Bird mortality at wind energy facilities has become the focus of diverse research to understand the causes of collisions and to find methods to reduce and/or eliminate mortality of birds. My study evolved out of 7 years of avian mortality research in the Altamont Pass and the Montezuma Hills, California. In 1992 Kenetech Windpower began installing their new variable speed wind turbine (KVS-33) with a blade diameter of 33 m. This turbine was designed to replace the older smaller KCS-56 turbine with a blade diameter of 18.5 m. The ratio of the area swept by the rotor (RSA) between the KVS-33 and the KCS-56 turbines is 3.18: 1. The U.S. Fish and Wildlife Service put forth the hypothesis that the larger KVS-33 wind turbine would potentially kill more birds by sweeping more area than the KCS-56 turbine. I tested the RSA hypothesis by surveying 53 KVS-33 turbines at 3 locations in the Altamont Pass and Montezuma Hills wind energy fields and compared them to randomly selected KCS-56 turbine strings for an RSA that was equivalent. Observers searched areas adjacent to all turbines twice weekly for 18 months. Eighty-five collision mortalities were confirmed during the time when turbines of both types were in operation. The avian mortality ratio between the RSA equivalent sets of KVS-33 and KCS-56 turbines was 1:3.47. This ratio was significantly different than the even ratio that would be expected from my experimental design. The evidence does not support the hypothesis that the larger rotor-swept-area of the KVS-33 wind turbines resulted in more mortalities.
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