We implemented a 2-year study to test the effectiveness of an ultrasonic acoustic deterrent for reducing bat fatalities at wind turbines at the Iberdrola Renewables Locust Ridge I and II Wind Farms located in Columbia and Schuylkill Counties, Pennsylvania. We randomly selected a set of control and treatment turbines that were searched daily in summer and fall 2009 and 2010 and estimates of fatality, adjusted for searcher efficiency, carcass persistence, and habitat and area adjustment, were compared between the two sets of turbines.
In the first year (2009), we randomly selected 10 turbines that were fitted with deterrent devices and 15 control turbines and searched each turbine daily for carcasses from 15 August to 10 October 2009. We did not assess inherent differences between sets of turbines in 2009. In 2010, we attempted to account for potential inherent differences between turbine sets and modified the design to reflect a Before-After Control-Impact (BACI) design. The same sets of turbines were monitored for a period of time prior to implementation of the deterrent treatment (1 May to 26 July 2010), then again during the deterrent implementation period (31 July through 9 October 2010). This design allowed for incorporating initial inherent differences between the two experimental treatment sets prior to implementation of the treatment as a reference for interpreting any differences detected during implementation of the treatment.
In 2009, we estimated 60% higher fatality (95% CI: 26%, 104%) per control turbine than per Deterrent turbine, or conversely, we estimated 21–51% fewer bats were killed per Deterrent turbine than per control turbine during this period. Without accounting for inherent differences, we estimated 18–62% fewer bats were killed per Deterrent turbine than per control turbine in 2010. However, there was marginal evidence that the ratio of control:Deterrent fatalities was greater during the treatment period than in the pre-treatment period; about 10% in the fatality rate between the two sets. Thus, when accounting for this inherent difference, between 2% more and 64% fewer bats were killed per Deterrent turbine relative to control turbines in 2010 after accounting for inherent turbine differences prior to treatment implementation.
We also determined species-specific response to deterrents for those species with adequate sample sizes. We estimated that twice as many hoary bats were killed per control turbine than Deterrent turbine, and nearly twice as many silver-haired bats in 2009. In 2010, although we estimated nearly twice as many hoary bats and nearly 4 times as many silver-haired bats killed per control turbine than at Deterrent turbines during the treatment period, these only represented an approximate 20% increase in fatality relative to the pre-treatment period for these species when accounting for inherent differences between turbine sets.
This study, and previous experiments with earlier prototypes, revealed that broadband ultrasound broadcasts may reduce bat fatalities by affect behavior of bats by discouraging them from approaching the sound source. Yet, the effectiveness of ultrasonic deterrents as a means to prevent bat fatalities at wind turbines is limited by the distance and area that ultrasound can be broadcast; ultra sound attenuates quickly and is heavily influenced by humidity. Humid conditions (nightly average of ~80%) contributed to limited affected airspace during our study. Also, we only deployed 8 deterrent devices on each turbine and did not cover the maximum amount of possible airspace bats could encounter. Also, during both years of the study water leakage caused some deterrents to malfunction and not all deterrents were operational at all times during the study period. Thus, we contend that our findings may represent a more conservative estimate of the potential reduction achievable through application of the deterrent we tested. However, we caution that we do not yet have a deterrent device ready for operational deployment at wind facilities. With further experimentation and modifications, this type of deterrent method may prove successful and broadly applicable for protecting bats from harmful encounters with wind turbine blades. We anticipate further research and development of acoustic deterrent devices in 2011 and a new field test of the effectiveness of the new prototype in 2013. Future research and development and field studies should attempt to optimize both placement and number of devices on each turbine that would affect the greatest amount of airspace in the rotor-swept area to estimate potential maximum effectiveness of this tool to reduce bat fatalities. Future efforts also must evaluate the cost-effectiveness of deterrents in relation to different curtailment strategies to allow a cost-benefit analysis for mitigating bat fatalities.