Widespread and often extensive fatalities of bats have increased concern about impacts of wind energy development on bats and other wildlife. Minimizing these fatalities is critically important to both bat conservation and public acceptance of wind-energy development. Currently, only operational mitigation (stopping turbine blades from spinning) during predictable high risk periods has demonstrated effective reductions of fatalities of bats.
We synthesized information from 10 different operational mitigation studies in North America. Most studies found at least a 50% reduction in bat fatalities when turbine cut-in speed (wind speed at which turbines begin producing electricity into the power grid) was increased by 1.5 m/s above the manufacturer’s cut-in speed. Similar reductions in bat fatality were reported by one study that implemented a raised cut-in speed given temperatures were above 9.5°C. One study demonstrated equally beneficial reductions with a low-speed idling approach, while another discovered that feathering turbine blades (pitched 90o and parallel to the wind) at or below the manufacturer’s cut-in speed resulted in up to 72% fewer bats killed when turbines produced no electricity into the power grid. Other studies that did not demonstrate statistically significant effects could be explained by lack of treatments being implemented during the study (i.e., winds were either too low or high to enable comparison of treatments), and the relatively low reduction in bat fatality (approximately 20–38%) at one study may be explained by the high percentage of Brazilian free-tailed bats (Tadarida brasiliensis), a species known to fly at higher wind speeds. Few studies have disclosed actual power loss and economic costs of operational mitigation, but those that have suggest that <1% of total annual output would be lost if operational mitigation were employed during high risk periods for bat fatalities. In addition to the lost power revenues, wind energy companies also incurred costs for staff time to set up the processes and controls, and to implement the curtailment from offsite 24-hour operations centers.
We conclude that increasing cut-in speed between 1.5 and 3.0 m/s or feathering blades and slowing rotor speed up to the turbine manufacturer’s cut-in speed yields substantial reductions in fatality of bats. Given the magnitude and extent of bat fatalities worldwide, the conservation implications of our findings are critically important. Research efforts should continue to focus on incorporating additional variables, in addition to wind speed (e.g., temperature, time of night, bat activity) into treatments and explore using automated systems to maximize wind production while still minimizing bat fatalities. Although additional studies are needed to optimize operational mitigation, we believe increasing cut-in speeds to the levels tested in these studies (generally 1.5–3.0 m/s) offers an ecologically sound and economically feasible strategy for reducing bat fatalities at wind energy facilities and should be implemented broadly.