Hatchet Ridge Wind, LLC is evaluating the feasibility of a wind energy development in Shasta County, California. The proposed site, Hatchet Ridge, is located approximately 6 miles west of Burney, California and along the primary ridgeline of Hatchet Mountain north of California State Highway 299. The proposed development would be located on private land and would consist of the installation, operation, maintenance, and eventual decommissioning of approximately 100 MW of turbines and supporting facilities.
In support of the environmental impact evaluation for the project, a detailed 12-month biological resources study plan was developed and implemented at the site. The study protocol was developed in cooperation with the California Department of Fish and Game and the U.S. Fish and Wildlife Service and based on expertise and experience of WEST, Inc. studying wind power effects on birds and wildlife. Objectives of the study were to provide data that was useful in evaluating potential impacts from the proposed project and assist in siting of project facilities within the project area. The field surveys were designed to: (1) describe and quantify seasonal avian use of the proposed project area; (2) describe and quantify raptor use of the proposed project; (3) describe and quantify seasonal bat use of the proposed project; (4) describe vegetation types and rare plant occurrence in the proposed project area.
Fixed-point avian use surveys were conducted to estimate the seasonal, spatial, and temporal use of the site by birds and in particular raptors. Surveys were conducted at six fixed survey stations located within the study area approximately once each week between November 15, 2005 and November 9, 2006, resulting in 270 30-minute point count surveys during the study. Seventy-nine avian species were observed during the fixed-point surveys. Passerines were the most numerous group and comprised 64% of all birds observed; dark-eyed junco, common raven, American robin, and mountain bluebird were the most numerous passerines observed. Raptors comprised approximately 10% of all birds observed. The most common raptors were red-tailed hawk, American kestrel, bald eagle, and Cooper’s hawk. Waterfowl comprised only 1% of all groups but 16% of all individual birds primarily because they tended to occur in large flocks. The most numerous waterfowl were tundra swan, greater white-fronted goose, and snow goose. Other birds (waterbirds, upland gamebirds, doves, and other non-passerine species) comprised approximately 9% of all birds observed.
To standardize the data for comparison between points, seasons, and with other studies, avian use, frequency of occurrence, and species composition were calculated from observations within 800 m of the survey point. Avian use by species was calculated as the mean number of observations per 30-minute survey. Over all seasons based on use, passerines were the most abundant group observed (6.2/survey), followed by waterfowl (1.3/survey), and raptors (1.0/survey). Waterfowl use was strongly influenced by observations of a few large flocks flying over the site. Over all seasons, dark-eyed junco was the most common bird observed with 0.98 detections per survey, followed by common raven (0.64), turkey vulture (0.51) and American robin (0.50). These four species comprised 28% of all bird use of the site for the year. Passerine and raptor use was significantly lower in the winter than in the summer and fall. There were no significant differences in use between seasons for all birds combined.
During the study 533 single birds or flocks totaling 1,581 individuals were observed flying during point count surveys. For all species combined, 25.1% of all flying birds observed were below the rotor-swept height, 63.4% were within the rotor-swept height, and 11.5% of birds were observed flying above the rotor-swept height of typical turbines that could be used in the project. Avian groups most often observed flying within the turbine rotor-swept height were buteos (98.5%), vultures (98.4%), doves (96.2%), and eagles (85.7%). For all flying raptors combined, 94.9% were observed flying within the rotor-swept height. For species with at least five separate observations of flying birds, those most often observed at rotor-swept heights were red-tailed hawk (98.4%), turkey vulture (98.4%), band-tailed pigeon (98.1%), unidentified bluebird (96.4%), and tree swallow (95.0%). Based on the use (measure of abundance) of the site by each species and the flight characteristics observed for that species, turkey vulture, common raven, snow goose, tree swallow and American robin had the highest probability of turbine exposure. The only raptor with a relatively high exposure index was red-tailed hawk, which ranked 7th of all species.
For all bird species combined, use was relatively uniform along Hatchet Mountain and no obvious flyways or concentration areas were observed. No strong association of use with topographic features of the site was noted for raptors or other large birds. The majority of large birds flew perpendicular to and across the prominent ridgeline, rather than parallel with the ridge, suggesting that the ridge is not an important migratory route for diurnal migrants. Although some differences in avian use were detected among survey points, the differences are not large enough to suggest that any portions of the project area should be avoided when siting turbines.
An aerial survey for raptor nests was conducted via helicopter on April 21, 2006. The nest survey area included the development area and the area within an approximate 2-mile buffer of the site, which totaled approximately 32,000 acres. Three active raptor nests were located in the project area, including two osprey nests located approximately 1.2 and 2.4 miles southeast of the development area and a bald eagle nest on the north side of Lake Margaret approximately 2 miles northeast of the turbine development area. Based on a survey area of 32,000 acres, active raptor nest density was 0.06/mi2, which is low compared to most other wind resource areas in the western U.S.
The objective of the bat use surveys was to estimate the seasonal and spatial use of the site by bats. Two Anabat® II echolocation detectors were used to monitor bat use at the project site on 78 nights during the period May 26 – October 18, 2006, resulting in a total of 145 detector nights. One AnaBat was placed on a met tower approximately 50 m above ground, and the other was placed on the ground at the base of the met tower. A total of 625 bat calls was recorded during 145 bat detector nights. Most (87%) of the calls were >35 kHz in frequency (e.g., Myotis bat species), and the remaining 13% were <35 kHz (e.g., big brown bat, hoary bat). The mean number of bat calls recorded per night per detector was higher for the AnaBat unit placed on the ground (5.6) than for the one placed 50 m in the air on the met tower (3.0). The number of bat calls per detector-night was similar in the summer breeding season (4.5) and fall migration (4.1). Peak activity levels for both detectors were in late-May to early-June and again from late-July through early-September. These periods correspond to bat migration periods and it is likely some bats migrate through the project area.
The objectives of the vegetation mapping were to identify the vegetation types (communities) that may be directly impacted by the project and characterize the habitat suitability of the study Ecological Baseline Study Hatchet Ridge Wind Project iii area for listed species and possible occurrence of rare plants. The vegetation of the project area was mapped on 1:24,000 scale USGS topographic maps based on aerial photos. Ten habitat types were identified within the entire mapping area; these included ceanothus scrub, clear cut/mixed conifer, developed, mixed conifer, oak scrub, open water, pasture, plantation-mixed conifer sapling, plantation-mixed conifer sapling/manzanita, and spring/seep-riparian scrub.
A pre-field evaluation was conducted to determine the number of and potential of rare plants possibly occurring in the project area. Field surveys were conducted for six rare plant species potentially occurring in the project area based on habitat and elevation. A large population of one species, Butte County morning glory, was located in the northern portion of the project area in an area composed of reddish, rocky volcanic soils with low overall plant cover. The majority of individuals encountered on site were observed in the barren interspaces between shrubs and other forbs, although some plants had established beneath manzanita shrubs. Tens of thousands of individuals were observed over a survey area comprising approximately 144 acres, and more plants are presumed to occur beyond this area to the south and west in similar habitat. Construction activity will impact individuals and patches of the plant but will not affect the viability of the population due to the large area over which it occurs.
Based on site specific avian use data collected for the Hatchet Ridge site, mean annual raptor use (adjusted as number of raptors observed per 20-minute survey to be comparable with other studies) was 0.69/survey with 50% of this value composed of turkey vulture use. Raptor/vulture use at Hatchet ridge is lower than 10 other wind resource areas (WRA) but higher than 17 other WRA evaluated in the U.S. using similar protocols. A regression analysis of raptor use and raptor collision mortality for several new-generation wind farms where similar methods were used to obtain raptor use estimates showed a significant (r2 = 90.3%) correlation between raptor use and raptor collision mortality. Using this regression to predict raptor collision mortality at the Hatchet Ridge project yields an estimated fatality rate of 0.06/MW/year, or 6 raptors per year for a 100-MW project. Based on species composition of the most common raptor fatalities at other western wind farms and species composition and timing of raptors observed at Hatchet Ridge during the studies, the majority of the fatalities of diurnal raptors would likely consist of red-tailed hawk and American kestrels during the summer and fall seasons.
Use of the Hatchet Ridge site by all bird species combined is low compared to 24 other WRA evaluated using similar protocols, as 20 of the 24 sites had higher bird use than that observed at Hatchet Ridge while only four sites had lower use. The data collected during this study suggest that the Hatchet Ridge project is not within a major migratory pathway, either for diurnal or nocturnal migrants. The project area also does not appear to provide important stopover habitat for migrant songbirds based on point count studies. Based on all survey data, song bird mortality at Hatchet Ridge would likely be lower than the national average of 2.3 birds/turbine/year or 3.1birds/MW/year. Although construction and operation of the wind farm may displace some groups of birds, because the Hatchet Ridge Wind Farm will be sited in previously altered habitats, and similar habitats are common in the region, it is unlikely that displacement of birds would result in any population impacts.
The mean number of bat passes per detector per night was compared to existing data at five WRA where both bat activity and mortality levels have been measured. The level of bat activity documented at the Hatchet Ridge site is much lower than the eastern and Midwestern U.S. wind farm sites, all of which had fairly high levels of bat mortality. However, it is higher than the Buffalo Ridge, Minnesota and Foote Creek Rim, Wyoming wind farms, both of which had relatively low levels of bat mortality. The data collected on site do not indicate that substantial numbers of bats migrate through the Hatchet Ridge Project Area. Some bat mortality will likely occur at the site, however, the available data indicate it would be lower than that experienced in the East, but potentially somewhat higher than that documented at other western U.S. sites.