In the study, scientists raised cut-in speeds on 12 out of 23 turbines at Iberdrola's 34.5MW Casselman Wind Project in Pennsylvania.
The researchers found no statistical difference between cut-in speeds of 5 and 6.5 metres per second (from a base of about 4 m/s).
Ed Arnett of BWEC and Bat Conservation International said that if the curtailment were applied to the entire farm, it would result in a 0.3% loss in annual output using the 5 m/s cut-in speed, and a 1% loss in annual output using the 6.5 m/s cut-in speed.
Arnett also revealed the preliminary results of a second BWEC study with Iberdrola, this time using devices that produce ultrasonic waves.
The study at Pennsylvania's Locust Ridge wind farm found that these deterrent devices have a much weaker effect than curtailment.
The deterrents reduced bat kills by 20% to 53%, and cost about $20,000 a turbine. Arnett warned that the researchers have not developed a device ready for commercial deployment.
Engineers are designing a fifth-generation prototype that BCI will use for further tests at Locust Ridge this summer.