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Turbine‐related bat mortality at commercial wind energy facilities may threaten populations of migratory tree‐roosting bat species in North America. Industry stakeholders and regulatory agencies alike are investigating strategies to reduce risk of population‐level consequences as the wind energy industry grows. Bats collide with turbines only when turbine rotors are spinning and curtailing turbine operation at low wind speeds can effectively reduce bat fatality rates. Nonetheless, few quantitative data exist to determine appropriate threshold wind speeds below which turbine operations should be curtailed. Carcass monitoring is labor‐intensive and does not provide information on factors linked to bat fatality rates on any scale finer than nightly. We tested whether acoustic bat data recorded at turbine nacelles could provide a more precise and sensitive measure of fatality risk to bats by analyzing acoustics, weather, turbine operation, and carcass data collected at 2 commercial wind energy facilities in West Virginia over 7 years. Each wind facility implemented several distinct curtailment treatments during our study, allowing us to compare fatality rates and acoustic bat activity across multiple operational strategies. We found that bat passes exposed to turbine operation explained close to 80% of the variation in carcass‐based estimates of bat fatality rates and accounted for significant variation in raw carcass counts per turbine and probability of finding bat carcasses during individual turbine searches. Conversely, bat activity occurring when turbines were not operating had little or no relationship to fatality rates. We also found that patterns in bat activity exposure could be predicted accurately among turbines and years. Our results demonstrate that measuring exposure of acoustic bat activity provides a quantitative basis for designing, evaluating, and adaptively managing curtailment strategies. This is an important advance towards using curtailment to reduce bat fatality rates strategically while allowing for increased generation of renewable energy. © 2021 The Authors. Wildlife Society Bulletin published by Wiley Periodicals LLC on behalf of The Wildlife Society.

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We wish to thank the Massachusetts Technology Collaborative (MTC) and PPM Energy for funding this study, and the numerous donors to BCI, member companies of AWEA, and the National Fish and Wildlife Foundation, for additional support. Kristen Goland, formerly with MTC and now with PPM Energy, was instrumental in pulling support together for this study. We also thank PPM Energy employees Andy Linehan and Scott McDonald for their support and efforts. Jack Waggert assisted with installation of detector systems on meteorological towers and helped with many logistical aspects of the study. We thank members of the BWEC Scientific Advisory Committee and the technical advisory committee for their review of this report. 2 EXECUTIVE SUMMARYWe initiated a multi-year pre-and post-construction study in mid-summer 2006 to determine patterns of bat activity and evaluate the use of acoustic monitoring to predict mortality of bats at a proposed wind energy facility in northwest Massachusetts. The primary objectives of this study are to 1) determine level and patterns of activity of different species groups of bats using the proposed wind facility prior to and after construction of turbines; 2) correlate bat activity with weather and other environmental variables; and 3) combine results from this study with those from similar efforts to determine if indices of pre-construction bat activity can be used to predict post-construction bat fatalities at proposed wind facilities. Here we report results from the first year of pre-construction data collection.We recorded echolocation calls of bats with Anabat II zero-crossing ultrasonic detectors programmed to record calls from 1900 to 0700 hr each day of the study from 26 July to 20 December 2006. We used meteorological (met) towers to vertically array detectors for acoustic sampling during this study (one detector at 10, 31, and 39 m above ground level (AGL) at each tower). We recorded a total of 4,816 bat calls from all detectors at all met tower locations combined from 26 July through 11 November, an average of 8.9 calls per tower per night. No bat calls were recorded between 11 November and 20 December 2006. Bat activity generally was highest immediately after sunset and declined through the night until just before sunrise the following morning. High (>35 kHz, mostly Myotis species and red bats [Lasiurus borealis]) and low (<35 kHz, mostly hoary bats [Lasiurus cinereus] and big brown bats [Eptesicus fuscus])frequency-emitting echolocating bats tended to fly at different heights on the study area. We estimated that activity rates for bats using high frequencies were 1.5-4.0 times higher than those using low frequencies at low altitudes (10 m). This trend was reversed at medium altitudes (31 m) where it was estimated that activity rates for bats using low frequencies were 5.75-22.9 times greater than those for high frequency users, and at 39 m AGL it was estimated that activity rates for bats using low frequencies were 11.2-38.8 times greater than those for high frequency users.The best...