Spatial Pattern of Summer Bat Mortality from Collisions with Wind Turbines in Mixed-Grass Prairie

Article excerpt

ABSTRACT.-

Spatial analysis that aims to identify site-specific hotspots of collision mortality from birds or bats striking wind turbines can potentially lead to mitigating measures that reduce mortality rates. During May-Jul. 2004 and 2005, we studied bird and bat mortality from collisions with wind turbines at a 102 megawatt, 68-turbine wind farm in the southern Great Plains, Oklahoma, USA, Standardized searches around turbine bases yielded 122 total carcasses of which 92 (75%) were found within 20 m of the turbine bases. We identified 111 carcasses of seven species of bats and 11 carcasses of 6 species of birds. Brazilian free-tailed bats (Tadarida brasiliensis) comprised 85% of bat carcasses collected. We corrected turbine collision estimates using searcher efficiency trials and a range of removal (i.e., "scavenging") rates reported in the literature. Estimated bat turbine collisions ranged from 1.19-1.71 fatalities/turbine (1.03-1.37/megawatt). These data provide some of the first evidence for a steady rate of collision mortality of Brazilian free-tailed bats at a North American wind farm, most likely due to the site's proximity (~15 km) to a maternity colony. Spatial analyses indicated no consistent pattern in mortality estimates relative to ground cover or topographic position; but collision mortality was higher at several individual turbines, all of which were located near the heads of eroded ravines.

INTRODUCTION

As the number of wind turbines has increased across North America, the potential influence of wind energy development on collision mortality of birds (Osborn et al., 2000; Johnson et al., 2002) and bats (Arnett et al., 2008) has received increasing study (Johnson et al., 2003; Kerns and Kerlinger, 2004). Bird mortality from collisions with anthropogenic structures has been well documented (see Coues, 1876; Cottam, 1929; Pulich, 1961; Avery, 1979; Klem, 1990; O'Connell, 2001; Erickson et al, 2001). Compared to these other causes of collision mortality, the number of birds killed by wind turbines is low (Erickson et al, 2001). For bats, reported rates of collision mortality with wind turbines vary in magnitude across the U.S. (Osborn et al., 1996; Erickson et al., 2003; Johnson et al, 2003, 2004; Williams, 2003; Kerns and Kerlinger, 2004; Kunz et al., 2007; Arnett et al., 2008) with the highest rates observed along forested ridges of the Appalachians (Williams, 2003; Fiedler, 2004; Kerns and Kerlinger, 2004; NWCC, 2004; Arnett et al, 2008).

To date, most published data on bat collisions with wind turbines address migratory species in passage (Erickson et al., 2002; Johnson et al, 2003; Kerns and Kerlinger, 2004), especially solitary tree roosters such as silver-haired (Lasionyctms noctivagans) , red (Lasiurus borealis) and hoary (L. dnereus) bats (Caire et al., 1989; Arnett et al., 2008; Baerwald and Barclay, 2009) . Research suggests that most collision mortality occurs during late summer and early autumn migration (Johnson et al., 2003; Fiedler, 2004; Arnett et al., 2005, 2008).

Contrasting the literature on collision mortality of migratory bats in passage, no study to date has specifically examined the influence of wind facilities near large maternity colonies such as those of Brazilian free-tailed bats ( Tadarida brasiliensis) in which single colonies can number up to 20 million individuals (Harvey et al., 1999). Individuals from such colonies have been shown to forage as far as 68 km from their roost cave (Best and Geluso, 2003). We have little information on the degree to which mortality during late spring and early summer (when females make daily flights from their roosts over a period of many weeks) can influence the viability of a given roost location. From a management perspective, we also lack information on the spatial distribution of collision mortality, which could vary from one wind turbine to the next as a function of local topography, vegetation cover and/or specific flight corridors habitually used by bats leaving and returning to their roosts. …