Effectiveness of Cut-in Speed Adjustments on Reducing Impacts to Mexican Free-tailed Bats at a Wind Energy Facility in Nevada

Direct impacts to bats at wind energy development projects are one of the biggest issues facing the wind industry; however, much of the research on reducing bat mortality has largely focused on migratory tree-roosting (e.g.,... [ view full abstract ]

Direct impacts to bats at wind energy development projects are one of the biggest issues facing the wind industry; however, much of the research on reducing bat mortality has largely focused on migratory tree-roosting (e.g., hoary bat [Lasiurus cinereus]) and listed bat species (e.g., Indiana bat [Myotis sodalis]) in the eastern US, where adjustments to turbine cut-in speeds have shown to be an effective tool for reducing impacts to these focal species. In general, direct impacts to bats have not been as great in the western US; therefore research on mitigation and minimization opportunities for bats have been less common in western parts of the country. The Mexican free-tailed bat (Tadarida brasiliensis) is among the most commonly found bat fatalities at some western wind energy facilities, whereas this species does not typically occur in the eastern US. In this presentation, we present the results of experimental studies conducted in the fall of 2012 and 2013 at a wind energy project in eastern Nevada that investigated the effects of increased turbine cut-in speeds on reducing bat mortality, with an emphasis on the strong-flying Mexican free-tailed bat. This project is located near a migratory stopover site/cave used by large numbers of migrating Mexican free-tailed bats from late spring through the fall and Mexican free-tailed bats account for a majority of the bat mortality at the Project. Forty of the project’s 66 turbines (60%) were searched daily for bat fatalities over a 64 day period each fall, with turbines randomly assigned to one of five cut-in speeds each night (one control and four treatments). Cut-in speed adjustments ranged from 4 m/sec to 6 m/sec during the two experimental periods, and included both partial- and all-night treatments. The studies demonstrated that increased cut-in speeds could significantly reduce bat mortality rates at the project. Based on the results of the experimental studies and other site-specific bat mortality monitoring data, the project implemented varied levels of cut-in speed adjustments during the summer and fall bat activity periods over a 3-year mortality monitoring study period from 2013 through 2015. Implementation of cut-in speed adjustments resulted in an estimated 83% reduction in bat mortality from year 1 to year 3 of the monitoring period, demonstrating how cut-in speed adjustments can be effectively used to reduce mortality of the common bat fatalities found at many western US projects.