The Horse Butte Wind Facility in Bonneville County, Idaho, developed by Utah Associated Municipal Power Systems (UAMPS) and operated by Horse Butte Wind I, LLC, became operational in August 2012. During the first three years of operation, SWCA Environmental Consultants (SWCA) conducted avian and bat carcass searches to document fatalities potentially attributable to turbine collision. This report presents observed (unadjusted) fatalities and adjusted fatality estimates for each of the survey years. Adjusted fatality estimates were calculated using an industry-accepted statistical estimator that corrects for carcass persistence and searcher efficiency (seasonal trials conducted during the first two survey years), search interval, and the proportion of turbines searched to the total number of turbines at the site.
Avian and bat fatalities are presented in the context of the project’s bird and bat conservation strategy (BBCS) and the project’s eagle conservation plan (ECP); both documents, which are currently under revision, present fatality thresholds developed as criteria for implementing phased operational and nonoperational mitigation. The thresholds, as currently written, are based on observed (unadjusted) fatalities.
Carcass searches were conducted every 2 weeks in Year 1 and, in Years 2 and 3, every 2 weeks during the winter and breeding seasons and weekly during the spring and fall migration seasons. These searches were conducted at 10 of 32 turbines during any one survey period, with nine turbines searched on a constant basis throughout the study. During searches, 18 birds and 26 bats were found in Year 1, nine birds and 17 bats were found in Year 2, and nine birds and 12 bats were found in Year 3. An additional six birds and two bats were found incidentally. No federally listed bird or bat was found during the formal searches or incidentally in any survey year. Two golden eagles (Aquila chrysaetos), protected under the Bald and Golden Eagle Protection Act of 1940, as amended, were found in Year 3 (one incidentally and one during searches). Five bird species—black rosy-finch (Leucosticte atrata), Brewer’s sparrow (Spizella breweri), eared grebe (Podiceps nigricollis), sharp-tailed grouse (Tympanuchus phasianellus), and Swainson’s hawk (Buteo swainsoni)—were considered “sensitive” under the project’s BBCS threshold categories because of their federal (birds of conservation concern) and/or state (Idaho species of greatest conservation need) sensitivity designations. However, none of the BBCS or ECP thresholds were met or exceeded in any survey year.
Avian and bat species composition and seasonal distribution patterns generally mirrored those observed at other wind energy facilities in the United States, with the exception that avian fatalities peaked in fall migration (42%) and in winter (33%; the majority of these winter fatalities were recorded in Year 1). Avian fatalities were distributed among several species (17 species were identified during this study), with passerines (33%) constituting more than other groups (waterbirds/waterfowl, 24%; diurnal birds of prey, 18%; upland gamebirds, 15%). More bat (55) than bird (36) fatalities were recorded. Bat fatalities were limited to specific species (four species were identified during this study and comprised 91% of all bat fatalities); these species were migratory tree bats, and most (82%) were found during late summer and fall migration.
Eared grebe (5) was the most common species found, followed by gray partridge (Perdix perdix; 4), and common redpoll (Acanthis flammea; 3). The two migratory tree bat species found during searches were hoary (Lasiurus cinereus; 26) and silver-haired bat (Lasionycteris noctivagans; 22). Among fatalities that could be identified to species, most have been commonly documented as fatalities at other wind energy facilities; however, some of the bird species have been infrequently (rough-legged hawk and Swainson’s hawk), rarely (Brewer’s sparrow, common redpoll, eared grebe, golden eagle, and sharp-tailed grouse), or never (black rosy-finch and gray-crowned rosy-finch) reported to our knowledge. One of the bats species (western small-footed myotis) has also never been reported to our knowledge.
Adjusted fatality estimates were comparably low for birds in Years 1 and 2 (1.9 and 2.6 birds/MW/year, respectively) and moderate for birds in Year 3 (3.5 birds/MW/year) and bats in all years (7.8, 9.3, and 5.1 bats/MW/years in Years 1-3, respectively) relative to other wind energy facilities in the United States. In this report, we discuss findings in the context of the project’s BBCS and ECP and in the context of data reported at other United States wind energy facilities (including the utility of comparing such data).