This study was designed to test the hypothesis that the newer KVS-33 turbines with a large blade diameter (33 m) would potentially kill more birds than the older KCS-56 (18.5 m blade diameter) turbines because the blades sweep more area. The ratio of the rotor swept area (RSA) of the KVS-33 to that of the KCS-56 is 3.46:1. Based on this hypothesis, it was predicted that a KVS-33 turbine would be three times as likely to cause an avian collision as a KCS-56. The study was designed so that within study sites the combined RSAs were equal between KVS-33 and KCS-56 sample turbines: in the Altamont Pass sample there were 36 of the larger KVS-33 turbines and 130 of the smaller KCS-56 turbines, and in the Montezuma Hills sample there were 17 of the larger and 59 of the smaller turbines. The number of sample turbines in each group at each site approximated the 3.46:1 RSA ratio. Bird deaths between the small and large turbine types were compared within study sites. A total of 70 bird deaths, 45 of which were raptors, were identified during the 13-month sampling period in Altamont Pass. In Montezuma Hills, surveyors found 13 dead birds, 12 of which were raptors, in five months of sampling. In Altamont Pass, avian mortality per turbine was roughly the same at both small and large turbines; the turbines appeared to present an obstacle to birds regardless of RSA. The evidence from Altamont Pass, the author states, does not support the hypothesis that the larger RSA of the KVS-33 turbines contributes to higher mortality. In the Montezuma Hills, however, avian mortality per turbine at the larger KVS-33 turbines was three times higher than at the smaller KCS-56 turbines, but the author states that this may be due to the smaller sample size and short duration of the Montezuma study.