Abstract
Odell Wind Farm, LLC (Odell) operates the Odell Wind Energy Project (Project) located in Cottonwood and Jackson counties, Minnesota. Odell contracted Western EcoSystems Technology, Inc. (WEST) to implement the Tier 4 post-construction mortality (PCM) monitoring study designed in the Avian and Bat Protection Plan (ABPP). As committed to in the ABPP, WEST quantified the direct impacts of the Project on birds and bats through PCM monitoring during the first year of operation.
The primary objective of the study was to estimate the level of bird and bat mortality attributable to collisions with wind turbines at the Project. The fatality study consists of four primary components: 1) standardized carcass surveys of selected turbines, 2) searcher efficiency trials to estimate the percentage of carcasses found by searchers, 3) carcass persistence trials to estimate the length of time that a carcass remains in the field for possible detection, and 4) adjusted fatality estimates for birds and bats calculated using the results from searcher efficiency trials and carcass persistence trials to estimate the approximate level of bird and bat mortality within the Project.
Fifteen turbines, representing 15% of all Project turbines were randomly selected to be searched. Searches were conducted within a full 120- x 120-meter (m; 394- x 394-foot [ft]) plot centered on the turbine. Vegetation at all search plots were regularly mowed to maintain a height of 10 centimeters (four inches) or less, providing relatively uniform searching conditions across all cleared plots. To the extent possible, turbine searches were rotated throughout the day such that all daylight periods were surveyed (i.e., morning, mid-day, and afternoon). Monitoring began on December 20, 2016, and continued through December 10, 2017. Searches were once a month during the winter (November 15 – March 14) and weekly during spring (March 15 – May 14), summer (May 15 – July 31), and fall (August 1 – November 14).
During the study, a total of 14 bird and 11 bat fatalities were documented during fatality searches and incidentally. Of these, 10 bird and 10 bat fatalities were included in the analysis. One bald eagle fatality was found outside of the search plot during the initial clearing search in December 2016 and was excluded from analysis since it was located outside of the search plot. No federal- or state-listed threatened, endangered, or candidate bird or bat species were found.
Searcher efficiency trials were conducted to estimate the proportion of casualties found by searchers. A combined total of 138 carcasses (58 large bird carcasses, 74 small bird carcasses and six bat or bat surrogate carcasses [i.e., house mouse]) were used for searcher efficiency trials. Searcher efficiency rates were 83.3% for large birds, 53.4% for small birds, and 80.0% for bats.
Carcass persistence trials were conducted to estimate the length of time that a carcass remained in the field for possible detection by searchers. A total of 112 carcasses (42 large birds, 53 small birds, and 17 bats or bat surrogates [house mouse]) were placed throughout the study period for persistence trials. Approximately 55% of large bird carcasses, 35% small bird carcasses, and no bat carcasses remained by day four; and by day 10, approximately 25% of all large bird carcasses and 10% of small bird carcasses remained.
Fatality estimates were calculated by adjusting search results for carcass persistence and searcher efficiency bias, and only included carcasses found within selected search plots. Two estimators, Huso and Shoenfeld, were used to calculate the fatality estimates for birds and bats. Bird fatality estimates were calculated for all four seasons, while bat fatality estimates were only calculated for the spring, summer, and fall seasons. The estimated fatality rates for all birds and bats were lower when using the Huso estimator compared to the Shoenfeld estimator. Using the Huso estimator all bird fatalities were estimated to be 9.38 bird fatalities per turbine per year (4.69 bird fatalities per megawatt [MW] per year) compared to 12.28 bird fatalities/turbine/year (6.14 bird fatalities/MW/year) using the Shoenfeld estimator. Using the Huso estimator, the bat fatality rate was estimated to be 13.48 bat fatalities/turbine/year (6.74 bat fatalities/MW/year) compared to 17.12 bat fatalities/turbine/year (8.56 bat fatalities/MW/year).