Acoustic exposure reveals variation in curtailment effectiveness at reducing bat fatality at wind turbines

As the global transition to renewable energy generation continues, so does the need to reduce wind turbine-related bat mortality. Curtailing turbine operation to prevent rotor movement at low wind speeds not only lowers risk but also decreases renewable energy production. Adjusting curtailment criteria according to seasonal patterns in bat activity could reduce energy loss, but determining whether the resulting curtailment alternative sufficiently lowered risk to bats would require a more sensitive measure of bat mortality than carcass counts can provide. We deployed turbine-mounted acoustic bat detectors at two wind energy facilities to (1) explore seasonal and spatial variation in bat activity in and near the rotor-swept zone of turbines, (2) confirm the efficacy of acoustic exposure to turbine operation as a measure of bat fatality risk, and (3) evaluate seasonal variation in reduction in acoustic exposure among curtailment alternatives with varying cut-in wind speeds. Biweekly distribution of acoustic bat activity was similar among facilities, and acoustic exposure to rotating turbine blades was closely correlated with bat fatality estimates, corroborating previous studies. Curtailment strategies with higher cut-in speeds reduced the percentage of acoustic exposure by a consistent margin across biweekly intervals, but differences in the rate of acoustic exposure among strategies were far greater during late summer and early fall, when bat activity levels were highest. In other words, the relative protectiveness of curtailment strategies did not vary greatly throughout the year, but the choice of curtailment strategy during periods of high bat activity could substantially affect bat fatality rates. Small changes in cut-in speed (e.g., 0.5 m/s) resulted in clear reductions in acoustic exposure that were measurable at biweekly intervals, providing sensitive feedback on curtailment effectiveness. Site-specific data from turbine-mounted acoustic detectors could therefore provide more sensitive feedback on curtailment effectiveness than carcass searches, which cannot typically detect differences in fatality rates among curtailment strategies with similar cut-in speeds. Acoustic exposure also provides useful practical feedback for wind energy facility operators on how best to design curtailment strategies around site-specific patterns in bat activity and balance the simultaneous goals of generating renewable energy and protecting bats.