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Patterns of Avian and Bat Mortality at a Utility-Scaled Wind Farm on the Southern High Plains


Wind energy has been utilized commercially in the United States since the 1970s. Empirical evidence suggests that direct collision mortalities of avian and bat species are consequences of wind energy development. Texas has the most installed wind energy generation capacity in the United States, yet no empirical data are available to assess the impact to local avian and bat populations. It is the goal of this study to determine the spatial and temporal distribution of avian and bat mortality at a utility-scale wind energy development along the Caprock Escarpment and to develop an accurate mortality estimate for avian and bat mortality at the site. Further, this study seeks to incorporate the year-long continuous mortality study, species use of the site and the site’s geophysical characteristics into a predictive model for wind energy development along the Caprock Escarpment.

From September 2006 to September 2007, I conducted standardized carcass searches at 28 turbines and 3 anemometer towers (n = 1,551). Additionally, I assessed removal rate of carcasses by scavengers and the efficiency of searchers in finding carcasses in trials concurrent to carcass searches. I calculated observer efficiency as the proportion of trial carcasses that were detected by observers, carcass persistence as the average length of time trial carcass remained onsite before complete removal and modeled mortality estimates using the Young et al. (2003) formula. I estimate mortality for bat species for an eight-month season of occupancy, and for avian species on a yearly basis. To identify spatial and temporal distributions, I conducted chi-squared test analysis of deviance for avian and bat taxa separately.

During standardized carcass searches, observers detected 25 avian carcasses and 47 bat carcasses. Turkey vultures (Cathartes aura) accounted for 36 percent of avian carcass detections, and Brazilian free-tailed bats (Tadarida brasiliensis) accounted for 94 percent of bat carcass detections. Using a 63 percent observer efficiency rate and a 9.5 day carcass persistence, I estimate avian mortality to be 0.5 individuals per MW per year (SE = 0.24). I estimate bat mortality to be 36.9 individuals per MW per season of occupancy (SE =111.89) using a 23 percent observer efficiency rate and 1 day carcass persistence. There was no significant spatial distribution of avian or bat carcasses within the wind energy development. Avian carcass detections were significantly higher during the fall season (χ² = 20.87, d.f. = 2, P = 0.0001). Bat carcass detections were significantly higher during the fall and spring season (χ² = 52.47, d.f. = 2, P < 0.0001).

The local impact of the Red Canyon Wind Energy Center on avian species appears to be low and similar to results seen in Oklahoma. In contrast, the relatively high mortality estimate for bat species indicates further study is required. These results are the first publicly available mortality estimates for the Caprock Escarpment region and identify the potential for population-level impacts of collision mortality to local bat species.

Amanda Miller
Master’s Thesis in Wildlife Biology
August 2008

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