Large numbers of bats are being killed at utility-scale wind energy facilities, especially along forested ridge tops in the eastern United States.  These fatalities raise important concerns about cumulative impacts of proposed wind energy development on bat populations. We implemented the first U.S.-based experiment on the effectiveness of changing turbine cut-in speed on reducing bat fatality at wind turbines at the Casselman Wind Project in Somerset County, Pennsylvania.  Our objectives were to 1) determine differences in bat fatalities at turbines with different cut-in-speeds relative to fully operational turbines, and 2) determine economic costs under different curtailment prescriptions and timeframes.  We employed three treatments at each turbine with four replicates (n = 12 turbines) on each night of the experiment: 1) fully operational, 2) cut-in speed at 5.0 m/s (C5 turbines), and 3) cut-in speed at 6.5 m/s (C6 turbines).  We used a completely randomized design and treatments were randomly assigned to turbines each night of the experiment, with the night when treatments were applied being the experimental unit.  We conducted daily searches at the 12 turbines from late July to mid-October in 2008 and 2009.  

Results/Conclusions

There was strong evidence that the estimated number of bat fatalities over 25 nights differed among turbine treatments during both years of the study.  We found no difference between the number of fatalities for C5 and C6 turbines during both years.  Total fatalities at fully operational turbines were estimated to be 5.4 times greater on average than at curtailed turbines (C5 and C6 combined; χ₁² = 14.11, p = 0.0005, 95% CI: 2.08–14.11) in 2008 and 3.6 times greater on average than at curtailed turbines (C5 and C6 combined; χ₁² = 12.93, p = 0.0003, 95% CI: 1.79–7.26) in 2009.  In other words, 82% (95% CI:  52–93%) of all fatalities at curtailment turbines likely occurred when the turbines were fully operational in 2008 and 72% (95% CI:  44–86%) in 2009.  The lost power output was only 0.3 to 1% of total annual output.  While more studies are needed to test changes in turbine cut-in speed among different sizes and types of turbines, wind regimes, and habitat conditions, we believe changing cut-in speeds to the levels we tested offers an effective mitigation strategy for reducing bat fatalities at wind facilities.  Given the magnitude and extent of bat fatalities worldwide, the conservation implications of our findings are critically important.