An estimated 5 to 80 million birds die annually in the United States by colliding with manmade objects (Banks 1979, Avery et al. 1980). Although generally considered environmentally friendly, windpower, at most locations, has been associated with low levels of avian fatalities caused by collisions with turbines and other windplant structures (e.g., Orloff 1992, Johnson et al. 1999a, Erickson et al. 2000). At a few specific locations, such as the Altamont Pass Wind Resource Area (WRA) in California, there have been higher levels of avian fatalities (Orloff and Flannery 1992). In comparison to TV/radio and other communications towers, the number of bird mortalities in wind generation facilities is relatively small (AWEA 1995). TV/radio towers often result in episodic mortality events that may number thousands of birds when inclement weather occurs during migration periods (Avery et al. 1980, Trapp 1998). Studies conducted to date
indicate that raptors and passerines are the most susceptible to turbine collisions in the U.S. (AWEA 1995).
Early wind energy facilities in the U.S., such as those in the Altamont Pass, were placed without regard to level of avian use, and some of these sites are located where birds are abundant and the risk of turbine collisions is high (AWEA 1995). As a result, extensive mortality has been reported at those facilities. In the Altamont Pass area near Livermore, California, where more than 7,000 turbines exist within the WRA, an estimated 567 raptors were killed over a 2-year period from colliding with turbines (Orloff and Flannery 1992). Researchers estimated 6,800 birds, primarily passerines, were killed annually at the San Gorgonio, California wind facility based on 40 dead birds found while monitoring nocturnal migrants. The 40 dead birds were comprised of 15 passerines, seven waterfowl, two shorebirds, and one raptor. Because most of these birds were passerines and large numbers of passerines migrate through this area, it was concluded that this level of mortality was not biologically significant (Southern California Edison Company, unpubl. data). Studies conducted on other wind generation facilities have shown that these levels of mortality do not routinely occur (e.g., Johnson et al. 1999a, Erickson et al. 2000), and numerous factors including avian abundance, species composition, presence of migration corridors, geographic area, landscape features, prey abundance, and wind plant features influence the potential for avian mortality (Nelson and Curry 1995, Orloff 1992).