Lack of sufficient information about avian mortality from collisions with wind turbines led to this study. Although most prior studies concluded losses represented small fractions of populations, it has become apparent that some species, such as golden eagles, are more susceptible to impacts from wind turbines. The average activity of raptors observed in the Altamont Pass was lower annually than expected from reports by bird watchers such as Richmond (1985). Activity rates were lower than rates observed in the Montezuma Hills, Solano County (Howell et al. 1988).
Over the 12 months of this study 359 turbines were sampled for bird mortality, and 42 birds and 21 mammal were recovered. Site differences between Dyer and Midway contributed some of the more interesting results of this study. Based on nest counts, Dyer had almost 3 times as many nests as Midway, which had significantly higher mortality. Of six young birds banded, none were recovered as turbine strikes. A prairie falcon was recovered in Billings, Montana several months after banding.
Midway and Dyer had locations with multiple strikes. Inspection of topographic maps and the distribution of turbine strings indicated that multiple strikes tended to occur at swales, that is depressions, and at shoulders of hills, that is where ridge lines have a stairstep effect. From this study and U.S. Windpower data from the two study sites 28 possible strikes occurred at the ends of strings and 56 occurred within the interior of strings.