Abstract
Bird and bat fatality estimates based on scientific monitoring are used to assess and compare impacts among wind energy projects. Fatality estimates are influenced by multiple factors, including variation in methodology. Variation in the interval between fatality searches exemplifies a monitoring decision that can potentially confound comparison of fatality estimates. A study at the Sand Hill portion of the Altamont Pass Wind Resource Area, California, USA, provided the first opportunity to compare fatality-rate estimates derived from 2 independent, experienced monitoring teams searching the same wind turbines at 2 different periodic intervals. Over 30 months of monitoring the same wind turbines, April 2012–October 2014, searches averaging 5-day intervals detected 308 additional fatalities (of 431 fatalities total) representing 20 additional species (of 32 species total) compared with the searches averaging 39-day intervals. Body mass explained most of the variation in discrepant fatality detections between the 2 search intervals, with the 39-day interval searches detecting only 10% of the bats and birds of 10–40g that were found by the 5-day interval searches. The 39-day search interval produced an estimate of annual bird fatalities/MW that was 39% lower than the estimate produced from the 5-day search interval. The 39-day search interval also resulted in many more species identification errors and greater errors in estimated time since death. The average search interval used in fatality monitoring strongly influences fatality estimates; long intervals can contribute to false impressions that wind projects have small or negligible effects on small birds and bats.