Ultrasonic deterrents have limited effects on reducing bat fatalities at wind facilities

Bats frequently collide with spinning wind turbine blades, especially during the fall migratory period, where hundreds of thousands of bats are estimated to occur at wind facilities annually. The two main strategies to reduce bat fatalities at wind farms are curtailment (stopping turbines during high-risk periods) and deterrents (discouraging bat activity around turbines). Research from this study by Clerc and colleagues suggests that ultrasonic deterrents had little effect on bats as a whole and may have actually increased mortality in eastern red bats. Due to the inconsistent results among species groups, researchers advise caution when using ultrasonic deterrents to reduce bat mortality, particularly in species for which the effects have not been studied.

Importance of reducing bat fatalities at wind facilities

Bat fatalities at wind facilities have the potential to impact populations, such as hoary bats, that are particularly susceptible to collisions. The most popular curtailment approach is to stop turbine operation during low wind speeds, when bat activity is typically higher. Stopping blades minimizes the likelihood of a bat collision. The most studied deterrents involve ultrasonic noises emitted from the wind turbine to deter bats from flying in proximity to the blades.

In past studies, curtailment has been shown to be effective in reducing bat fatalities by an average of 62% when turbines are not spinning at wind speeds of 5 m/s or less; thus, wind-only curtailment is currently the most widely accepted approach to curtailment. Despite the success of curtailment in reducing bat fatalities, there is a loss of energy generation; thus, there is interest in developing deterrents that reduce bat fatalities while minimizing or eliminating energy production loss. Furthermore, using curtailment and deterrents together may provide additional fatality reduction compared to using either method alone.

How was the study done?

Researchers Clerc and colleagues set out to study the effectiveness of curtailment and deterrents by comparing the number of fatalities found under turbines that used no treatment, curtailment alone, ultrasonic deterrents alone, or a combination of the two. During the study period, each of the four treatments was randomly assigned nightly to 4 of the 16 total turbines used for the study. The researchers searched for carcasses underneath all 16 turbines every morning during the study period and used models to understand the effectiveness of curtailment and deterrents on bat mortality.

What did the study find?

The study showed mixed results for the effectiveness of deterrents on bat mortality. When researchers examined all bat species combined and low-frequency calling species (hoary bats, silver-haired bats, and big brown bats), there was some evidence that ultrasonic deterrents reduced mortality; however, the relationship was equivocal and not as strong as expected. Conversely, researchers were surprised that ultrasonic deterrents increased eastern red bat mortality by a factor of two compared to when the ultrasonic deterrents were not active.

What do the results mean?

With a lack of definitive evidence regarding the effectiveness of ultrasonic deterrents, this approach is unlikely to be more effective than curtailment for either of the species groups examined in this study, especially for eastern red bats. Previous studies have found that ultrasonic deterrents have been effective in natural settings; however, the same results were not observed in this study at a wind facility. Researchers speculate that these differences may be due to the deterrents not fully covering the large wind turbines or because bat behavior differs around wind turbines compared to other habitats.

Regardless of the mechanism behind the differences in responses to ultrasonic deterrents among bat species groups, the researchers suggest caution with their use, especially with species where the effects have not been studied. This caution is especially heightened given the significant increase in eastern red bat fatalities that occurred when ultrasonic detectors were used.

References

Clerc, J., M. Huso, M. Schirmacher, M. Whitby, and C. Hein. 2025. Ultrasonic deterrents provide no additional benefit over curtailment in reducing bat fatalities at an Ohio wind energy facility. PLOS ONE 20:e0318451.

Ultrasonic deterrents provide no additional benefit over curtailment in reducing bat fatalities at an Ohio wind energy facility

Wind energy is important for achieving net-zero greenhouse gas emissions but also contributes to global bat mortality. Current strategies to minimize bat mortality due to collision with wind-turbine blades fall broadly into two categories: curtailment (limiting turbine operation during high-risk periods) and deterrence (discouraging bat activity near turbines). Recently, there has been interest in combining these strategies to achieve greater reductions in bat fatalities than either strategy might achieve in isolation. To investigate the effectiveness of combining curtailment with ultrasonic deterrent minimization strategies, we deployed six ultrasonic deterrents at nacelle height on 16 experimental turbines at Avangrid Renewables’ Blue Creek Wind Energy Facility. We rotated between four conditions (normal operations, curtailment only, deterrent only, curtailment and deterrent) randomly assigned to four wind turbines each night between 15 June and 3 October 2017. We found that bat mortality at wind turbines was independent of wind speed. The effectiveness of ultrasonic acoustic deterrents varied between high-frequency-calling species (eastern red bats) and low-frequency-calling species (hoary bats, silver-haired bats, and big brown bats). When deterrents were active, mortality was twice as high for eastern red bats compared to the control. Conversely, deterrents had a weak dampening effect on bat mortality for low-frequency species. We found no additive effects on mortality reduction for turbines operating both curtailment and deterrents compared to either approach in isolation. Our findings suggest that ultrasonic acoustic deterrents may not be effective for both high and low frequency echolocating bats. The increase in fatalities of eastern red bats is alarming and underscores the importance of considering site- and species-specific effects of minimization solutions.

PLOS OneJeff Clerc ,

Friedenberg, N. A., and W. F. Frick. 2021. Assessing fatality minimization for hoary bats amid continued wind energy development. Biological Conservation 262:109309. https://www.sciencedirect.com/science/article/abs/pii/S000632072100361X?via%3Dihub

Gilmour LRV, Holderied MW, Pickering SPC, Jones G. Comparing acoustic and radar deterrence methods as mitigation measures to reduce human-bat impacts and conservation conflicts. PLoS One. 2020;15(2):e0228668.

Comparing acoustic and radar deterrence methods as mitigation measures to reduce human-bat impacts and conservation conflicts

Where humans and wildlife co-exist, mitigation is often needed to alleviate potential conflicts and impacts. Deterrence methods can be used to reduce impacts of human structures or activities on wildlife, or to resolve conservation conflicts in areas where animals may be regarded as a nuisance or pose a health hazard. Here we test two methods (acoustic and radar) that have shown potential for deterring bats away from areas where they forage and/or roost. Using both infrared video and acoustic methods for counting bat passes, we show that ultrasonic speakers were effective as bat deterrents at foraging sites, but radar was not. Ultrasonic deterrents decreased overall bat activity (filmed on infrared cameras) by ~80% when deployed alone and in combination with radar. However, radar alone had no effect on bat activity when video or acoustic data were analysed using generalised linear mixed effect models. Feeding buzzes of all species were reduced by 79% and 69% in the ultrasound only treatment when compared to the control and radar treatments, but only the ultrasound treatment was significant in post-hoc tests. Species responded differently to the ultrasound treatments and we recorded a deterrent effect on both Pipistrellus pipistrellus (~40–80% reduction in activity) and P. pygmaeus (~30–60% reduction), but not on Myotis species. However, only the ultrasound and radar treatment was significant (when compared to control and radar) in post-hoc tests for P. pipistrellus. Deterrent treatment was marginally non-significant for P. pygmaeus, but the ultrasound only treatment was significant when compared to radar in post-hoc tests. We therefore suggest that acoustic, but not radar methods are explored further as deterrents for bats. The use of acoustic deterrence should always be assessed on a case-by-case basis, with a focus on bat conservation.

PLOS OneLia R. V. Gilmour ,