats. Seasonal involvement of species with shared behaviors indicates that behavior plays a key role in the susceptibility of bats to wind turbines, and that migratory tree bats might actually be attracted to wind turbines.
Despite patterns indicating that bat fatalities at turbines are somehow associated with migration and mating behavior, and that attraction may occur, these hypotheses have not been tested. At present, it is unclear whether bats killed by turbines are local residents, migrants moving through the area, bats actively mating, or some combination of these things. Further complicating matters is the fact that some of the species that die in smaller numbers at wind turbines are not known to migrate, although migratory habits of many bat populations are unknown and some of these species may actually do so. Addressing these important topics is crucial to future efforts aimed at mitigating the impact of wind turbines on bat populations. Knowing whether most of the bats killed by wind turbines are actively migrating, mating, or attracted to turbines will make it easier to direct effective mitigation efforts and assess risk before turbines are built.