The Hatchet Ridge Wind Farm (Project) is a 44 turbine wind energy facility located in Shasta County California. In October 2010, Tetra Tech, Inc. (Tetra Tech) was contracted to develop and implement a post-construction mortality monitoring (PCMM) study plan which incorporated methods consistent with the California Energy Commission’s California Guidelines for Reducing Impacts to Birds and Bats from Wind Energy Development. The study plan incorporated fatality monitoring at all turbines in the form of standardized carcass searches (biweekly and monthly), searcher efficiency and carcass persistence trials to adjust for inherent biases in estimating Project-related fatality rates, avian use surveys in Year One, and a Wildlife Education and Incidental Reporting. In October 2012, the Technical Advisory Committee providing Project oversight and guidance recommended a consecutive third year of the study.
This report presents the results of the third year of monitoring and the overall 3 years of PCMM, and includes a summary of documented fatalities, estimates of searcher efficiency and carcass persistence, and estimated annual fatality rates adjusted for bias for each year. Additionally, observed trends in Project-related fatalities are discussed along with trends relating to SpecialStatus Species and Groups and sources of study bias.
Specific to Year Three, a total of 51 fatalities were detected during 4 seasons of mortality monitoring. When sorted by search frequency, 38 fatalities (27 birds, 11 bats) were detected during biweekly (2 week interval) searches, 8 fatalities (3 birds, 5 bats) were detected during monthly searches, and 5 fatalities were detected incidentally (3 birds, 2 bats). Fatalities included bird fatalities from 19 species and 2 bird fatalities not identifiable to a species, as well as 18 bat fatalities from 4 species. The avian species groups with the highest number of fatalities include waterfowl (n=13; 25 percent of fatalities) and songbirds (n=9; 17 percent of fatalities). Four raptor fatalities (Special-Status Group: Other Raptors) including red-tailed hawk (n=2), Cooper’s hawk (n=1) and great horned owl (Special Status Group: Owls; n=1) were detected during biweekly searches. One raptor fatality, a sharped-shinned hawk, was detected during monthly searches. Seasonal composition of fatalities varied, with the highest number of avian fatalities (n=14) occurring in winter and the highest number of bat fatalities (n=15) occurring in summer. No Special-Status Species (bald eagle, sandhill crane, yellow warbler) were detected.
Year Three searcher efficiency and carcass persistence trials were conducted in each season. Searcher efficiency ranged from 0. 61 (90 percent CI=0.52–0.68) for bats to 0.92 (90 percent CI=0.88–0.96) for large birds. Carcass persistence times ranging from 1.55 days (90 percent CI=1.23–1.87) for bats to 40.80 days (90 percent CI=30.05–59.38) for large birds.
In Year Three fatality estimates were calculated for 5 groups: All birds, non-raptors, large birds, small birds and bats using fatalities detected during the biweekly carcass searches. The annual fatality estimates are presented in Table ES-1. Fatality estimates were not calculated for individual species or species groups with less than 5 fatalities detected (at biweekly searched turbines) due to the estimation model sample size requirement (n≥5).
Over 3 years of monitoring, a total of 98 avian fatalities from 39 species were detected at the Project with 42 avian fatalities not identifiable to species due to the condition of the remains. Estimated annual fatality rates for all birds ranged from 1.93 birds/turbine (90 percent CI=1.49– 2.50) to 5.74 birds/turbine (90 percent CI=4.53–7.74). Estimated annual avian fatality rates at the Project were comparable to other reported fatality rate estimates regionally. The majority of avian fatalities were waterfowl in the spring. Waterfowl fatalities at the Project be attributable to these species making localized movements under high wind and/or low visibility conditions. Songbird fatalities were also documented, but at lower rates than other wind facilities. Population-level impacts to the species most commonly detected as fatalities are unlikely due to secure populations of these species. All but one of the identified avian species (European starling) are protected under the MBTA.
A total of 63 bats from 4 species were detected at the Project over the three years of monitoring with 4 fatalities not identifiable to species due to the condition of the remains. Estimated annual bat fatality rates ranged from 5.13 bats/turbine (90 percent CI = 1.92–9.75) to 12.02 bats/turbine (90 percent CI = 6.74–20.85). Estimated annual bat fatality rates at the Project were comparable to other reported fatality rate estimates regionally and were consistent among the years of the study. Bat fatalities were highest during July-September and included hoary, silver-haired, and Brazilian free-tailed bats, as has been found at other wind facilities. Project-related fatalities are unlikely to have population-level impacts for the Brazilian free-tailed bat given their population stability. Limited population data for the hoary and silver-haired bat is available; however, the relatively low number of fatalities detected for these species indicate that population-level impacts are unlikely.
MM BIO-6 Special Status Species and Groups fatalities were detected; however, no bald eagle or sandhill crane fatalities were documented during the 3 years of the study. Fatalities were documented for MM BIO-6 Special-Status Groups Other Raptors (biweekly searched turbines, n=5; monthly searched turbines, n=4), Owls (biweekly searched turbines, n=1) and SpecialStatus Species Yellow Warbler (biweekly searched turbines, n=1; monthly searched turbines, n=1). Statistically-reliable per turbine annual fatality estimates could not be calculated for these groups or species due to an insufficient sample size (n≥5); therefore, comparisons against Project-specific thresholds for these species and groups cannot be made.