Bird mortality in the Altamont Pass Wind Resource Area, California

The 165-km(2) Altamont Pass Wind Resource Area (APWRA) in west-central California includes 5,400 wind turbines, each rated to generate between 40 kW and 400 kW of electric power, or 580 MW total. Many birds residing or passing through the area are killed by collisions with these wind turbines, We searched for bird carcasses within 50 m of 4,074 wind turbines for periods ranging from 6 months to 4.5 years. Using mortality estimates adjusted for searcher detection and scavenger removal rates, we estimated the annual wind turbine-caused bird fatalities to number 67 (80% CI = 25-109) golden eagles (Aquila chrysaetos), 188 (80% CI 116-259) red-tailed hawks (Buteo jamaicensis), 349 (80% CI = -49 to 749) American kestrels (Falco sparverius), 440 (80% CI = -133 to 1,013) burrowing owls (Athene cunicularia hypugaea), 1,127 (80% CI = -23 to 2,277) raptors, and 2,710 (80% CI = -6,100 to 11,520) birds. Adjusted mortality estimates were most sensitive to scavenger removal rate, which relates to the amount of time between fatality searches. New on-site studies of scavenger removal rates might warrant revising mortality estimates for some small-bodied bird species, although we cannot predict how the mortality estimates would change. Given the magnitude of our morolity estimates, regulatory agencies and the public should decide whether to enforce laws intended to protect species killed by APWRA wind turbines, and given the imprecision of our estimates, directed research is needed of sources of error and bias for use in studies of bird collisions wherever wind farms are developed. Precision of mortality estimates could be improved by deploying technology to remotely detect collisions and by making wind turbine power output data available to researchers so that the number of fatalities can be related directly to the actual power output of the wind turbine since the last fatality search.