Curtailment and acoustic deterrents reduce bat mortality at wind farms.

The impacts of wind energy on bat populations is a growing concern because wind turbine blades can strike and kill bats, and wind turbine development is increasing. We tested the effectiveness of 2 management actions at 2 wind‐energy facilities for reducing bat fatalities: curtailing turbine operation when wind speeds were <5.0 m/second and combining curtailment with an acoustic bat deterrent developed by NRG Systems. We measured the effectiveness of the management actions using differences in counts of bat carcasses quantified by daily and twice‐per‐week standardized carcass searches of cleared plots below turbines, and field trials that estimated searcher efficiency and carcass persistence. We studied turbines located at 2 adjacent wind‐energy facilities in northeast Illinois, USA, during fall migration (1 Aug–15 Oct) in 2018. We estimated the effectiveness of each management action using a generalized linear mixed‐effects model with several covariates. Curtailment alone reduced overall bat mortality by 42.5% but did not reduce silver‐haired bat (Lasionycteris noctivagans) mortality. Overall bat fatality rates were 66.9% lower at curtailed turbines with acoustic deterrents compared to turbines that operated at manufacturer cut‐in speed. Curtailment and the deterrent reduced bat mortality to varying degrees between species, ranging from 58.1% for eastern red bats (Lasiurus borealis) to 94.4 for big brown bats (Eptesicus fuscus). Hoary (Lasiurus cinereus) and silver‐haired bat mortality was reduced by 71.4% and 71.6%, respectively. Our study lacked a deterrent‐only treatment group because of the expense of acoustic deterrents. We estimated the additional reduction in mortality with concurrent deployment of the acoustic deterrent and curtailment under the assumption that curtailment and the acoustic deterrent would have reduced mortality by the same percentage at adjacent wind‐energy facilities. Acoustic deterrents resulted in 31.6%, 17.4%, and 66.7% additional reductions of bat mortality compared to curtailment alone for eastern red bat, hoary bat, and silver‐haired bat, respectively. The effectiveness of acoustic deterrents for reducing bat mortality at turbines with rotor‐swept area diameters >110 m is unknown because high frequency sound attenuates quickly, which reduces coverage of rotor‐swept areas. Management actions should consider species differences in the ability of curtailment and deterrents to reduce bat mortality and increase energy production. [ABSTRACT FROM AUTHOR]