Abstract
Alta Wind X, LLC (Alta Wind X) has constructed a wind energy facility in Kern County, California, referred to as the Alta X Wind Energy Project (“Alta X” or the “Project”). Consistent with Mitigation Measure (MM) #63 of the Alta East Wind Project Draft Environmental Impact Report (DEIR), Alta Wind X is committed to conducting avian and bat mortality monitoring at the Project during the first, second, and third years of operation. Following construction in the spring of 2014, Alta Wind X contracted Western EcoSystems Technology, Inc. (WEST) to develop and implement a study protocol for post-construction monitoring at the Project for the purpose of estimating the impacts of the wind energy facility on birds and bats. The following report describes the methods and results of mortality monitoring conducted during the initial year of operation of the Project, March 2014 to March 2015.
As stated in MM #63 of the DEIR, the ultimate goal of the mortality monitoring study is to demonstrate that the level of incidental mortality does not result in an unanticipated long-term decline in populations of avian or bat species in the vicinity of the Project. To this end, WEST designed and implemented a 3-year study to determine the level of bird and bat mortality attributable to collisions with wind turbines at the facility on an annual basis. The monitoring study consists of four components: 1) standardized carcass surveys of selected turbines; 2) searcher efficiency trials to estimate the percentage of carcasses found by searchers; 3) carcass removal trials to estimate the length of time that a carcass remains in the field for possible detection; and 4) adjusted mortality estimates for birds and bats, calculated using the results from searcher efficiency trials and carcass removal trials to estimate the approximate level of bird and bat mortality within the Project.
A sample of 11 plots surrounding turbines was searched every two weeks at the Project. Search plots consisted of a 240-meter by 240-meter (m; 787-foot by 787-foot [ft]) plot beneath a randomly selected turbine, plus an additional area extending to the center point of the adjacent turbine or turbines if present. This level of effort included searching all or portions of 26 of the Project’s 48 total turbines and covered an area equivalent to searching approximately 15 240-m by 240-m plots or 31% of Project-wide turbines. Surveyors walked parallel transects within the search plots while scanning the ground for bird and bat mortalities.
During the study 33 birds, representing 17 identifiable species, and two bats representing two identifiable species, were found during standardized carcass searches within the Project. An additional 20 bird carcasses were also found incidentally. The most common bird carcasses able to be identified to species that were found as mortalities during searches or incidentally were greater roadrunner (three mortalities) and band-tailed pigeon (three mortalities). During scheduled searches or incidentally, 15 small birds unidentifiable to species were found in addition to four unidentified warblers. Two diurnal raptor mortalities (one red-tailed hawk and one unidentified raptor) were found in total during the study. The unidentified diurnal raptor was found not to be a golden eagle. Bird mortalities were distributed throughout the year, with somewhat lower rates of mortality occurring in winter. The two bat mortalities (one hoary bat and one Mexican free-tailed bat) were found during the fall. One hoary bat carcass was also found incidentally in the spring.
A total of 159 bird carcasses were placed for searcher efficiency trials. Overall searcher efficiency rates were 70% for small birds and 93% for large birds. A total of 153 bird carcasses and 26 bat carcasses were placed for carcass removal trials, which showed that by day 10, roughly 40% of large birds, 10% of small birds, and 3% of bats remained in the search area. The mean carcass removal time for both large and small birds varied by season. For large birds, the carcass removal time was 12.09 days in summer and fall, and 36.87 days in winter and spring. For small birds, the mean carcass removal time was 8.54 days in summer and winter, and 2.10 days in spring and fall. For bats, the mean carcass removal time was 2.92 days throughout the year.
Mortality estimates were adjusted based on the corrections for carcass removal and observer detection bias. For small birds, the probability that a carcass would remain in the search plot and be found by a searcher ranged from 0.10 during spring and fall to 0.36 during winter and summer. For large birds, this probability was higher, and ranged from 0.56 during summer and fall, to 0.81 during winter and spring. For bats, the probability that a carcass would remain in the search plot and be found by a searcher remained constant throughout the year at 0.14. Based on the 2.85-megawatt (MW) capacity of the turbines at the Project, the estimated mortality rate for birds was 4.88 birds per MW per year, with an estimated mortality rate of 0.04 diurnal raptors per MW per year. The estimated mortality rate for bats was 0.42 bats per MW per year. No state or federally listed bird or bat species or species of concern were found during the study.