Survey of Bat Mortalities at a Wind-Energy Facility in North Dakota

Wind energy is quickly becoming a critical technology for providing Americans with renewable energy (Pasqualetti et al. 2004). However, bats have been found to be susceptible to injury and fatality near wind turbines (Baerwald et al. 2008; Horn et al. 2008) and impacts have been extensively documented (Kerns and Kerlinger 2004; Fiedler et al. 2007; TRC Environmental Corporation 2008; Gruver et al. 2009). The first wind-energy–related bat fatalities in the United States were recorded during avian fatality searches at California facilities (e.g., Orloff and Flannery 1992; Thelander et al. 2000). Since then, bat deaths at wind-energy facilities have been documented at many sites across the country, with fatality rates ranging from relatively low (1.2 bats · turbine⁻¹ · y⁻¹, Klondike Wind Farm, Oregon; Johnson et al. 2003b) to alarmingly high (63.9 bats · turbine⁻¹ · y⁻¹, Buffalo Mountain Wind Farm, Tennessee; Fiedler et al. 2007). Research has shown that the species experiencing the highest number of fatalities at turbines are hoary bats Lasiurus cinereus, silver-haired bats Lasionycteris noctivagans, and eastern red bats Lasiurus borealis (Johnson 2005; Cryan and Brown 2007; Kunz et al. 2007). These tree-dwelling species exhibit autumn migratory behavior (Findley and Jones 1964; Izor 1979; LaVal and LaVal 1979; Koehler and Barclay 2000; Cryan 2003), and fatalities at wind facilities have been consistently highest during the migration period of late summer and autumn (Johnson 2005; Cryan and Brown 2007; Arnett et al. 2008).

As of January 2012, North Dakota ranks 10th in the United States, with 1,469 MW of installed wind-generating capacity and proposed projects exceeding 11,000 MW (AWEA 2012). Most turbines on wind farms in North Dakota have a 1.5-MW generating capacity; therefore, these numbers represent approximately 980 installed turbines and >7,000 proposed turbines. This rapid increase in wind-energy development, coupled with our knowledge of bat fatalities at other wind facilities, suggests a need to assess potential risks to local bat populations. Yet, we know little about the ecology of bats in North Dakota; detailed studies of the distribution and habitat preferences of bat species have only begun in the past 3 y. Further, only a handful of preconstruction monitoring reports for industry have been produced in the state, and to our knowledge, no postconstruction assessments of bat fatalities have been conducted. The extensive presence and predicted growth rate of the wind industry in North Dakota point to the potential for a large impact on regional bat populations. Hence, the objective of this research was to conduct a preliminary survey of bat fatalities at an operational wind-energy facility in South-central North Dakota.

This study took place in the North Dakota portion of Tatanka Wind Farm, which is located in a region of the Missouri Coteau covering approximately 5,698 ha along the North Dakota–South Dakota border. Five bat species are known to occur in the study area: big brown bats Eptesicus fuscus, little brown bats Myotis lucifugus, eastern red bats L. borealis, silver-haired bats L. noctivagans, and hoary bats L. cinereus (Seabloom 2011). Vegetation consists primarily of nonnative pasture and Conservation Reserve Program grasslands. Other vegetation consists of deciduous shelterbelts, as well as tree stands around farmlands, wetlands, and springs, although these stands are rare. The North Dakota portion of Tatanka Wind Farm consists of 61 1.5-MW turbines, which are 80 m tall and have a rotor diameter of 77 m (Acciona-NA 2011).

We conducted fatality searches at 12 of the 61 turbines at the North Dakota Tatanka Wind Farm during the presumed autumn migration period. We searched each turbine six times between 23 July and 12 September 2010, with the exception of two turbines that could not be accessed on 10 September due to weather.

Turbine sites selected for carcass searches included various habitat and vegetation types. We centered square plots measuring 100 × 100 m on the turbine to ensure that a minimum of 50 m around the turbine was searched in all directions. Studies conducted at other wind farms suggest that the majority of carcasses are found within 40 m of the turbine (Anderson et al. 1999; Johnson et al. 2003a, 2003b). We marked parallel transects by flags placed every 6 m, with a maximum searching distance of 3 m on each side of the transect.

We conducted searches each day beginning at sunrise and walked transects at a rate of approximately 45–60 m/min. Recorded data included date and time collected; turbine; species; sex (when possible); condition (e.g., intact or scavenged); visible injuries, such as a broken wing or limb; distance and direction from the turbine; and distance from transect (Johnson et al. 2004). We labeled, triple-bagged, and froze all carcasses found, for further examination in the lab.

We found eight hoary bats and one silver-haired bat during the study period (Table 1). Carcasses were found at 5 of the 12 turbines. The largest number of bats located at one turbine in one search period was two bats. The greatest number of carcasses found over the entire study period at one turbine was three bats. Carcass distance from the turbine base ranged from 1 to 38 m, with an average distance of 10.1 m from the turbine base. We found most carcasses on the gravel pad surrounding the turbine base. We found carcasses an average of 1.3 m from transects. We identified two female hoary bats and two male hoary bats; the remaining carcasses could not be sexed due to scavenging or degree of decomposition.

Although we found no red bat carcasses during searches, the hoary bat and silver-haired bat carcasses were consistent with previous findings that demonstrate the vulnerability of these species to wind-energy development (Johnson 2005; Kunz et al. 2007). We found no carcasses after 27 August, which corresponds with the expected southward migration patterns of hoary, eastern red, and silver-haired bats (Cryan 2003); this might suggest that most bats had already moved through North Dakota by late August. We also found 78% of carcasses within 10 m of the turbine base, which aligns with previous studies that show the majority of bats killed fall close to the turbine base (Gauthreaux 1994). This interpretation should be taken with caution, because carcasses are inevitably easier to locate on gravel pads than within the vegetation surrounding a turbine.

These data do not account for searcher efficiency or carcass removal rates; thus, we could not estimate facility fatality rates. Without this information, we must assume that many carcasses may have been missed during searches. We are also making the assumption that these fatalities were turbine-related, because no controls were in place to assess otherwise. Future studies of high importance would involve coupling acoustic monitoring with carcass surveys at multiple locations, which would allow for calculation of facility-specific fatality rates in different cover types across the state (drift prairie, Missouri Coteau, etc.); searcher efficiency and carcass removal trials for accurate estimation of overall fatality rates; and extending the sampling period to include the spring migration period.

Although fatality rates vary greatly between facilities, high numbers of fatalities have continued to be documented around the country (Johnson 2005; Kunz et al. 2007; Arnett et al. 2008) In 2005, there were an estimated 63.9 bat fatalities· turbine⁻¹ · y⁻¹ at the Buffalo Mountain Windfarm (Fiedler et al. 2007); an estimated 1,206 bats were killed at the Judith Gap Wind Energy Center in Montana during the study period of August 2006–October 2006 and February 2007–May 2007, with 97% percent of fatalities occurring during autumn migration (TRC Environmental Corporation 2008); and in a study conducted at the Blue Sky Green Field Wind Energy Center, Wisconsin, researchers found 247 carcasses during the study period (Gruver et al. 2009).

While baseline data collection on the activity levels, distribution, and composition of bat populations in North Dakota is ongoing, populations of susceptible species have already been documented across the state (Seabloom 2011). The preferred habitats of bats are limited on the plains of North Dakota, so disruptions to these existing habitats may significantly impact local and/or migratory bat populations. Continued research will be necessary to monitor changes in populations and to document any potential wind-facility impacts so that mitigation measures may be taken, if necessary. We hope that this preliminary work will encourage researchers to conduct more complex studies of pre- and postconstruction bat activity and fatalities in the area. In addition, our study may be of value to researchers examining the migratory pathways of North American bat species, because migratory corridors are generally not well-documented, and our results confirm that at least two bat species are migrating through North Dakota during the autumn months.

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We would like to thank the Department of Biological Sciences and the College of Science and Mathematics at North Dakota State University for funding.

We would also like to thank Acciona-NA for permitting us to sample at their site. We would like to thank Paul Barnhart and Josiah Nelson for help in the field, as well as Wendy Reed, Chris Biga, Manuel Huso, two anonymous reviewers, and the Subject Editor for comments on an earlier version of this manuscript. Finally, we thank Eric Britzke for his invaluable help in study design and data analysis.

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