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 “Project”). Consistent with 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 postconstruction 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 second year of operation of the Project, April 2015 to April 2016.
As stated in the DEIR, the goal of the mortality monitoring study is determine the level of incidental injury and mortality to 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 consisted 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 search plots surrounding selected 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 19 birds, representing 11 identifiable species, and two bats representing two identifiable species, were found during standardized carcass searches within the Project. An additional three bird carcasses and one bat carcass were found incidentally. The most common bird species found during searches or incidentally were northern flicker (three mortalities) and mourning dove (two mortalities). No diurnal raptor mortalities were found during the second year of operation. Bird mortalities were distributed throughout the year, with the highest rates occurring in spring for both large birds and small birds. The two bat mortalities (one California myotis and one Mexican free-tailed bat) were found during summer and winter, respectively. One hoary bat carcass was also found incidentally in the fall.
A total of 239 bird carcasses were included in searcher efficiency trials. Overall searcher efficiency rates were 81% for small birds and 94% for large birds. A total of 320 carcasses were used in carcass removal trials (110 large birds, 157 small birds, 46 mice, and seven bats), which showed that by day 10, roughly 45% of large birds, 5% 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 and ranged from 19.03 days in fall to 30.42 days in the winter for large birds and from 2.38 days to 3.81 days for small birds. For bats/bat substitutes, the mean carcass removal times ranged from 1.42 days in the fall to 2.27 days in the winter.
Mortality estimates were adjusted 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 fall to 0.16 during winter. For large birds, this probability was higher, and ranged from 0.60 during fall, to 0.72 during winter. For bats, the probability that a carcass would remain in the search plot and be found by a searcher remained low and ranged between 0.05 in summer and fall to 0.08 in winter. Based on the 2.85-megawatt (MW) capacity of the turbines at the Project, the estimated mortality rate for birds was 2.17 birds/MW/year. No raptor mortalities were found during the study period, therefor the raptor fatality estimate was zero/MW/year. The estimated mortality rate for bats was 0.8 bats per MW per year.
Estimated overall bird mortality decreased from 3.96 birds/MW/year in Year 1 to 2.17 birds/MW/year in Year 2, which was primarily the result of a relatively large reduction in small bird mortality compared to a small increase in large bird mortality. Raptor fatality rates were consistently low across years, with only one diurnal raptor mortality found during the two years of study, while bat mortality estimates increased from Year 1 to Year 2, even though the same number of fatalities (two) was found each year.
Results of the second year of mortality monitoring at Alta X illustrate the annual variability inherent in estimating bird and bat mortality. Although bird and bat mortality during the first and second years of monitoring did vary some, diurnal raptor and bat mortality estimates were generally low compared to other wind energy facilities in California, while all bird mortality was considered to be more moderate among California projects. Results from the two years of monitoring at Alta X do not suggest that levels of bird and bat mortality will result in substantial impacts to bird and bat populations. However, a third year of mortality monitoring is ongoing at Alta X (2016-2017), which upon completion, will be assessed along with the first two years of data to provide a clearer picture of the impacts of Alta X on birds and bats.