Prepared in cooperation with the U.S. Fish and Wildlife Service

Bats and Wind Energy—A Literature Synthesis and 

Annotated Bibliography

Open-File Report 2012–1110

U.S. Department of the Interior

U.S. Geological Survey
Cover photograph: Hoary bat (Lasiurus cinereus), photo by Paul Cryan 
Bats and Wind Energy—A Literature Synthesis and 

Annotated Bibliography

By Laura E. Ellison

Open-File Report 2012–1110

U.S. Department of the Interior

U.S. Geological Survey
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iii

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

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Suggested citation:

Ellison, L.E., 2012, Bats and wind energy—A literature synthesis and annotated bibliography: 

U.S. Geological Survey Open-File Report 2012–1110, 57 p.

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Contents

Executive Summary....................................................................................................................................................... 1

Format of Report ........................................................................................................................................................... 2

Literature Review Methods ............................................................................................................................................ 2

Acknowledgments ......................................................................................................................................................... 3

I. Literature Synthesis................................................................................................................................................... 3

Causes of Mortality ................................................................................................................................................. 3

Fatalities and/or Activity Patterns............................................................................................................................ 4

Fatality Search and Estimation Techniques ........................................................................................................ 7

Migration ................................................................................................................................................................. 8

Mitigation and Curtailment ...................................................................................................................................... 9

Review Papers and Syntheses ..............................................................................................................................10

II. Annotated Bibliography............................................................................................................................................11

III. Additional References..............................................................................................................................................44

International References ........................................................................................................................................46

Unpublished Reports and Theses..........................................................................................................................47

Wind Resources.....................................................................................................................................................56
Bats and Wind Energy—A Literature Synthesis and 

Annotated Bibliography

By Laura E. Ellison

Executive Summary

Turbines have been used to harness energy from wind for hundreds of years (Gipe, 2004). 

However, with growing concerns about climate change, wind energy has only recently entered the 

mainstream of global electricity production. In 2010, the global installed capacity for wind energy 

reached 196,630 Megawatt (MW), which represents approximately 2.5 percent of the total global energy 

consumption (World Wind Energy Report, 2010). In the United States, the total utility-scale wind power 

capacity through the 3rd quarter of 2011 totaled 43,461 MW and this represents more than 20 percent of 

the world’s installed wind power (American Wind Energy Association, 2012). In 2011, the electricity 

produced from wind energy in the United States amounted to 120 Terawatt-hours (thousand MW) or 2.9 

percent of total global electricity demands (U.S. Energy Information Administration, 2012). Canada is 

the ninth largest producer of wind energy in the world with current installed capacity at 4,862 MW, 

representing about 2.1 percent of Canada’s total electricity demand (Canadian Wind Energy 

Association, 2012). 

Since early on in the development of wind-energy production, concerns have arisen about the 

potential impacts of turbines to wildlife; these concerns have especially focused on the mortality of 

birds. Early styles of turbines appeared to pose a greater risk to birds in terms of collision mortality than 

more modern turbines do (Erickson and others, 2002; Young and others, 2003). Early turbines were 

smaller, had a higher blade-rotation rate, and had a lower energy output. This resulted in more turbines 

being needed for significant electricity production, thereby increasing the chances of birds encountering 

turbines (Curry and Kerlinger, 2000; Erickson and others, 2002; Howell, 1995). The lattice towers of 

smaller turbines also provided birds with perching opportunities, which was thought to further increase 

mortality (Kerlinger, 2002; Orloff and Flannery, 1992; Osborn and others, 1998). Structural changes 

and improved turbine design have been instrumental in reducing mortality in birds (Johnson and others, 

2002; Smallwood and Karas, 2009). For example, during a study at the Altamont Pass Wind Resource 

Area in California, it was found that when comparing the concurrently operating old-generation, smaller 

turbines during 2005–2007, adjusted fatality rates in the newer, larger, and taller turbines were 66 

percent lower for all birds combined.

Despite the improvements to turbines that have resulted in reduced mortality of birds, there is 

clear evidence that bat mortality at wind turbines is of far greater conservation concern. Larger and 

taller turbines actually seem to be causing increased fatalities of bats (Barclay and others, 2007). Bats of 

certain species are dying by the thousands at turbines across North America, and the species consistently 

affected tend to be those that rely on trees as roosts and most migrate long distances (Cryan and 

Barclay, 2009). Bat mortality at wind-energy facilities was first documented in Australia, where 22 

white-striped mastiff-bats (Tadarida australis) were found at the base of turbines over 4-year (yr) period 

(Hall and Richards, 1972). In 1999, 45 dead bats were found at a wind energy facility in Carbon County, 
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Wyoming; 10 dead bats were found at a wind energy facility in Umatilla County, Oregon; and 34 dead 

bats were found within a wind energy facility in Wisconsin (Keeley and others, 2001). Small numbers 

of dead bats have also been found at wind-energy facilities in California (Orloff and Flannery, 1992; 

Howell, 1997; Anderson and others, 2000; Thelander and Rugge, 2000). Turbine-related bat mortalities 

are now affecting nearly a quarter of all bat species occurring in the United States and Canada. Most 

documented bat mortality at wind-energy facilities has occurred in late summer and early fall and has 

involved tree bats, with hoary bats (Lasiurus cinereus) being the most prevalent among fatalities.

Populations of bats are difficult to monitor (O’Shea and others, 2003). Because of this, there is 

insufficient information on the population status of the 45 species of bats in the United States, especially 

for migratory foliage- and tree-roosting bats (O’Shea and others, 2003). With this lack of understanding 

of total population sizes, demographics, and impacts of fatalities from wind turbines on the viability of 

affected bat populations, it is currently not possible to determine the influence of any single source of 

mortality or of any effects of mitigation strategies on these bat populations. In addition to the direct 

effects of wind-energy development on bat mortality, indirect effects may occur as well. Bats have low 

reproductive rates and generally give birth to a single individual once a year. This results in bat 

populations growing slowly and an inability to quickly rebound after rapid declines in population size. 

Bat populations therefore rely on high adult survival rates to compensate for low reproductive rates and 

prevent declines. Therefore, substantial cumulative impacts of wind-energy development on certain bat 

species, especially tree-roosting bats, are expected, and these populations would be slow to recover from 

any population declines (Barclay and Harder, 2003). 

Numerous research opportunities exist that pertain to issues such as: (1) identifying the best and 

worst placement of sites for turbines and (2) mitigation strategies that would minimize impacts to 

wildlife (birds and bats). Unfortunately, to date, very little research of this type has appeared in the peerreviewed scientific literature; much of the information exists in the form of unpublished reports and 

other forms of gray literature. This literature synthesis and annotated bibliography focuses on refereed 

journal publications and theses about bats and wind-energy development in North America (United 

States and Canada). Thirty-six publications and eight theses were found, and their key findings were 

summarized. These publications date from 1996 through 2011, with the bulk of publications appearing 

from 2007 to present, reflecting the relatively recent conservation concerns about bats and wind energy. 

The idea for this Open-File Report formed while organizing a joint U.S. Fish and Wildlife 

Service/U.S. Geological Survey “Bats and Wind Energy Workshop,” on January 25–26, 2012. The 

purposes of the workshop were to develop a list of research priorities to support decision making 

concerning bats with respect to siting and operations of wind-energy facilities across the United States. 

This document was intended to provide background information for the workshop participants on what 

has been published on bats and wind-energy issues in North America (United States and Canada). 

Format of Report

This report is divided into three sections: (1) a literature synthesis; (2) an annotated 

bibliography; and, (3) additional references. The literature synthesis and annotated bibliography focus 

on North America and on refereed journal publications. Additional references include a selection of 

citations on bat ecology, international research on bats and wind energy, and unpublished reports.

Literature Review Methods

A detailed literature review was conducted using Internet resources and databases. The 

keywords chosen for these searches included “bats,” “Chiroptera,” “wind,” and “wind energy.”
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Keywords were used alone or in combination with the other terms. Databases and search engines used 

included Google (http://www.google.com), Google Scholar (http://scholar.google.com), SciVerse 

Scopus (http://www.scopus.com), ISI Web of Knowledge (http://apps.isiknowledge.com), and the USGS 

Library’s Digital Desktop (http://library.usgs.gov). The literature-cited sections of publications obtained 

from keyword searches were also cross-referenced to identify additional citations or gray literature that 

were missed by the Internet search engines.

Acknowledgments

The work on which this Open-File Report is based was conceived while organizing a joint U.S. 

Fish and Wildlife Service (FWS)/U.S. Geological Survey (USGS) “Bats and Wind Energy Workshop,” 

on January 25–26, 2012. I thank the participants of the workshop for their suggestions and valuable 

input on the development of this document: From FWS, Paul Barrett (Southwest, R2), Gabriela 

Chavarria (Washington), Jeremy Coleman (Northeast, R5), Megan Cook (Washington), Craig Hanson 

(Mountain-Prairie, R6), David Kampwerth (Pacific, R1), TJ Miller (Midwest, R3), Scott Pruitt (R3), 

and Jennifer Szymanski (R3). From USGS, Laurie Allen (USGS Ecosystems Mission Area), Dan 

Manier [Fort Collins Science Center (FORT)], Nina Burkhardt (FORT), Paul Cryan (FORT), Robb 

Diehl (Northern Rocky Mountain Science Center), Jay Diffendorfer (Rocky Mountain Geographic 

Science Center), Marcos Gorresen (Pacific Islands Ecosystem Research Center), Manuela Huso [Forest 

and Rangeland Ecosystem Science Center (FRESC)], Doug Johnson (Northern Prairie Wildlife 

Research Center), Sue Phillips (FRESC), and Wayne Thogmartin (Upper Midwest Environmental 

Sciences Center). Ed Arnett (Bat Conservation International), Mark Hayes (FORT), and Patty Stevens

(FORT) provided helpful peer review comments. Jenny Shoemaker (USGS, FORT) assisted with 

formatting the final report. Funding for this Open-File Report was provided by Laurie K. Allen and 

William A. Lellis (USGS Ecosystems Mission Area).

I. Literature Synthesis

I found 36 refereed journal articles and eight theses during this literature search. The earliest 

article was published in 1996 (Osborn and others, 1996) and the number of publications per year 

increased until the present year. There was one publication in 1996, one in 2002, one in 2003, two in 

2004, one in 2005, six in 2007, seven in 2008, four in 2009, six in 2010, nine in 2011, and finally, one 

in 2012. Five of the eight theses were annotated and included in this synthesis because they were 

intended for publication. Three of the theses were later published and annotated in this report. I 

identified five broad categories of topics: (1) Causes of mortality; (2) Fatalities and/or activity patterns; 

(3) Migration; (4) Mitigation and curtailment; and, (5) Review papers and syntheses. I summarize the 

literature for each of these topics in the following five subsections.

Causes of Mortality

Eight papers addressed the causes of bat mortality at wind-energy facilities. Kunz and others 

(2007a) and Cryan and Barclay (2009) provided overviews of the hypothesized causes of bat fatalities at 

turbines. These hypotheses fall into two categories: proximate and ultimate. Proximate causes explain 

the direct means by which bats die at turbines and include bats colliding with turbine towers, colliding 

with rotating blades, or barotrauma (that is, internal injuries suffered after being exposed to rapid 

pressure changes near the trailing edges and tips of the moving blades). Hypotheses of ultimate causes 

are numerous and include three general categories: random collisions, coincidental collisions, and 

collisions that occur because bats are attracted to turbines. Random collisions are those that occur due to 
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chance alone or involve no assumptions of circumstance or attraction. Coincidental collisions involve 

bats being victims of unfortunate behavioral circumstances that put them at risk of colliding with 

turbines (for example, turbines are located along migratory pathways). Hypotheses of attraction propose 

that there is some attractor or combination of attractors drawing bats to wind turbines (Kunz and others, 

2007). 

The obvious prominent cause of bat deaths at wind turbines are direct collision (that is, bluntforce trauma) and barotrauma. Baerwald and others (2008) proposed that barotrauma was a significant 

cause of bat fatalities at wind turbines in southwestern Alberta, Canada. They found that 46 percent of 

bats killed at turbines had no discernible external injuries that would have been fatal, and 92 percent of 

bats necropsied had hemorrhaging in the thoracic and/or abdominal cavities. In a follow-up study at this 

same location, a small percentage (2.5 percent) of bats found while conducting fatality searches were 

alive and only 30 percent of those live bats had visible signs of skeletal damage or considerable soft 

tissue damage. However, Grodsky and others (2011) used veterinary diagnostic procedures to 

investigate bat fatalities in southeastern Wisconsin and found that the exact cause of death (that is, 

barotrauma or direct collision) could not be determined in most bats due to the variability of injuries and 

a lack of exclusively attributable lesions. They concluded that the cause of death for bats killed at 

turbines was not exclusively or predominantly barotrauma or direct collision but rather an indiscernible 

combination of both. Simply using a visual inspection of a bat carcass is not adequate for conclusively 

diagnosing fatal injuries, including broken bones. Rollins and others (2012) examined the causes of lung 

damage from salvaged bats and found that results could be confounded by common postmortem 

artifacts (for example, decomposition, temperature differences) as well as the competing hypothesized 

causes of traumatic injury (for example, blunt force trauma, barotrauma, laceration). Because these 

competing factors could not be confidently excluded during a pathological examination, pulmonary 

barotrauma should not be diagnosed from salvaged bat carcasses unless assessments are also made for 

ruptured ear drums. The authors conclude that their results point to a need for improvements in the 

current monitoring procedures at wind-energy facilities and in the experimental approaches used to 

determine whether barotrauma or other causes contribute to bat mortality. 

Horn and others (2008) used thermal infrared (TIR) cameras to monitor the activity of bats at 

Mountaineer Wind Energy Center in West Virginia. They found clear evidence of bats avoiding turbine 

blades and of bats being directly struck by blades. One of the most important observations from this 

study was that bats were observed actively investigating both moving and motionless turbine blades; 

this suggests that they may be attracted to the turbines as potential roosting trees. 

Kunz and others (2007) first proposed the hypothesis that tree bats are attracted to turbines. 

Cryan (2008) further elaborated on this hypothesis by proposing that tree bats collide with turbines 

while engaging in mating behaviors that center on the tallest trees in the landscape. Tree bats may use 

the highest trees in a landscape as rendezvous points during the mating season and this behavior places 

them at risk of collision with the blades of wind turbines. Jameson’s (2010) thesis called this the 

“Reproductive Landmarks Hypothesis” and found evidence that tree bats were attracted to tall structures 

during the fall migration, the same time of year of most documented mortality at wind-energy facilities. 

Direct evidence of mating at communication towers and turbines was not observed during this study; 

however, high levels of bat activity were recorded at communication towers during migration.

Fatalities and/or Activity Patterns

Eleven papers and five theses investigated patterns of fatality and/or activity of bats at windenergy facilities. Four papers and one thesis looked at fatalities alone, while the remaining studies 

examined both bat activity at and near turbines and bat mortalities. Patterns that emerged from these 
5

fatality and activity studies were the following: (1) fatality rates for bats at wind turbines and the species 

most affected differed by geographical region; (2) there was a tendency for more males to be killed than 

females (with a few exceptions); (3) most mortality occurred during fall migration; (4) the majority of 

fatalities were tree bats (hoary, silver-haired, and eastern red bats); (5) searcher detection and scavenger 

rates for bat carcasses are highly variable and depend on the region, vegetation, and methodology used; 

(6) there was no consistency in how mortality rates were presented; (7) bat-activity (acoustical) patterns 

around turbines and in nearby habitats varied by geographical region; and, (8) there was a tendency for 

bat fatalities to increase in low wind speeds.

Prior to Osborn and others (1996), most studies on the biological impacts of wind energy were 

designed to assess the impacts to birds. Bat collisions with turbines were infrequent and had not 

received attention by natural resource agencies or the scientific community. While conducting a study to 

assess impacts to birds from wind turbines at the Buffalo Ridge Wind Resource Area in southwestern 

Minnesota in 1994–1995, Osborn and others (1996) found 13 bat carcasses of 5 different species, 11 of 

which were tree bats (hoary, silver-haired, and red bats). A high percentage of these fatalities (85

percent) occurred during the summer. Johnson and others (2003) continued to document mortality of 

bats at this same location from 1996 to 1999 and found 163 fatalities: 89 percent of the fatalities were 

hoary and eastern red bats, and 97 percent occurred from July 15 to September 15. In 2001 and 2002, 

Johnson and others (2004) continued to conduct fatality searches as in the previous two studies, but 

included acoustic and mist-netting surveys to investigate bat activity and composition in relation to the 

wind facility. They found more bat activity at foraging and roosting areas than at turbines. Mortality of 

bats at wind turbines did not appear to involve foraging bats, but instead corresponded with postbreeding southward migration of hoary (Lasiurus cinereus), eastern red (L. borealis), and silver-haired 

bats (Lasionycteris noctivagans). 

At about the same time as the work being done at Buffalo Ridge, Minnesota, Gruver (2002) 

conducted a study examining bat community structure and bat fatalities at the Foote Creek Rim Wind 

Power Facility in south-central Wyoming from 2000 to 2001. He found that although bats in the genus 

Myotis represented approximately 81 percent of all bats captured using mist nets, bat fatalities consisted 

of 22 hoary bats and 1 big brown bat over the course of this study. (Hoary bats and big brown bats are in 

different genera than Myotis.)

Previous to 2003 or 2004, mortalities were reported as numbers of carcasses found and were 

summarized by species. Around 2004, fatality surveys began to address the sampling and statistical 

issues of estimating mortality using carcass surveys. Studies began to incorporate searcher efficiency 

and scavenger removal trials into their methodology. However, there was still a lot of variability in how 

mortality statistics were presented. For example, some studies lumped tree bats and reported an adjusted 

mortality rate for the year and for the entire location (Fiedler, 2004), some reported a rate of bats killed 

for the sampling period of the study [(for example, from April to December 2003; Jain and others 

(2011)], and some reported the number of bats killed per turbine per day and/or per MW per day 

(Grodsky, 2010).

Bat mortality was investigated at Buffalo Mountain Windfarm in eastern Tennessee from 2000

to 2003 (Fiedler, 2004). At this location, eastern red bats made up the majority of bat fatalities (61.3

percent), followed by tricolored bats (22.4 percent), and hoary bats (10.1 percent). An adjusted mortality 

rate for this study and location was estimated to be 20.82 bats/turbine/year. On nights where fresh 

fatalities occurred at turbines, bat activity levels, determined by acoustic monitoring, were greater than 

on nights where no fatalities occurred. However, turbine collisions of red and hoary bats were 

disproportionately greater than the acoustical monitoring would have indicated if acoustic activity was 

directly proportional to fatality risk.
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Jain and others (2011) examined bat mortality and activity at Top of Iowa Wind Resource Area 

in north-central Iowa during 2003–2004. They calculated an adjusted mortality rate for the entire windresource area for each year of the study. In 2003, mortality was estimated to be 396.05 plus or minus

71.71 (95 percent CI) bats (4.45 bats/turbine, 4.94 bats/MW) and in 2004, that estimate increased to 

635.83 plus or minus 111.89 (95 percent CI) bats (7.14 bats/turbine, 7.94 bats/MW). There was no 

difference between bat activity at turbine sites and adjacent crop fields without turbines during both 

years.

Another Midwestern study took place in Wisconsin. Grodsky (2010) studied bird and bat 

mortality at the Forward Energy Center in southeastern Wisconsin. Daily fatality rates were calculated 

for migratory tree bats as 0.158 bats/turbine/day and 0.105 bats/MW/day. In addition to daily fatality 

rates, the author also corrected estimates for tree bat mortality to the number of bats per turbine and bats 

per MW over the entire study in order match the standards from other studies (Jain and others, 2007; 

Kerlinger and others, 2007; Gruver and others, 2009). These estimates were 47.865 bats/turbine/entire 

study and 31.875 bats/MW/entire study. By his estimation, approximately 4,454 total bats, 3,019 

migratory bats, and 912 non-migratory bats were killed during the 2 springs and 2 falls of the study.

Piorkowski and O’Connell (2010) studied the spatial patterns of summer bat mortality from 

collisions with wind turbines at the Oklahoma Wind Energy Center in the southern Great Plains, 

Oklahoma. This study provided the first evidence of a steady rate of collision mortality of Brazilian 

free-tailed bats (Tadarida brasiliensis). Eighty-five percent of all bat fatalities were Brazilian free-tailed 

bats and rates of mortality adjusted for probability of detection and incorporating published removal 

estimates yielded an estimate of 74–92 individuals (0.73–0.90/MW) in 2004 and 67–85 (0.66–

0.83/MW) in 2005. 

Until Miller’s (2008) thesis, no studies of bat mortality at wind-energy facilities had been 

completed in Texas, despite Texas leading the United States in installed wind-energy capacity. Miller 

(2008) looked at patterns of avian and bat mortality at the Red Canyon Wind Energy Center in the 

southern portion of the Texas Panhandle from September 2006 to September 2007. As with Piorkowski 

and O’Connell’s (2010) Oklahoma study, the Brazilian free-tailed bat represented the majority of all bat 

fatalities (94 percent), followed by hoary bats (4 percent), and eastern red bats (2 percent). The author 

classified turbines into three categories: low, medium, and high mortality. Turbines were assigned these 

three mortality categories based on elevation, aspect, and percent of the turbine site that was a part of 

the Caprock Escarpment edge, a distinct palisade-like escarpment. High mortality turbines were 

southwest-facing, less than 853 meters (m) in elevation with greater than 38 percent Caprock 

Escarpment edge within the turbine site boundary. Medium mortality turbines were south-facing, 

between 853 and 865 m in elevation, and with between 34 and 38 percent of the site boundary as 

Caprock edge. Low mortality turbines were southeast-facing, greater than 865 m in elevation, and less 

than 34 percent of the site boundary as Caprock edge. Low mortality turbines were estimated to kill 

15.29 bats/MW, medium mortality turbines were estimated to kill 45.00 bats/MW, and high mortality 

turbines were estimated to kill 106.67 bats/MW. Total mortality was estimated for the entire wind 

energy development to be 36.87 bats/MW. 

At the Altamont Pass Wind Resource Area (APWRA) in California, Smallwood and Karas 

(2008) examined avian and bat mortality rates at older-generation turbines compared to repowered 

(larger and taller) turbines over several years (1998–2003 and 2005–2007). Adjusted fatality rates for 

bats increased from zero at the old vertical-axis turbines to 16.4/year at the new, repowered turbines. 

This was nearly 800 percent greater bat mortality at the new, larger turbines when compared to the 

older-generation turbines. However, the newer turbines were found to reduce bird fatality rates a great 
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deal. The authors extrapolated the average adjusted bat fatality rate from the repowered turbines to a 

completely repowered APWRA and estimated about 454 bat fatalities/year might result. 

Weller and Baldwin (2011) combined the results of continuous echolocation and meteorological 

monitoring at multiple stations to model the conditions that explain the presence of migratory bats at a 

wind energy facility in southern California. Despite low overall bat-activity levels, the authors were able 

to build models that successfully predicted conditions when migratory bats were present. Using acoustic 

detectors and occupancy modeling, they found that occupancy of bats was associated with lower nightly 

wind speeds and higher nightly temperatures, mirroring results from other wind-energy facilities in 

other regions and habitats. The results also highlight the importance of modeling conditions that explain 

bat activity on a seasonal basis. There were marked differences in activity levels seasonally; the relative 

importance of explanatory variables also differed seasonally. The authors conclude that using this 

model-based approach could improve the effectiveness and efficiency of mitigations for bat fatalities at 

wind-energy facilities. If properly conducted, echolocation monitoring and meteorological monitoring 

may provide a reliable index to expected bat fatality levels at wind-energy facilities. 

Canadian bat mortality studies report relatively high fatalities at wind-energy facilities 

(Baerwald and Barclay, 2009; 2011; Jameson, 2010; Klug and others, 2011). Baerwald and Barclay 

(2009; 2011) examined the geographical variation in activity levels and fatality rates of bats at one 

proposed and eight existing wind-energy facilities across southern Alberta, Canada, from 2006 to 2007. 

Corrected fatality rates varied among the 9 sites sampled, from 31.41 plus or minus 3.04 

bats/turbine/year to 1.26 plus or minus 0.18 bats/turbine/year, with higher rates at western wind-energy 

facilities and lower rates at eastern facilities. Fatality rates did not appear to be influenced by migratory 

bat activity at ground level or at 30 m. Jameson (2010) examined the factors influencing bat mortality at 

the St. Leon Wind Energy Project in south-central Manitoba, Canada, in 2008 and 2009. In 2008, 

adjusted mortality rates reported for this study were 11.5 plus or minus 4.9 bats/turbine (7.2 plus or 

minus 3.1 bats/MW) for silver-haired bats, 15.7 plus or minus 5.5 bats/turbine (9.9 plus or minus 3.4 

bats/MW) for hoary bats, and 2.0 plus or minus 1.0 bats/turbine (1.2 plus or minus 0.6 bats/MW) for 

eastern red bats. In 2009, adjusted mortality estimates were 7.7 plus or minus 2.1 bats/turbine (4.6 plus 

or minus 1.3 bats/MW) for silver-haired bats, 5.8 plus or minus 2.1 bats/turbine (3.5 plus or minus 1.3 

bats/MW) for hoary bats, and 1.1 plus or minus 0.8 bats/turbine (0.7 plus or minus 0.5 bats/MW) for 

eastern red bats. 

Fatality Search and Estimation Techniques

Arnett (2006) conducted a baseline effort to assess the efficiency of dog-handler teams to 

recover bat fatalities at two wind-energy facilities in West Virginia and Pennsylvania. Overall doghandler efficiency (percent of trial bats found) for all trials and bats combined, and using combined 

findings from both dogs, was 71 percent at Mountaineer (W. Va.) and 81 percent at Meyersdale (Pa.), 

compared to 42 percent and 14 percent for humans, respectively. Significant variability was found in 

searcher efficiency (both dog-handler and humans) based on vegetative cover, terrain, and amount of 

high-visibility habitat found at the two sites. Although the findings from this pilot study were promising, 

the author suggested that more research was needed to better understand patterns and to account for 

limitations and biases influencing the efficiency of dog-handler teams as a tool to improve fatality 

estimates for bats at wind-energy facilities.

Huso (2010) addressed the problem of estimating numbers of wildlife fatalities from observed 

carcasses. She proposed a fatality estimator that accounts for searcher efficiency, scavenger removal, 

search interval, density-weighted area searched, and visibility within search transects. The proposed 

estimator’s bias and precision were compared to two other estimators in common use during bat-carcass 
8

surveys. Bias is the difference between a population mean of the estimator and an accepted reference or 

true value (Bainbridge, 1985). Precision is the measure of the statistical variance of an estimation 

procedure (West, 1999). None of the three estimators compared were unbiased under all conditions. 

However, bias in the proposed estimator never exceeded plus or minus 27 percent, whereas bias in the 

other two estimators was always negative and exceeded that of the proposed estimator in 98 percent and 

93 percent of the simulated conditions, respectively. The proposed estimator was relatively robust to 

variation in sources and magnitudes of imperfect detectability, but was sensitive to distributional 

assumptions regarding carcass-removal rates and searcher efficiency. The proposed estimator offers 

significant improvement over two current estimators and provides relatively unbiased estimates of 

fatality that can be applied under a variety of conditions and survey protocols.

Migration

Four publications examined migration patterns of bats in relation to wind-energy facilities. A 

fifth paper provided a short review of how certain aspects of bird migration could be useful for 

designing research on bat migration and wind-energy interactions (Larkin, 2006). Reynolds (2006) 

sampled spring migration at the Maple Ridge Wind Project in the Tug Hill Plateau ecoregion of western 

New York in 2005. He sampled migratory activity using acoustic monitoring at two sites and found that 

most of the migratory bat activity occurred at minimum wind speeds below 1.3 meters per second (m/s). 

He also found that days with high bat-migration activity had a mean maximum temperature of 23.9 plus 

or minus 4.4°C and lower bat-migration activity at temperatures of 9.8 plus or minus 4.8°C. Acousticmonitoring results from this study strongly suggested that the Tug Hill Plateau region did not support a 

large bat population in terms of either species diversity or total bat abundance. Most of the migratory bat 

activity occurred at wind speeds below the cut-in speeds of a typical commercial wind turbine. 

However, this study did not sample fall migration: the period in which most bat fatalities occur at windenergy facilities, including the high fatality rates observed at the Maple Ridge Wind Farm where 

Reynolds monitored.

Cryan and Brown (2007) investigated migration of bats at the Southeast Farallon Island off the 

coast of California west of the Bay Area. They analyzed a 38-year (yr) data set of observations of hoary 

bats arriving and departing on this island. They also investigated whether moonlight and weather 

conditions influenced the occurrence of bats on the island. They found that hoary bats were observed 

more frequently in September than any other month. Bats were more likely to arrive and depart from the 

island on evenings when wind speeds were relatively low, during darker phases of the moon, and during 

overcast nights. These results are relevant to the problem of bat fatalities at wind turbines because (1) 

arrivals of migrating tree bats at particular sites may be predictable events, (2) there was conclusive 

evidence that tree bats migrate over offshore water where future construction of wind turbines has been 

proposed, (3) migratory hoary bats use vision to navigate during migration and are drawn toward visual 

stimuli, (4) there was a clear relationship between the timing of autumn migration by hoary bats and 

collisions with anthropogenic structures, and (5) there was a possible behavioral explanation for bat 

collisions with wind turbines.

Johnson and others (2011a) examined the seasonal bat activity on Assateague Island National 

Seashore off the coast of Maryland, in 2005 and 2006. They used acoustic monitoring stations at three 

locations on the island to sample bat activity. More than half the total bat passes were from eastern red 

bats (59.33 percent), followed by big brown bats (3.05 percent), hoary bats (0.10 percent), tricolored 

bats (0.10 percent), and silver-haired bats (0.07 percent). They found that bat activity generally 

increased in April, peaked in August, and declined gradually until December. Bats were less active 

during periods of low temperatures and high wind speeds, possibly above thresholds for wind turbine 
9

cut-in speed. The results from this study provided evidence that bats migrate along the Atlantic coastline 

and use barrier islands, possibly for navigation or as stopover sites, and this has implications for 

proposed offshore wind-energy facilities. 

Johnson and others (2011b) examined seasonal and geographic trends in acoustic detection of 

migrating tree-roosting bats at one existing and thirteen proposed wind-energy facilities in seven eastern 

states in the United States between April and November of 2007 and 2008. Acoustic surveys were 

placed into one of three geographic regions to allow for latitudinal comparisons: northeast, northern 

Allegheny Plateau, and mid-Atlantic. Nightly silver-haired, hoary, and eastern red bat 

calls/detector/night at two proposed sites in West Virginia were also compared to the number of 

mortalities at Mount Storm Wind Energy Facility, W. Va., using linear correlations. The authors found 

that nightly detection rates of silver-haired and hoary bats at the two sites in West Virginia were 

positively correlated with the number of dead silver-haired and hoary bats at the Mount Storm Wind 

Facility. However, no significant correlation was found between detection rate of eastern red bats and 

the mortality at Mount Storm. The nightly detection rates of silver-haired, hoary, and eastern red bats 

during fall migration peaked earlier in the year in the two northernmost geographic regions compared to 

the mid-Atlantic region. Conversely, nightly detection rates of the three species during the spring 

migration peaked earlier in the year in the mid-Atlantic region, compared to geographic regions farther 

north. In all regions, eastern red bats were more frequently detected later in the year than were hoary 

bats. This has implications for wind-energy facilities, in that the seasonal detection rates of hoary bats, 

silver-haired bats, and eastern red bats likely reflect their different migratory patterns, and these patterns 

may be useful in predicting the timing of mortality events at turbines. 

Mitigation and Curtailment

Two publications examined mitigation and curtailment at wind-energy facilities to reduce the 

number of bat fatalities. Baerwald and others (2009) conducted a mitigation experiment at a wind 

energy facility in southwestern Alberta, Canada. Normal operating turbines at this facility had a cut-in 

speed of 4 m/s: the wind speed at which the turbine begins to generate electricity. The authors

specifically tested the hypothesis that reducing the amount that turbine rotors turn in low wind speeds 

will reduce bat fatalities. They operationally altered randomly chosen turbines in two different ways. 

Fifteen turbines had their rotor start-up speed increased to 5.5 m/s, meaning that the turbines were idle 

and motionless during low wind speeds. Another six turbines were manipulated to change the pitch 

angle of their blades to reduce rotor speed in low wind, thereby lowering the generator speed required to 

start energy production, which caused turbines to be motionless similar to the other experimental 

treatment but with different implications for turbine operations. Both sets of experimental turbines killed 

fewer bats than did the control (normal operating) turbines, and there was no difference between the two 

experimental treatments. The corrected fatality rates for each species of migratory bat were reduced by 

between 50 percent and 70 percent at the experimental turbines. The effect of changes in operation was 

different between the two experimental turbines, with the increased rotor start-up speed (5.5 m/s) being 

more costly than changing the pitch angle of the blades.

Arnett and others (2011) also tested the effectiveness of raising the wind-turbine cut-in speed to 

reduce bat fatality. Wind-turbine cut-in speed was defined to be the lowest wind speed at which turbines 

generate power to the utility system. Raising the wind-turbine cut-in speed renders turbines nonoperational until the higher cut-in speed is reached; then turbines begin to spin and produce power. This 

study took place at the Casselman Wind Project in Somerset County, Pa., during 2008 and 2009. Three 

turbine treatments were defined and assigned (in a randomized block design) to 12 turbines. Treatments 

were that turbines became operational and began to spin with (1) a cut-in speed of 3.5 meters per second 
10

(m s-1; normal operations), (2) cut-in speed of 5.0 m s-1, and (3) cut-in speed of 6.5 m s-1. Treatmentrelated mortality was measured as the sum of all individual carcasses of bats estimated to have been 

killed during the previous night observed along transects near the turbines. The results of this study 

found no difference between the number of fatalities for the two raised cut-in speed treatments for both 

years. Observed bat mortality at fully operational turbines was, on average, 5.4 (2008) and 3.6 (2009) 

times greater than mortality associated with curtailed (that is, non-operating) turbines. The authors 

conclude that relatively small changes to wind-turbine operation resulted in nightly reductions in bat 

mortality, ranging from 44 percent to 93 percent, with marginal annual power loss (less than or equal to

1 percent of total annual output). 

In both of the curtailment studies described above, all species of bats were combined, and 

species-specific effects of curtailment are unknown. Most fatalities at these curtailment sites were 

migratory tree bats.

Review Papers and Syntheses

Eleven papers found during this literature search were reviews or syntheses of bats and windenergy issues. The first review paper published was in 2005 (Johnson, 2005). As of 2004, only 11 

studies had been conducted in the United States to estimate fatality rates of bats at wind farms (Johnson, 

2005). The numbers of fatality studies continued to increase in succeeding years. Five synthesis and 

technical guidance papers were published in 2007 (Arnett and others, 2007; Barclay and others, 2007; 

Kunz and others, 2007a; 2007b; Kuvlevsky and others, 2007). By 2006, Arnett and others (2008) were 

able to summarize the results from 21 studies of patterns of bat fatalities at 19 wind-energy facilities

across North America. The patterns of bat fatalities that emerged from these various syntheses provided 

the scientific community the ability to construct numerous hypotheses about the causal reasons bats 

collide with turbines. These hypotheses and their resulting predictions were summarized by Cryan and 

Barclay (2009). Sovacool (2009) compared bird fatalities from wind energy, fossil fuel, and nuclear 

electricity in an attempt to contextualize the negative effects to bird populations. According to the 

author, fossil-fueled facilities are about 17 times more dangerous to birds on a per Gigawatt hour (GWh)

basis than wind and nuclear power stations. Bats were included in the title of this paper; however, they 

were lumped with birds, and their unique life-history strategies were not considered in the paper. Willis 

and others (2010) provided a response to Sovacool (2009) that examined the various problems with 

lumping birds and bats together when comparing mortality resulting from the three methods of 

electricity generation. The most recent review publication was Cryan (2011), and his review 

summarizes bat mortality caused by wind turbines and the potentially significant effects of this mortality 

on certain bat populations.

The following general patterns and findings common to all of the review papers are listed below:

• Bat-fatality rates at wind turbines are variable across sites and regions of North America. Estimates 

of bat fatalities were highest at wind-energy facilities located on forested ridges in the eastern 

United States and the prairies of eastern Canada, and were lowest in the Rocky Mountain and 

Pacific Northwest regions.

• Bat-fatality rates range from just below one bat per installed megawatt per year (bats/MW/year) to 

as high as 70 bats/MW/year.

• Three species of migratory tree bats compose the majority of fatalities (hoary, eastern red, and 

silver-haired bats), and hoary bats compose about half of all documented fatalities in North America.
11

• Silver-haired bat fatalities were more frequently found in Canada, Iowa, and the Pacific Northwest 

relative to the eastern United States, whereas eastern red bats were commonly found in eastern 

forested sites.

• In Texas and Oklahoma, wind-energy development has the potential to negatively impact migratory 

Brazilian free-tailed bats (see Miller, 2008; Piorkowski and O’Connell, 2010).

• Fatalities were skewed toward males of the four most commonly killed species at most facilities.

• Most fatalities occur during a period of time that coincides with autumn migration, although few 

studies spanned the entire season when bats were active.

• Larger, taller turbines killed more bats per turbine when compared to smaller turbines. The highest 

bat-fatality rates occurred at turbines with towers of 65 m or taller.

• Bats collide with spinning turbine blades and do not strike stationary blades or towers.

• The majority of bats appeared to be killed on low wind-speed nights when power production was 

proportionally low but turbine blades were still moving, often at or close to fully operational speed.

• There appeared to be no difference in bat-fatality rates between turbines equipped with Federal 

Aviation Administration (FAA) lights and those that were unlit.

• Factors influencing fatality rates remain unclear, and it is not known yet how to identify and avoid 

high-risk sites for bats and wind energy.

• Population sizes of migratory tree bats are unknown because of their cryptic habits and limitations in 

how their numbers can be estimated. There is currently no established method of assessing how 

quickly the current rates of bat mortality at turbines could cause population declines.

• Studies in Canada and the United States have independently shown that curtailment, or preventing 

turbine blades from turning during relatively low wind speeds in late-summer and autumn, can 

reduce bat fatalities by as much as 40–90 percent.

II. Annotated Bibliography 

The following is an annotated bibliography of 36 peer-reviewed publications and five 

unpublished theses in alphabetical order. For research papers, annotations include an overview of 

objectives, methods, key findings, and conclusions. For review or synthesis papers, the main highlights 

are provided.

Arnett, E.B., 2006, A preliminary evaluation on the use of dogs to recover bat fatalities at wind-energy 

facilities: Wildlife Society Bulletin, v. 34, p. 1440–1445.

This study was a baseline effort to assess the efficiency of dog-handler teams to recover bat 

fatalities at two wind-energy facilities in West Virginia and Pennsylvania. Objectives: (1) to train dogs 

to find bat carcasses; and, (2) to conduct pilot studies to determine the search efficiency of dog-handler 

teams under different vegetation conditions. Methods: The study took place at Mountaineer Wind 

Energy Center in West Virginia and the Meyersdale Wind Energy Center in Pennsylvania. The author 

trained two chocolate labs, one male and one female, for seven days prior to field testing. Dogs were 

trained by seeding a transect with bat carcasses representing different species at various stages of decay. 

Search efficiency of the dog-handler team was tested simultaneously with human searchers during 

scheduled trials at the two wind-energy facilities in late summer 2004. 

Key Findings: 
12

• Overall dog-handler efficiency (percent of trial bats found) for all trials and bats combined, and 

using combined findings from both dogs, was 71 percent at Mountaineer and 81 percent at 

Meyersdale, compared to 42 percent and 14 percent for humans, respectively. 

• Quite a bit of variability was found in searcher efficiency (both dog-handler and humans) based on 

vegetative cover, terrain, and amount of high-visibility habitat found at the two sites. 

Conclusions: Although the findings from this pilot study were promising, more research is warranted to 

better understand patterns and account for limitations and biases influencing the efficiency of doghandler teams as a tool to improve fatality estimates for bats at wind-energy facilities.

Arnett, E.B., Inkley, D.B., Larkin, R.P., Manes, S., Manville, A.M., Mason, J.R., Morrison, M.L., 

Strickland, M.D., and Thresher, R., 2007, Impacts of wind-energy facilities on wildlife and wildlife 

habitat: Bethesda, Md. , The Wildlife Society, Wildlife Society technical review 07-1, 

This technical review summarizes information on the impacts of wind-energy facilities on

wildlife and wildlife habitat, including state and federal permitting processes, wildlife fatality, habitat 

loss and modification, animal displacement and fragmentation, offshore development, and issues 

surrounding monitoring and research methodology, including the use of technological tools. In the 

review, a summary section is provided on bats and bat fatalities from wind turbines. This review 

provided 12 recommendations that should help managers and decision-makers meet the challenges of 

developing wind energy responsibly: (1) improve state agency involvement and consistency for 

requirements and regulation; (2) renewable portfolio standards (for example, regulations that require 

increased production of energy from renewable energy sources, such as wind, solar, biomass, or 

geothermal); (3) develop federal and state guidelines; (4) avoid siting wind facilities in high-risk areas; 

(5) reduce fragmentation and habitat effects; (6) conduct priority research; (7) evaluate pre-construction 

assessments and predicted impacts; (8) conduct more consistent, longer-term studies; (9) develop and 

evaluate habitat-related mitigation strategies; (10) employ principles of adaptive management; (11) 

conduct regional assessments and forecasting of cumulative land-use and impacts from energy 

development; and, (12) improve public education, information exchange, and participation.

Arnett, E.B., Brown, W.K., Erickson, W.P., Fiedler, J.K., Hamilton, B.L., Henry, T.H., Jain, A., 

Johnson, G.D., Kerns, J., Koford, R.R., Nicholson, C.P., O’Connell, T.J., Piorkowski, M.D., and 

Tankersley, R.D., Jr., 2008, Patterns of bat fatalities at wind-energy facilities in North America: 

Journal of Wildlife Management, v. 72, p. 61–78. 

This synthesis examined the patterns of bat fatalities from post-construction studies at windenergy facilities. Objectives: (1) present unifying and unique patterns from the studies; (2) discuss the 

scope, biases, and limitations of existing efforts; and, (3) identify information gaps and offer 

suggestions for future research needed to develop mitigation strategies to minimize or eliminate bat 

fatality at wind facilities. Methods: Authors reviewed 21 studies from 19 different wind-energy facilities

in 5 regions of the United States and one province in Canada. These studies took place from 1996 to 

2006 and represent only those published in peer-reviewed journals or unpublished reports available 

publicly. 

Key Findings: 

• The duration of most studies was short. 

• Only eight studies used bat carcasses to quantify searcher bias and scavenger removal. 
13

• Most studies calculated an overall bias correction factor for the entire study period and across 

different habitat conditions. 

• Estimates of bat fatalities were highest at wind-energy facilities located on forested ridges in the 

eastern United States and lowest in the Rocky Mountain and Pacific Northwest regions. 

• In most regions and individual studies, bat fatalities were heavily skewed to migratory foliageroosting species including hoary bats (Lasiurus cinereus), eastern red bats (L. borealis), and 

migratory tree and cavity roosting silver-haired bats (Lasionycteris noctivagans). 

• Hoary bats constituted high proportions of fatalities at most facilities. 

• Silver-haired bats were found more frequently in Canada, Iowa, and the Pacific Northwest relative 

to the eastern United States, whereas eastern red bats were commonly found in eastern forested sites 

in the midwestern United States. 

• Fatalities were skewed toward males for the four most commonly killed species at most facilities.

• The highest bat fatalities were consistently reported during late summer and early fall, although few 

studies spanned the entire season when bats are active (generally April–November). 

• Evidence was found that larger, taller turbines killed more bats per turbine compared to smaller 

turbines. 

• Findings from these studies support the contention that bats collide with spinning turbine blades and 

do not strike stationary blades or towers. 

• The majority of bats appear to be killed on low wind nights when power production seemed 

insubstantial but turbine blades were still moving, often at or close to full operational speed. 

• There appeared to be no statistically significant differences in fatality between turbines equipped 

with FAA lights and those that were unlit. 

Conclusions: The authors end by addressing the limitations of the studies reviewed: (1) most work 

conducted was short term, and the frequency of the study was inadequate to assess seasonal and annual 

variability; (2) fatality estimates could not be directly compared among studies because of different 

sampling protocols; (3) sizes of search plots varied among studies; (4) search effort, intensity of 

searches, and experience of observers varied across studies; and, (5) most studies failed to adequately 

account for habitat variability during fatality searches. 

Arnett, E.B., Huso, M.M.P., Schirmacher, M.R., and Hayes, J.P., 2011, Altering wind turbine speed 

reduces bat mortality at wind-energy facilities: Frontiers in Ecology and the Environment, v. 9, p. 

209–214.

This paper describes an experiment to test the effectiveness of raising the turbine cut-in speed to 

reduce bat mortality at a wind facility in Pennsylvania during 2008 and 2009. Wind-turbine cut-in speed 

was defined to be the lowest wind speed at which turbines generate power to the utility system. Raising 

the wind-turbine cut-in speed renders turbines non-operational until the higher cut-in speed is reached;

then turbines begin to spin and produce power. Objectives: (1) to determine if rates of bat fatality 

differed between fully operational turbines with a cut-in speed of 3.5–5.0 m s-1 and turbines with raised 

cut-in speeds of either 5.0 m s-1 and 6.5 m s-1, and (2) to quantify the economic costs of different 

curtailment programs and timeframes. Methods: The study took place at the Casselman Wind Project in 

Somerset County, Pa., over a 2-yr period (2008–2009). Three turbine treatments were defined and 

assigned in a randomized block design to 12 turbines. Treatments were (1) fully operational, (2) cut-in 

speed of 5.0 m s-1, and (3) cut-in speed of 6.5 m s-1. Treatment-related mortality was measured as the 

sum of all individual carcasses of bats estimated to have been killed during the previous night observed 

along transects near the turbines. 
14

Key Findings: 

• No difference was found between number of fatalities for the two cut-in speed treatments for both 

years. 

• Observed bat mortality at fully operational turbines was, on average, 5.4 (2008) and 3.6 (2009) 

times greater than mortality associated with curtailed (that is, non-operating) turbines. 

Conclusions: Relatively small changes to wind-turbine operation resulted in nightly reductions in bat 

mortality, ranging from 44 percent to 93 percent, with marginal annual power loss (less than or equal to

1 percent of total annual output). 

Baerwald, E.F., and Barclay, R.M.R., 2009, Geographic variation in activity and fatality of migratory 

bats at wind-energy facilities: Journal of Mammalogy, v. 90, p. 1341–1349.

This study examined geographic variation in activity levels and fatality rates of bats at one 

proposed and eight existing wind-energy facilities across southern Alberta, Canada. The authors 

hypothesized that migratory bats use specific migration routes associated with landmarks and roost 

availability. They predicted that migratory bat activity would vary among the different habitats and that 

bat fatalities at wind turbines would correlate with activity rate. Objectives: to determine if bat activity 

and fatality were concentrated in certain areas or evenly distributed across the landscape. Methods: Bat 

activity was monitored acoustically from July 15 to September 15, 2006 and 2007, at seven proposed or 

existing wind-energy installations. Anabat II detectors were installed in pairs (one approximately 1.5 m 

from the ground and the other 30 m from the ground) on meteorological towers. The authors compiled 

fatality data collected between 2001 and 2007 from nine wind-energy facilities. They also searched for 

bat carcasses at two sites from July 15 to September 30, in 2006, and 2007, and at another site from July 

15 to September 30, 2007. Fatality rates were adjusted per turbine for searcher efficiency and scavenger 

removal rates. 

Key Findings: 

• Bat activity varied among sites with the highest activity of migratory-bat passes detected at Site 1, 

the westernmost site near the Rocky Mountains. 

• Pooled across all sites, total migratory bat activity did not differ between detection heights. 

However, when separated by species, the activity of hoary bats and silver-haired bats differed 

between the two detection heights with hoary bats being less common at ground level than at 30 m 

and silver-haired bats more common at ground level than at 30 m. 

• Searcher efficiency was estimated to be 78 percent and carcasses lasted an average of 5.6 days. 

• Corrected fatality rates varied among the nine sites sampled, from 31.41 plus or minus 3.04 

bats/turbine/year to 1.26 plus or minus 0.18 bats/turbine/year, with higher rates at western windenergy facilities and lower rates at eastern facilities. Fatality rate increased with turbine height. 

• Fatality rate was not influenced by migratory bat activity at ground level, migratory bat activity at 

30 m, or the interaction between migratory bat activity at ground level and turbine height.

Conclusions: Migratory bats in southern Alberta appear to be concentrating along select routes and 

activity rates of both hoary and silver-haired bats were higher near the foothills of the Rocky Mountains 

to the west than on the prairie grasslands farther east. Fatality rates of bats varied among all sites, partly 

due to differences in turbine tower height, but also due to the differences in activity of migratory bats at 

30 m. The results of this study indicated that factors in addition to turbine height and migratory bat 

activity influence fatality rates at wind turbines. The authors suggested that the number of turbines may 

play a role: facilities with many turbines may experience a dilution effect with lower fatality rates per 

turbine than facilities with few turbines. 
15

Baerwald, E.F., and Barclay, R.M.R., 2011, Patterns of activity and fatality of migratory bats at a 

wind energy facility in Alberta, Canada: Journal of Wildlife Management, v. 75, p. 1103–1114.

Using acoustic monitoring and carcass searches, the authors examined temporal and spatial 

variation in activity levels and fatality rates of bats at one wind-energy facility in southern Alberta, 

Canada, in 2006 and 2007. Objectives: to test the hypotheses regarding the influence of weather 

variables and turbine characteristics on migratory bat activity and fatality. The authors predicted that 

weather conditions conducive to migration would increase activity of migratory bats and therefore

would increase fatality rate. They also predicted that weather and turbine characteristics would affect 

fatality rate independent of the number of bats migrating on a particular night (for example, bats 

attracted to turbines). Methods: Weather data were collected from a meteorological tower located on the 

wind energy facility and included wind speed and direction data at 50 m above ground level and 

temperature and barometric readings at 2 m above ground-level. Activity was monitored using Anabat II 

detectors on meteorological towers and turbine nacelles from July 15 to September 30, 2006 and 2007. 

Nacelles are the covers at the top of the turbine that house all of the generating components of a wind 

turbine, including the generator, gearbox, drive train, and brake assembly. Carcass searches were also 

conducted during the same time periods for both years and were adjusted for searcher efficiency and 

scavenger removal rates. Four different statistical analyses examined whether: (1) activity rate varied 

with weather; (2) variation in fatality rate was influenced by turbine location and characteristics; (3) 

variation in fatality rate was influenced by activity rate; and finally, (4) fatality rate depended on 

weather variables, while controlling for variation in activity of bats. 

Key Findings: 

• Activity of migratory bats was greater in low wind speeds and was lower when the wind was from 

the north or northeast. Hoary bats were more active with falling barometric pressure but were not 

influenced by any other weather variable. Silver-haired bat activity was greater in warmer ambient 

temperatures and low wind speeds and decreased when the wind was from the north or northeast.

• Fatality rates differed among turbines; higher rates were found at turbines in the north end of the 

facility in comparison to the south end. There were no differences in fatality rates at turbines on the 

east side versus the west side of the facility, at turbines on the end of a row versus in the middle of a 

row, or at turbines lit by aviation lighting versus those unlit. 

• Bat fatalities increased with total migratory bat activity; however, when separated by species, hoary 

bat fatalities per night did not increase with their activity and silver-haired bat fatalities per night did 

increase with their increased activity. 

• Total nightly fatalities increased with increased moon illumination and were influenced by the 

interaction between barometric pressure change and the activity of migratory bats. Although fatality 

rates increased as activity increased, if activity was low and there was a large drop in barometric 

pressure, the fatality rate was high. Hoary bat fatality rate increased with falling barometric pressure 

but not with any other variable or interaction tested. Silver-haired bat fatality rate increased with 

increasing activity, increasing fraction of moon illuminated, and southeasterly winds.

Conclusions: Migration behavior of bats is a very complex issue, but it offers potential ways to reduce 

fatalities at wind-energy facilities if we can understand the relationships between weather, activity, 

and fatality. As an example, if increasing moon illumination and falling barometric pressure increase 

the risk of fatality, then matching these parameters with changes to wind-turbine operations might 

reduce fatalities. 
16

Baerwald, E.F., D’Amours, G.H., Klug, B.J., and Barclay, R.M.R., 2008, Barotrauma is a significant 

cause of bat mortalities at wind turbines: Current Biology, v. 18, p. 695–696.

The authors describe a study where they found that barotrauma was a significant cause of bat

fatalities at wind turbines in southwestern Alberta, Canada. The decompression hypothesis, as described 

by Baerwald and others, 2008, proposes that bats are killed by barotrauma caused by rapid air-pressure 

reduction near moving turbine blades. Objective: to test the decompression hypothesis. Methods: Hoary 

bats and silver-haired bats killed at a wind facility were examined for external and internal injuries. 

Key Findings: 

• Of 188 bats killed at turbines the previous night, 87 had no external injuries that would have been 

fatal. 

• Of 75 fresh bats necropsied, 32 had obvious external injuries, but 69 had hemorrhaging in the 

thoracic and/or abdominal cavities. 

Conclusions: Barotrauma helps explain the high fatality rates of bats at some wind-energy facilities. 

Even when echolocation allows bats to avoid turbine blades, they may still be incapacitated or killed by 

the internal injuries caused by rapid pressure reductions.

Baerwald, E.F., Edworthy, J., Holder, M., and Barclay, R.M.R., 2009, A large-scale mitigation 

experiment to reduce bat fatalities at wind-energy facilities: Journal of Wildlife Management, v. 73, p. 

1077–1081.

This paper describes the results of a large-scale mitigation experiment to reduce bat fatalities at a 

wind facility in southwestern Alberta, Canada, during the peak period of migration for hoary and silverhaired bats in 2006 and 2007. Normal operating turbines at this facility had a cut-in speed (the wind 

speed at which the turbine begins to generate electricity) of 4 m/s. Below that wind speed, the turbine 

rotor rotates at a slow rate that increases with wind speed until the rotor is turning at a rate which 

triggers generator rotation, a wind speed of 4 m/s. Objective: to test the hypothesis that reducing the 

amount that turbine rotors turn in low wind speeds reduces bat fatalities. Methods: Twenty-one 

randomly chosen turbines were operationally altered in two ways in 2007. Fifteen had their rotor startup speed increased to 5.5 m/s, meaning that the turbines were idle and motionless during low wind 

speeds. The other six turbines were manipulated to change the pitch angle of their blades to reduce rotor 

speed in low wind, thereby lowering the generator speed required to start energy production, which 

caused turbines to be motionless. This procedure was similar to the other experimental treatment but had 

different implications for turbine operations. Fatality searches were conducted once per week using 

transects and two searchers. Corrected fatality rates were compared between experimental and control 

turbines both in 2006 (when no experiment was conducted) and in 2007. 

Key Findings: 

• In 2006, there was no difference in corrected bat-fatality rates between turbines later selected as 

experimental or control. 

• In 2007, both sets of experimental turbines killed fewer bats than did control turbines, and there was 

no difference between the two experimental treatments. 

• Corrected fatality rates for each species of migratory bat were reduced by between 50 percent and 70

percent at the experimental turbines. 

• By increasing the rotor start-up wind speed, the amount of time the turbines produced electricity was 

reduced by an average of 42.3 percent. 
17

Conclusions: The two experimental changes made to operation of turbines had a similar effect on their 

operation at low wind speeds and therefore resulted in lower bat fatalities. However, the effect of 

changes in operation was different in terms of costs with increased rotor start-up speed (5.5 m/s) being 

more costly than changing the pitch angle of the blades.

Barclay, R.M.R., Baerwald, E.F., and Gruver, J.C., 2007, Variation in bat and bird fatalities at windenergy facilities: assessing the effects of rotor size and tower height: Canadian Journal of Zoology, v. 

85, p. 381–387. 

The size and height of wind turbines have increased over the years, from 18-m diameter rotors 

on top of 24-m towers, to the newest turbines with 90-m diameter rotors and towers as tall as 94 m.

Objective: to test the hypothesis that wind turbine size and height influence fatality rates of bats and 

birds and to determine whether the effect differs between the two groups. Methods: Authors compiled 

data on the characteristics of 33 North American wind-energy facilities and on bat and bird fatalities. 

Data were available from some published scientific papers, but primarily from unpublished government, 

industry, and consultant reports, and from personal communications. Fatality rates were determined 

based on searches for carcasses under turbines. 

Key Findings: 

• Increased rotor-swept area was not a significant factor in fatality of birds or bats. 

• Fatality rates of both bats and birds were relatively low at short turbines (less than 65 m high), but 

bat fatalities increased exponentially with turbine height, while bird fatalities did not change. 

• The highest bat-fatality rates occurred at turbines with towers 65 m or taller. 

Conclusions: Increased rotor diameter and tower height of turbines may result in greater energy output 

per turbine, but these larger turbines have higher bat-fatality rates than bird-fatality rates.

Cryan, P.M., 2008, Mating behavior as a possible cause of bat fatalities at wind turbines: Journal of 

Wildlife Management, v. 72, p. 845–849.

This commentary proposes a hypothesis that tree bats collide with turbines while engaging in 

mating behaviors that center on the tallest trees in a landscape. The author further proposes that these 

behaviors occur because of two different mating systems: resource defense polygyny and lekking. Key 

points: Tree bats may use the highest trees in a landscape as rendezvous points during the mating period,

and this behavior places them at risk of collision with the blades of wind turbines. Reproductive bats 

could be attracted to the turbines when looking for mating opportunities and mistake the turbines for the 

tallest trees. Male European tree bats have been observed defending roosting structures needed by 

females during the mating season (late summer and autumn). This mating strategy is called resource 

defense polygyny. However, tree bats in North America (species of Lasiurus or lasiurines) differ in 

roosting habits and behavior from European species because they roost alone or in small family groups 

in tree foliage during spring and early summer. Foliage roosts used by female lasiurines during fall 

migration are most likely abundant and not easily defended by male bats. Lekking is a mating system 

for species in which females range widely or resources (food or roost) are too finely distributed for 

males to defend. Lekking could explain why so many lasiurines are recovered in greater number and 

more frequently than any other species of bats killed by wind turbines in North America. An interesting 

characteristic of lekking that makes this high casualty rate among lasiurines plausible is that males tend 

to create dense aggregations during late summer and autumn to sites visited by females for mating. 

Conclusions: Research focused on hypotheses of attraction should be prioritized. If courtship and 
18

mating activities occur at wind turbines, turbines have the potential to create population sinks that attract 

bats from far away, and selectively kill bats that are primed for reproduction.

Cryan, P.M., 2011, Wind turbines as landscape impediments to the migratory connectivity of bats: 

Environmental Law, v. 41, p. 355–370.

This review summarizes bat mortality caused by wind turbines and the potentially significant 

effects of this mortality on certain bat populations. The article includes an overview of bat migration, 

discusses natural and human-caused mortality of migrating bats, provides background on bat fatalities at 

wind turbines, and highlights the importance of future innovative research. 

Key points: 

• Many species of temperate-zone bats migrate long distances; however, the details of their migratory 

movements and seasonal whereabouts are mostly lacking. This information is lacking for several 

reasons: (1) bats migrate at night; (2) they are inactive during the day; (3) they roost in concealed 

and frequently inaccessible locations; and, (4) there have been few recorded observations of actively 

migrating bats. 

• Migration in bats most likely evolved in response to their distinctive thermoregulatory strategies and 

life histories. They mate during autumn and winter, and females often migrate in spring while they 

are pregnant, with some moving to habitats and regions of the continent separate from males during 

the early summer. Many species of bats can also drop their metabolism throughout the year and 

enter torpor to save energy when climate, weather, or food availability is unfavorable. Bats also have 

high rates of adult survival, resulting in longevity. Reproductive success is also high due to their 

putting considerable time and energy into taking care of a small number of young. These life-history 

strategies result in bat populations growing slowly and possessing an inability to quickly rebound 

after rapid declines in population size. 

• Migration can be a perilous endeavor. Migrating bats may experience higher natural mortality than 

species that do not migrate. 

• Collisions of migrating bats with buildings, communication towers, and aircraft have not been an 

obvious conservation issue in the past.

• Wind turbines are an emerging threat to bat migrations. 

• Bat-fatality rates at wind turbines are variable across sites and regions in the United States. Fatality 

rates range from just below one bat per installed megawatt per year to as high as 70 bats per 

megawatt per year. Based on published fatality rates and the number of currently installed wind 

turbines, an estimated 450,000 bats may already perish at turbines each year in North America.

• Three species of migratory tree bats compose the majority of fatalities, and hoary bats compose 

about half of all documented fatalities in North America. 

• Factors influencing fatality rates remain unclear, and studies at sites in North America have not 

found any consistent relationships between local landscape features and fatality rates, nor do we 

have a clear understanding of the regions or types of habitats where fatalities most likely occur. 

Therefore, it is not known yet how to identify and avoid high-risk sites for bats and wind energy. 

Most fatalities occur during a period of time that coincides with mating and autumn migration.

• Population sizes of migratory tree bats are unknown because of their cryptic habits and limitations in 

how their numbers can be estimated. Therefore, there is no way of assessing how quickly the current 

rates of bat fatality at turbines could cause catastrophic population declines and possible species 

extinctions. However, more hoary bats have been estimated to die at certain wind-energy facilities in 
19

the United States in two to three years than have ever been collected and preserved as scientific 

study specimens in the museums of the Americas. 

• Research on the proximate causes for bat fatalities at wind turbines is essential to help gauge the 

gravity of the situation and the need for conservation measures. Turbines may act as barriers to 

migration and disrupt necessary migratory connectivity. They may also act as population sinks and 

bottlenecks. Without knowing the exact reasons why bats are susceptible to turbines, scientists 

trying to find ways to minimize fatalities will continue to grope for solutions. 

• None of the bat species most affected by wind energy are protected by federal conservation laws 

such as the Endangered Species Act (ESA). Neither are these bats protected by legislation pursuant 

to international treaties such as the Migratory Bird Treaty Act. 

• Studies in Canada and the United States have independently shown that curtailment, or preventing 

turbine blades from turning during relatively low wind speeds in late-summer and autumn, can 

reduce bat fatalities by as much as 40 to 90 percent over the course of the studies. 

Conclusion: The author concludes that a legal framework for proactively monitoring and protecting all 

migratory wildlife before their populations become endangered may be the type of foresight needed to 

help drive scientific advances. This may also allow us to better predict and deal with emerging threats to 

migratory wildlife. 

Cryan, P.M., and Barclay, R.M.R., 2009, Causes of bat fatalities at wind turbines—Hypotheses and 

predictions: Journal of Mammalogy, v. 90, p. 1330–1340. 

This synthesis reviews the hypothesized causes of bat fatalities at wind turbines. The hypotheses 

of cause fall into two categories: proximate and ultimate. Proximate causes explain the direct means by 

which bats die at turbines and include bats colliding with turbine towers, colliding with rotating blades, 

or suffering internal injuries after being exposed to rapid pressure changes near the trailing edges and 

tips of the moving blades (barotrauma). Hypotheses of ultimate causes are numerous and include three 

general categories: random collisions, coincidental collisions, and collisions that result because bats are 

attracted to turbines. Random collisions are those that occur due to chance alone or no assumptions of 

circumstance or attraction. Coincidental collisions involve bats being victims of unfortunate behavioral 

circumstances that put them at risk of colliding with turbines (for example, turbines are located along 

migratory pathways). Hypotheses of attraction propose that there is some attractor or combination of 

attractors drawing bats to wind turbines. Table 1 of this publication provides an overview of the 

hypotheses and their corresponding predictions. The authors conclude that the conservation implications 

of bat attraction to wind turbines are by far the most serious of the causes and suggest that testing the 

general hypotheses of attraction be a top research priority. 

Cryan, P.M., and Brown, A.C., 2007, Migration of bats past a remote island offers clues toward the 

problem of bat fatalities at wind turbines: Biological Conservation, v. 139, p. 1–11.

Little is known about the behavior of bats during migration and what aspects of this seasonal 

behavior make them particularly susceptible to mortality from wind turbines. Objective: to analyze a 38-

yr data set of observations on hoary bats on Southeast Farallon Island to determine if moonlight and 

weather conditions influenced the occurrence of bats on the island. The authors also examined the 

temporal relationships of migration events involving hoary bats and similar species, and then related 

those findings to observed bat fatalities at wind turbines. Methods: This study took place at Southeast 

Farallon Island (SEFI), part of the Farallon National Wildlife Refuge. Personnel at the Point Reyes Bird 
20

Observatory recorded daily observations from routine censuses of birds and marine mammals and 

recorded regular observations of hoary bats from as early as 1968. Logistic regression was used to 

model the influence of weather and moonlight on bat arrival to the island and bat departure from the 

island. To summarize the seasonal timing of known events involving migration by species of Lasiurus, 

the authors compiled records from the literature. 

Key Findings: 

• Bats were observed more frequently in September than any other month and usually visited the 

island between the last week of August and the end of October. 

• For describing the arrivals of bats, the model that included wind speed, barometric pressure, cloud 

cover, and moon illumination ranked the best.

• The top-ranked model describing departures included only wind speed, cloud cover, and moonlight.

• Low wind speeds, moon illumination, and relatively high degrees of cloud cover were important 

predictors of both bat arrivals and departures, whereas low barometric pressure was an additional 

condition that helped predict arrivals. 

• The majority of records compiled from the literature occurred on days between early September and 

late October, with a peak in mid-September. 

Conclusions: The results from this study show that bats were more likely to arrive and depart from the 

island on evenings when wind speeds were relatively low. There was a predominance of hoary bat 

arrivals and departures on SEFI during darker phases of the moon and during overcast nights. There 

were five reasons this study’s findings are relevant to the problem of bat fatalities at wind turbines: (1) 

arrivals of migrating tree bats at particular sites may be predictable events; (2) there is conclusive 

evidence that tree bats migrate over offshore waters, on which future construction of wind turbines has 

been proposed; (3) migratory hoary bats use vision to navigate during migration and are drawn toward 

visual stimuli; (4) there is a clear relationship between the timing of autumn migration by species of 

Lasiurus and collisions with anthropogenic structures; and, (5) there is a possible behavioral explanation 

for bat collisions with wind turbines.

Fiedler, J.K., 2004, Assessment of bat mortality and activity at Buffalo Mountain Windfarm, eastern 

Tennessee: Knoxville, Tenn., University of Tennessee, Unpublished Master’s Thesis. 166 pp.

This thesis describes an investigation of bat mortality at Tennessee Valley Authority’s Buffalo 

Mountain Windfarm (BMW) in eastern Tennessee. Objectives: (1) to document the extent of bat 

mortality at BMW and then examine the mortality for spatial, temporal, and species patterns; (2) to 

determine bat activity using bat detectors and mist netting; and, (3) to determine the relationships 

between bat activity and mortality. Methods: Mortality monitoring was conducted during a three-year 

period beginning late September 2000 and concluded the end of September 2003. Carcass searches were 

conducted twice weekly from April 1 through November 1. Logistic regression was used to identify 

possible explanatory variables to predict nights when a fatality event would likely occur. Explanatory 

variables included daily generation of the wind facility, average wind speed, average wind direction, 

average temperature, minimum and maximum temperature between 1800 and 0600 hours, and the 

difference between minimum and maximum temperature. Biases in searcher efficiency, proximity and 

density of scavengers in the area, and changes in vegetation structure were accounted for by conducting 

six search bias trials. Bat activity data were collected with mist-netting and acoustic monitoring (Anabat 

II detectors). 

Key Findings: 

• Over the course of the study, a total of 120 bat fatalities of six species were found. 
21

• Seventy percent of bat fatalities occurred from August 1–September 15. 

• An adjusted mortality rate of 20.82 bats/turbine/year was calculated for BMW.

• Eastern red bats (Lasiurus borealis) made up the majority of bat fatalities overall at 61.3 percent. 

Tricolored bats (Perimyotis subflavus) and hoary bats were the next two most common fatalities 

overall (22.4 percent and 10.1 percent, respectively). The remaining bat fatalities consisted of three 

species: big brown bats (Eptesicus fuscus), silver-haired bats, and one Seminole bat (Lasiurus

seminolus). 

• More males than females were killed. 

• The peaks in bat mortality coincided with months of low winds and less wind-energy-facility

generation. 

• A fatality occurrence was more likely during slightly cooler nights with calmer, less variable winds. 

• Searcher detection rates for carcasses was highly variable and ranged from 9 to 75 percent for the 

six trials. 

• The average number of days a scavenged carcass remained onsite for the six trials ranged from 3.2 

to 16.7 days.

• Thirteen possible scavenger species were identified, and 36 percent of planted carcasses were 

removed from the site before the search the next morning. 

• Acoustical monitoring of bat activity showed that annual bat-activity levels were consistently greater 

during late summer and early fall than spring and summer. 

• Bat activity near water was greater than away from water.

• On nights where fresh fatalities occurred, bat-activity levels were greater than on nights where no 

fatalities occurred.

• Turbine collisions of red bats and hoary bats were disproportionately greater than acoustical 

monitoring indicated at BMW.

Conclusions: Several biases may affect estimates of bat mortality rates at wind facilities. These include 

searcher efficiency, scavenging rates, search interval, background mortality rates, crippling bias, plot 

size, and seasonal vegetation cover. The author recommends that individual wind-energy facilities

estimate these biases and their precision when calculating mortality rates, as both regional and sitespecific variables can influence them.

Grodsky, S.M., 2010, Aspects of bird and bat mortality at a wind facility in southeastern Wisconsin—

Impacts, relationships, and cause of death: Madison, Wis., University of Wisconsin, Unpublished 

Master’s Thesis. 99 pp.

This thesis presents results of a post-construction study of bird and bat mortality at wind turbines 

at the Forward Energy Center in southeastern Wisconsin. The thesis consists of two chapters both 

intended to be submitted for publication in peer-reviewed journals. The first chapter describes a study 

assessing bird and bat mortality at a wind energy facility in southeastern Wisconsin. The second chapter 

investigates the cause of death for wind turbine-associated bat fatalities. The results of the second 

chapter are summarized in the publication following this thesis (see below), so this summary will 

describe the results of the first chapter only. Objectives: (1) to determine estimated mortality rates for 

birds and bats, both migratory and non-migratory; (2) to examine species composition of wildlife 

mortality; and, (3) to correlate variables associated with bird and bat mortality. Methods: This study 

took place in the fall from July 15–October 15, 2008 and 2009, and the spring from April 15–May 31, 

2009 and 2010. Twenty-nine wind turbines were randomly selected and searched for fatalities using a 

stratified sampling technique. Searcher efficiency trials were conducted to estimate percentage of 
22

carcasses not seen and recovered by individual searchers. Scavenger removal rates were estimated using 

removal trials. A mortality estimator was used that accounted for searcher efficiency, scavenger 

removal, search interval, density-weighted area searched, and visibility within search transects. 

Covariates included in the analysis were weather variables, turbine operating status, bat activity data 

from Anabat detectors, and distances to a nearby marsh and mine. A generalized linear mixed model 

was used to look at the relationship between bird and bat mortality and covariates. 

Key Findings: 

• A total of 20 birds were recorded during carcass searches, and the estimated bird-mortality rates 

averaged across all study periods were 0.0255 birds/turbine/day and 0.0170 birds/MW/day. 

• Most bird species recorded as fatalities were nocturnally migrating passerines.

• A total of 122 bat carcasses were found (of five species). 

• More than half of the bat mortalities were hoary bats and silver-haired bats. When eastern red bat 

carcasses were included, the migratory tree bats accounted for 69 percent of the bat mortality. 

• Most bats were adults, and females outnumbered males. 

• Averaged across all study periods, migratory bat mortality rates were 0.158 bats/turbine/day and

0.105 bats/MW/day. Non-migratory bat mortality rates were 0.0382 bats/turbine/day and 0.0255 

bats/MW/day. 

• Most bat mortality occurred from late August through the second week of September. 

• Modeling suggested that temperature affected bat mortality significantly and distance to the marsh 

affected bat mortality marginally. All other covariates modeled did not influence bat mortality.

• Searcher detection rate was 38 percent for bats and 63 percent for birds. 

Conclusions: The underlying trend to all current mortality studies in the Midwest is that there is 

significantly higher bat mortality rates compared to bird mortality rates. 

Grodsky, S.M., Behr, M.J., Gendler, A., Drake, D., Dieterle, B.D., Rudd, R.J., and Walrath, N.L., 

2011, Investigating the causes of death for wind turbine-associated bat fatalities: Journal of 

Mammalogy, v. 92, p. 917–925.

This study investigated the causes of death for wind-turbine-associated bat fatalities at the 

Forward Energy Center in southeastern Wisconsin. Objectives: (1) to use veterinary diagnostic 

procedures to determine the cause of lesions found on bats killed by wind turbines; (2) to correlate 

patterns of injuries with causes of death; and, (3) to create guidelines for future studies. Methods: Bat 

carcasses were collected July 15–October 15, 2009, using techniques described above in Grodsky 

(2010). Thirty-nine bat carcasses were radiographed and evaluated by a board-certified veterinary 

radiologist who was unaware of the postmortem findings. Data recorded included the location, number, 

and type (that is, fracture classification, dislocations) of skeletal injuries and any abdominal or 

diapharagmatic hernias. The thoracic cavity was examined for radiographic signs of pneumothorax (air 

in the thoracic cavity), hemothorax (blood in the thoracic cavity), chest wall trauma, atelectasis, or 

pleural fluid accumulation. Eighteen bat carcasses were also dropped off the top of the turbine nacelle 

91.44 m above ground level to see whether bats could break bones from a free fall. These dropped bats 

were then radiographed and the same veterinary radiologist reviewed the radiographs. Bats were also 

necropsied, and histologic sections of the lung and middle ear tissues were made. All brain samples 

were also tested for rabies. 

Key Findings: 

• Of the 39 bats radiographed, at least one broken bone was detected in 74 percent of the bats. 

Fractures to the wing bones were the most common, and the majority of the fractures were
23

comminuted (that is, shattered or crushed into many pieces). Fractures to the skull, scapula, lumbar 

vertebrae, ribs/sternum, and pelvis were the next most common injuries.

• Of the 18 bats dropped, no new bones were fractured in most of the bats postdrop (80 percent). 

• Of 33 bats necropsied, the most common lesions found were pneumothorax and hemothorax. An 

equal mix of inguinal and diaphragmatic hernias was the next most common injury. 

• Of the 28 bats processed for histopathology, 5 (21 percent) had no pulmonary hemorrhage, 3 (13

percent) had mild hemorrhages, and 8 (33 percent) had moderate and severe hemorrhages.

• Middle ears from 24 bats were sampled and 11 (48 percent) bats had no hemorrhage in middle ears 

or surrounding structures, 4 (17 percent) bats had middle ear hemorrhage, and 8 (35 percent) bats 

had hemorrhage in and around the middle or inner ears.

• All bats tested negative for rabies.

• Thirty-eight bat-carcass locations were measured relative to distance from base of the turbine. The 

majority of the bats (71 percent) were found within 30 m of the base of the turbine. The highest 

percentage of bats (29 percent) was found within 10 m of the base of the turbine. 

Conclusions: The exact cause of death (that is, barotrauma or direct collision) could not be determined 

in most bats due to the variability of injuries and a lack of exclusively attributable lesions. The authors 

suggest that the cause of death for bats killed at wind turbines is not exclusively or predominantly 

barotrauma or direct collision but rather an indiscernible combination of both. Visually inspecting bat 

carcasses is not adequate for conclusively diagnosing fatal injuries, including broken bones. The authors 

conclude that there may be delayed lethal effects of wind turbines on bat mortality. Bats could become 

crippled by either inner ear damage or by bone fractures, but survive long enough to exit the search area 

of mortality studies. 

Gruver, J.C., 2002, Assessment of bat community structure and roosting habitat preferences for the 

hoary bat (Lasiurus cinereus) near Foote Creek Rim, Wyoming: Laramie, Wyo., University of 

Wyoming, Unpublished Master’s Thesis. 164 pp.

This thesis examined bat community structure and roosting habitat preferences for the hoary bat 

(Lasiurus cinereus) proximate to the Foote Creek Rim Wind Power Facility in south-central Wyoming. 

Objectives: (1) assess bat-community composition proximate to the wind-energy facility; and, (2) 

quantify roosting habitat for the hoary bat. Methods: Mist-netting surveys were conducted over streams 

and ponds, and acoustic surveys (Anabat II) were conducted on rim-top, over ponds surrounding the 

rim, and over streams and ponds on the Medicine Bow National Forest. Searches for carcasses were 

conducted on transects within 30×30-m plots centered on selected turbines on Foote Creek Rim. 

Proportions of bats recovered during carcass searches in 2000 and 2001 were compared to those 

captured in mist-nets during the same period using a Chi-square test. Radio-telemetry was used to track 

hoary bats to daytime roosts, and habitat surrounding known roosts was quantified and compared to 

habitat surrounding randomly selected sites.

Key Findings: 

• Bats in the genus Myotis represented approximately 81 percent of all bats captured (Myotis volans

and M. lucifugus) in mist nets. 

• The hoary bat was the fourth most abundant species captured with mist nets. 

• Bat activity on the rim was exceedingly low compared to sites off the rim. 

• Acoustic surveys revealed similar proportions of each group of bats found in mist-netting surveys, 

indicating that the assemblage of bats in the study area was comprised primarily of Myotis species 

(M. lucifugus and M. volans). 
24

• Carcass searches in 2001 found 22 hoary bats and one big brown bat (Eptesicus fuscus). 

• Over 3 yr of searches, hoary bats represented the overwhelming majority of recovered bats. 

• Bat mortality at wind turbines by species was disproportionate to the relative abundance determined

by mist netting. 

• Radio transmitters were attached to 14 adult hoary bats, 6 of which were never located. Eight 

individuals were tracked to 17 different roosts. Instrumented bats roosted primarily in live lodgepole 

pine trees, although two individuals were found in medium-sized understory deciduous shrubs. 

• Hoary bats preferred trees that were taller, had more canopy cover, and were near water.

Conclusions: The author concludes this thesis by summarizing bat-turbine interactions. Deaths of bats at 

wind-energy facilities at the time of this thesis had only recently been reported and addressed. Records 

of turbine-related bat mortality were steadily increasing in the United States, and the cumulative results 

suggested two noteworthy trends: (1) most deaths are detected from late summer to early fall; and (2) 

bats in the genus Lasiurus (specifically L. borealis and L. cinereus) appear to represent the majority of 

recovered carcasses. 

Horn, J., Arnett, E.B., and Kunz, T.H., 2008, Behavioral responses of bats to operating wind turbines: 

Journal of Wildlife Management, v. 72, p. 123–132.

This study directly addresses the causal factors of why bat fatalities occur at wind-energy 

facilities. The authors carried out this study by using thermal infrared (TIR) cameras to monitor the 

activity of bats at the Mountaineer Wind Energy Center in West Virginia. Objectives: (1) to document 

how bats behaved while flying within the rotor-swept zone where there was potential for direct contact

with turbine rotors; (2) to examine temporal patterns of activity and to determine if environmental 

variables such as wind speed and temperature were associated with variation in bat activity levels; and, 

(3) to compare activity levels between lighted and unlighted turbines at the facility and to test if aviation 

obstruction lighting mounted on turbines attracts bats (either directly, or indirectly through increased 

insect abundance). Methods: Three TIR cameras were mounted on tripods and were placed 0.5 m apart 

to form a single observation point. Real-time streams of radiometric data from each of the three cameras 

was captured using software and laptop computers. This observation station was placed near the base of 

wind turbines at dusk and was positioned as close to 30 m as possible from the base of each turbine. 

Three 9-hr video sequences were recorded nightly for 10 nights. To test the potential of aviation lighting 

on the turbines to attract bats, cameras were placed alternatively at randomly selected lighted and 

unlighted turbines for 5 nights each. Data were analyzed by manually observing playback of all video 

sequences in real time and recording the appearance and timing of flying objects. 

Key Findings: 

• For all cameras combined, 4,568 moving objects were observed: 1,810 were bats (39 percent), 872 

insects (19 percent), 46 birds (1.0 percent), 5 aircraft (0.1 percent), and 1,835 unknown (40 percent). 

• Most bat activity near wind turbines occurred in the first 2 hr after sunset.

• Among the flying bats, the altitude above ground level (AGL) was highly variable, with some 

individuals flying within 10 m AGL and others foraging at or above the height of the turbine nacelle 

(70 m AGL). 

• The vast majority of bats observed were flying at the medium-altitude band (within the rotor-swept 

zone; 65.0 bats/night). This was greater than three times the number observed flying at low or high 

altitudes (22.7 and 11.2 bats/night, respectively). 

• Aviation lighting did not appear to affect the incidence of foraging bats around turbines. 
25

• Mean turbine RPM and insect abundance were the most significant predictors of the number of bat 

passes observed. 

• Clear instances of bats avoiding turbine blades and of bats being directly struck by blades were 

recorded. From 998 total bat observations, avoidance was observed 41 times (4.1 percent). Direct 

contact with blades was seen 5 times (0.5 percent). 

• Exploratory behavior of bats was observed; bats often made several check passes or made repeated 

flight loops near the moving blades. 

Conclusions: The rate of mortality at the Mountaineer facility suggests that bats are more abundant near 

turbines, and this study provides evidence that collisions may be nonrandom interactions between bats 

and moving turbine blades. One of the most important observations from this study was that bats were 

observed actively investigating both moving and motionless turbine blades; this suggests that the bats

may be attracted to the turbines as potential roosting trees. The authors suggest that this study has 

ramifications for mitigating bat fatalities at wind facilities. For example, curtailment operations could 

occur during predictable nights or periods of high bat kills and could reduce fatalities considerably, with 

potentially modest reduction in power production and associated economic impact on project 

operations.

Huso, M.M.P., 2010, An estimator of wildlife fatality from observed carcasses: Environmetrics, v. 22, 

p. 318–329.

This research article addresses the problems of estimating numbers of wildlife fatalities from 

observed carcasses. Objectives: (1) formalize the conceptual model of fatality and define relevant 

parameters to provide a unifying framework for discussion; (2) develop an estimator of fatality when 

probability of detection is less than 1; (3) evaluate the bias and precision of the estimator using 

simulation and examine its sensitivity to magnitude of parameters as well as to assumptions regarding 

distributions of parameters; and, (4) compare the bias and precision of this estimator with two other 

estimators in common use. Methods: Simulations were conducted and included four major components: 

(1) simulate the fatality process; (2) simulate the carcass persistence (CP) using three distributional 

assumptions; (3) simulate the search process, using six search intervals; and, (4) simulate the 

observation process using two distributional assumptions to produce simulated carcass counts. Recorded 

bat echolocation calls at proposed turbine locations in Casselman, Pennsylvania were used in the 

simulation. 

Key Findings: 

• The proposed fatality estimator accounts for searcher efficiency, scavenger removal, search interval, 

density-weighted area searched, and visibility within search transects.

• None of the three estimators compared were unbiased under all conditions.

• Bias in the proposed estimator never exceeded plus or minus 27 percent, whereas bias in the other 

two estimators was always negative and exceeded that of the proposed estimator in 98 percent and 

93 percent of the simulated conditions, respectively. 

• The proposed estimator was relatively robust to variation in sources and magnitudes of imperfect 

detectability, but was sensitive to distributional assumptions regarding carcass removal rates and 

searcher efficiency.

• The proposed estimator offers significant improvement over two current estimators and provides 

relatively unbiased estimates of fatality that can be applied under a variety of conditions and survey 

protocols.
26

Jain, A.A., Koford, R.R., Hancock, A.W., and Zenner, G.G., 2011, Bat mortality and activity at a 

northern Iowa wind resource area: American Midland Naturalist, v. 165, p. 185–200.

This study examined bat collision mortality, activity, and species composition at an 89-turbine 

wind-resource area in farmland of north-central Iowa from mid-April to mid-December, 2003, and midMarch to mid-December, 2004. Objectives: (1) estimate the number of bat fatalities due to turbine 

collisions; (2) compare bat echolocation activity between areas with wind turbines and adjacent crop 

fields; and, (3) describe any trends in bat activity and mortality related to seasonal, weather, and habitatrelated factors. Methods: The study took place at Top of Iowa Wind Resource Area (WRA). Twenty-six 

of 89 wind turbines were searched for evidence of collision-induced mortality. A stratified, randomized 

design was used to sample turbines. Three-m wide, 76 m-long transects under the turbines were kept 

free of vegetation by using herbicides and manual weeding. The average total area searched under each 

turbine was 1,742 square meters (m

2

), which was about 30 percent of the 5,776 m2 search plot. Searches 

occurred every 2 to 3 days. Fatality estimates were adjusted for search probability, carcass retention 

while exposed to scavengers, search efficiency, and proportion of turbines searched. Search efficiency 

and scavenge rates were estimated using small birds, since bat carcasses were not available. Mortality 

was compared between turbines with three different types of FAA lighting. Bat activity was monitored 

with two Anabat II detectors at 14 of the 26 turbines. One detector was placed at the base of the turbine 

and another was placed at the edge of an adjacent field. Total passes recorded at turbines versus adjacent 

sites were analyzed using a paired t-test. Geographic Information System (GIS) was used to estimate the 

distance of the nearest woodlot to each turbine monitored. Bat activity at turbines with three different 

types of FAA lighting was also compared. 

Key Findings: 

• In 2003, 30 bat carcasses of 5 species were found (11 hoary, 9 little brown, 5 eastern red, 3 big 

brown, and 2 silver-haired bats). In 2004, 45 bats of 6 species were found (11 hoary, 13 eastern red, 

8 little brown, 5 big brown, 7 silver-haired, and 1 eastern pipistrelle sic tri-colored bat). 

• All bats were found from June to October in 2003 and from June to September in 2004.

• Eighty-three percent of the bat carcasses in 2003 and 67 percent in 2004 showed obvious signs of 

trauma with bone fractures and hemorrhaging.

• Two of the 19 carcasses in 2003 had internal thoracic hemorrhaging without other evidence of 

physical trauma (barotrauma), but there was no evidence of barotrauma in 2004.

• Adjusted estimates of bat mortality of Top of Iowa WRA were 396.05 plus or minus 71.71 (95

percent CI) bats (4.45 bats/turbine, 4.94 bats/MW) between April, 2003, and December 15, 2003,

and 635.83 plus or minus 111.89 (95 percent CI) bats (7.14 bats/turbine, 7.94 bats/MW) between 

March 24, 2004, and December 15, 2004.

• No significant difference in bat mortality was observed between turbines with the three different 

types of FAA lighting. 

• There was no significant difference between bat activity at turbine sites and adjacent crop fields 

without turbines in 2003 and 2004. 

• There was no significant effect of distance to nearest woodlot on the number of bat calls recorded at 

turbines. 

• No difference was found in bat activity at turbines with red blinking and non-blinking beacons.

Conclusions: The estimated mortality at this wind facility was higher than those of other wind facilities 

in the Midwest. Several factors were discussed that may cause bats to be susceptible to collision with 

wind turbines. These included local and regional habitat, population levels of bats, flight heights, 
27

weather and visibility conditions, turbine lighting, and use of the regions immediately around the 

turbines.

Jameson, J.W., 2010, Tall structure attraction and mortality at wind turbines of migratory tree bats in

Manitoba Canada: Winnipeg, Manitoba, Canada, The University of Winnipeg, Unpublished Master’s 

Thesis. 135 pp.

This thesis consists of four chapters: a general introduction, a chapter on testing the reproductive 

landmarks hypothesis, a chapter on factors influencing bat mortality at a wind-energy facility in central 

Canada, and a concluding chapter with recommendations. This summary focuses on Chapters 2 and 3. 

Chapter 2 tests the reproductive landmarks hypothesis to explain mortality of bats at wind 

turbines. The “Reproductive Landmarks Hypothesis (RLH)” posits that bats may be attracted to tall 

structures during migration and that this attraction serves in finding mates. Objectives: (1) to confirm 

the attraction of migratory tree bats to turbines and to other tall anthropogenic structures; and, (2) to 

determine whether this attraction is associated with migratory and/or reproductive behavior. Methods: 

This study took place during the summer and fall of 2008 and 2009 at the St. Leon Wind Energy Project 

in south-central Manitoba, Canada. Bat activity was sampled at four groups of three habitat types: 

telecommunication service towers, open fields, and woodlots. Acoustic activity was monitored using 

Pettersson D240X time expansion bat detectors. Detectors were placed at 30 m on the towers. 

Generalized linear models were used to compare bat activity among habitat types and sites. Feeding 

activity (feeding buzzes) was also quantified and compared among habitat types using generalized linear 

models. Bat activity was also compared between ground level and 30 m communication towers. 

Mortality surveys were conducted at five wind turbines between July 6 and September 27, and bat 

mortality was correlated with bat activity at ground level and 30 m at communication towers. Carcass 

surveys were also conducted at communication towers.

Key Findings:

• In 2008, 4.8 percent of total bat passes were collected at open fields, 54.1 percent at woodlots, and 

41.0 percent at communication towers.

• In 2008, 64.0 percent of passes were identified as silver-haired bats, 8.7 percent were hoary bats, 

19.0 percent were eastern red bats, 7.2 percent were big brown bats, and 1.1 percent were little 

brown bats.

• In 2009, 9.6 percent of total bat passes were collected open fields, 45.1 percent at woodlots, 37.6

percent at communication towers, and 7.7 percent at wind turbines.

• In 2009, 64.1 percent of passes were identified as silver-haired bats, 24.1 percent were hoary bats, 

8.1 percent were eastern red bats, 3.2 percent were big brown bats, and 0.65 percent were little 

brown bats.

• High levels of bat activity were recorded at communication towers during migration. 

• Activity of silver-haired bats at communication towers during the pre-migration period was below 

activity levels at woodlots and equal to activity levels at open habitat—this is consistent with the 

second prediction of the RLH, that activity at towers would be relatively low during pre-migration, 

as it is at wind turbines. Hoary bat activity was also consistent with this prediction.

• There were fewer tree-bat passes and less variability in activity at turbines when compared to 

communication towers.

• Equal proportions of feeding buzzes were observed across all habitats—this does not support the 

hypothesis that tall structure attraction in bats is linked to foraging behavior.
28

Chapter 3 of this thesis examines the factors influencing bat mortality at the St. Leon Wind 

Energy Project. Objectives: (1) to see if night-to-night variation in bat mortality is correlated with 

weather patterns in the vicinity of the turbines; (2) to see if night-to-night variation in mortality is 

correlated with insect abundance in the vicinity of the turbines; (3) to see if night-to-night patterns in 

mortality are correlated with bat activity at turbines; and, (4) to see if vegetation at the base of turbines 

significantly influences the accuracy of fatality estimates. Methods: Carcass surveys were conducted for 

31 days in 2008 and 79 days in 2009. Turbines were selected randomly and carcass searches were 

conducted by a team of two searchers. Searchers surveyed a 40-m-radius circular plot at the base of each 

turbine. Searcher efficiency tests were conducted using bat carcasses at mowed and non-mowed 

turbines. Scavenging trials were conducted at mowed turbines in 2009. An effective daily-fatality rate 

was calculated for each turbine. Weather variables, insect abundance, and acoustic-activity data were 

also collected and used as predictor variables for mortality using generalized linear models.

Key Findings: 

• In 2008, 55 bat carcasses were recovered and 32.7 percent were silver-haired bats, 52.7 percent were 

hoary bats, and 9.1 percent were eastern red bats. 

• In 2008, adjusted mortality rate over the study period was 11.5 plus or minus 4.9 bats/turbine (7.2 

plus or minus 3.1 bats/MW) for silver-haired bats, 15.7 plus or minus 5.5 bats/turbine (9.9 plus or 

minus 3.4 bats/MW) for hoary bats, and 2.0 plus or minus 1.0 bats/turbine (1.2 plus or minus 0.6 

bats/MW) for eastern red bats. 

• In 2009, only 33 bat carcasses were recovered and 57.6 percent were silver-haired bats, 36.4 percent

were hoary bats, and 6.1 percent were eastern red bats. 

• In 2009, adjusted mortality estimates were 7.7 plus or minus 2.1 bats/turbine (4.6 plus or minus 1.3 

bats/MW) for silver-haired bats, 5.8 plus or minus 2.1 bats/turbine (3.5 plus or minus 1.3 bats/MW) 

for hoary bats, and 1.1 plus or minus 0.8 bats/turbine (0.7 plus or minus 0.5 bats/MW) for eastern

red bats.

• No weather variables measured were correlated with bat mortality in 2008.

• In 2009, mortality of silver-haired bats was highest on calm, cold, humid nights when insect 

abundance was low. 

• A significant positive correlation was found between bat activity measured at ground level and 

mortality of silver-haired and hoary bats in 2009.

• Adult males dominated mortality numbers in 2008; however, in 2009, most mortalities were 

subadult silver-haired bats and adult female hoary bats.

• Fatality estimates were considerably less reliable when vegetation was high.

Conclusions: Although the author was not able to directly test if tree bats were mating at communication 

towers or turbines, the results from this study were consistent with the Reproductive Landmarks 

Hypothesis. The author concluded that this study showed evidence that tree bats exhibit an attraction to 

tall structures during fall migration, the same time of year that they are killed at wind turbines. Mortality 

of silver-haired bats was high on calm, cold, humid nights when insect abundance was low. 

Johnson, G.D., 2005, A review of bat mortality at wind-energy developments in the United States: Bat 

Research News, v. 46, p. 45–49. 

This review describes the patterns of bat mortality at wind-energy developments in the United 

States as of 2004. In 2004, wind-power capacity in the United States was 6,740 MW, or enough to 

supply electricity to over 1.6 million homes. 
29

• Eleven of the 45 species of bats in North America were included among the fatalities at wind farms, 

although none of these were classified as threatened or endangered by the federal government. 

• Most (83.2 percent) fatalities involved migratory tree bats. Hoary bats (Lasiurus cinereus) 

comprised 45.5 percent of the 1,440 bats identified to species. Eastern red bats (L. borealis) were 

26.3 percent of the fatalities and silver-haired bats (Lasionycteris noctivagans), 11.4 percent. The 

remaining fatalities were tri-colored bats (formerly eastern pipistrelles; Perimyotis subflavus, 8.5

percent), little brown bats (Myotis lucifugus, 5.9 percent), and big brown bats (Eptesicus fuscus, 1.9

percent). Six northern long-eared bats (Myotis septentrionalis) and single specimens of the western 

red bat (Lasiurus blossevillii), Brazilian free-tailed bat (Tadarida brasiliensis), long-eared myotis 

(Myotis evotis), and Seminole bat (Lasiurus seminolus) were also found killed at U.S. wind farms. 

• In the Upper Midwest (UM), hoary and eastern red bats comprised most fatalities, whereas eastern 

red bat, tricolored bats, and hoary bats were most commonly killed in the East. In the Pacific 

Northwest (PNW), equal numbers of hoary and silver-haired bats were killed, whereas most 

fatalities in the Rocky Mountain West (RMW) were hoary bats. 

• Approximately 90 percent of fatalities occurred from mid-July through late September. Collisions 

were lower during the breeding season and during spring migration. 

• As of 2004, 11 studies had been conducted in the United States to estimate fatality rates of bats at 

wind farms. Estimated fatality rates adjusted for searcher efficiency and scavenger removal were 1.2 

bats/turbine in the PNW and RMW, 1.7 bats/turbine in the UM, and 46.3 bats/turbine in the East.

• Studies suggested that mortality rates were highest in forested environments, moderate in open areas 

close to forests, and lowest in open areas. 

• Inclement weather did not appear to correlate with mortality of bats. 

• Warning lights for airplanes that were placed on turbines did not increase fatalities. 

Johnson, G.D., Erickson, W.P., Strickland, M.D., Shepherd, M.F., and Sarappo, S.A., 2003,

Mortality of bats at a large-scale wind power development at Buffalo Ridge, Minnesota: American 

Midland Naturalist, v. 150, p. 332–342.

This paper documents mortality of bats at a large-scale wind-power development at Buffalo 

Ridge, southwestern Minnesota, from 1996–1999. Objectives: (1) to estimate the number of bat 

mortalities attributable to collisions with wind turbines for the entire Buffalo Ridge wind-energy 

facility; (2) to determine the species and groups at highest risk; and, (3) to determine what factors may 

be related to collision mortality. There were three phases to this study and the phases were based on the 

installation of different types of turbines. Methods: Fatality searches were conducted at a select number 

of turbines during each phase of the project. Each turbine was searched every 14 days from March 15 to 

November 15 each study year. A 100-m×100-m square plot was centered around each turbine and 

variable transect widths were used based on visibility within each habitat type. Carcass-removal trials 

and searcher-efficiency trials were conducted to estimate the time that bat fatalities remained in the 

search area and to estimate a rate of searcher efficiency. 

Key Findings: 

• Of the 163 bat fatalities that could be identified to species, hoary bats (Lasiurus cinereus) comprised 

66 percent, and eastern red bats (L. borealis) comprised 23 percent. The remaining fatalities were 

silver-haired bats (Lasionycteris noctivagans), tricolored bats (Perimyotis subflavus), little brown 

bats (Myotis lucifugus), and big brown bats (Eptesicus fuscus). 

• Both the hoary and eastern red bat samples were primarily males. 

• Ninety-seven percent of the fatalities occurred from July 15 to September 15. 
30

• Injuries sustained by bats included fractured wings, legs and necks; head wounds; and abrasions and 

abdominal injuries. 

• Bat fatalities were fairly widespread throughout the study area. 

• Fifty-four percent of all bat carcasses were found less than or equal to 10 m from a turbine, 43

percent were found from 10 m to 20 m, 3 percent were found from 20 m to 30 m and one (0.5

percent) was found greater than 30 m from a turbine. 

• The mean number of fatalities at lighted turbines was not significantly higher than the mean number 

of fatalities at unlit turbines. 

• For all three phases of the study, 541 bat collision fatalities were estimated to occur each year.

Johnson, G.D., Perlik, M.K., Erickson, W.P., and Strickland, M.D., 2004, Bat activity, composition 

and collision mortality at a large wind facility in Minnesota: Wildlife Society Bulletin, v. 32, p. 1278–

1288.

The authors investigated bat activity, composition, and collision mortality at the Buffalo Ridge 

wind farm in southwestern Minnesota from June 15 to September 15, 2001 and 2002. Objectives: (1) to 

estimate relative abundance and composition of bat populations in the study area; (2) to estimate the 

number of bat fatalities attributable to wind-energy-facility development; (3) to determine whether the 

mortality was occurring to resident breeders, migrants, or both; and, (4) to determine what physical (for 

example, turbine characteristics) and biological (for example, habitat) factors might be associated with 

bat collision mortality in the study area. Methods: Bat activity was monitored using Anabat II bat 

detectors at potential foraging and roosting sites within commuting distance of the wind-energy 

facilities. To monitor bat activity within the wind-energy facilities, detectors were placed at each of 

three turbines each survey night and selection of turbine locations were made using a systematic design 

with a random start and removed to different turbines each night. Mist-netting surveys were conducted 

in woodlands and over ponds and other wetlands within 3.6 kilometers (km) of the wind facilities to 

obtain data on bat-species composition in the study area. Fatality searches were conducted by searching 

for carcasses daily at the three turbines where the Anabat detectors were placed the previous night. This 

activity was performed to determine relationships between bat activity at turbines and collision 

mortality levels. Eighty (in 2001) and 100 (in 2002) additional turbines were selected for fatality 

searches using a systematic design and random start. Turbines were searched every 14 days throughout 

the study. Fatality estimates were adjusted for removal bias and searcher efficiency. Poisson regression 

models were used to investigate the relationship between bat activity and mortality levels and cover type 

within 100 m of the turbines, distance from each turbine to the nearest wetland and woodland, and 

presence of FAA lighting. 

Key Findings:

• More bat activity was found at foraging and roosting areas than at turbines. 

• Peak bat activity at turbines followed the same trend as mortality and occurred from mid-July 

through the end of August. 

• Distance to woodland was the only landscape variable significantly related to bat activity, and bat 

activity decreased with increasing distance from woodlands.

• There was no significant difference between number of bat passes and whether turbines were 

lighted.

• There was no relationship between bat activity and bat mortality at turbines.

• Mist-netting surveys captured big brown bats (64), silver-haired bats (17), little brown bats (11), 

eastern red bats (8), and hoary bats (3).
31

• Fatality searches found 151 bat casualties during the study, including 115 hoary bats, 21 eastern red 

bats, 8 big brown bats, 4 silver-haired bats, and 3 little brown bats. 

• Eighty-two percent of fatalities occurred during the period July 16–August 31.

• Bat collisions occurred during inclement weather as well as during clear weather.

• Mean number of bat fatalities did not differ between lighted and unlighted turbines.

• Estimated total bat mortality for both wind facilities combined was 849 in 2001 and 364 in 2002, 

which equates to an estimated mean of 3.02 fatalities/turbine in 2001 and 1.30 fatalities/turbine in 

2002.

Conclusions: Mortality of bats at wind turbines does not appear to involve foraging bats. The peak in 

collision mortality corresponds to the period of post-breeding southward migration for hoary, eastern 

red, and silver-haired bats. Both Anabat and mist-net data indicate that relatively large breeding 

populations of bats were close to the wind facility when collision mortality was low to nonexistent.

Johnson, J.B., Gates, J.E., and Zegre, N.P., 2011a, Monitoring seasonal bat activity on a coastal 

barrier island in Maryland, USA: Environmental Monitoring and Assessment, v. 173, p. 685–699.

This study used acoustic monitoring to examine the seasonal bat activity on the Assateague 

Island National Seashore, a barrier island off the coast of Maryland, from 2005 to 2006. Objectives: (1) 

to establish seasonal bat activity patterns on Assateague; (2) to determine if bats use the barrier island 

during migration; and, (3) to determine if bat-activity patterns were correlated with weather conditions. 

Methods: Anabat II detectors were used to monitor bat echolocation passes at three bat-monitoring 

stations. The total number of bat passes was summed for each monitoring station as an index of bat 

activity. A time-series analysis was used to account for weather and to model nightly and seasonal bat 

passes. The time-series analysis accounted for autocorrelation. Generalized least squares was used to 

develop a predictive model of bat activity and to account for seasonal and climatic influences. 

Key Findings: 

• 39,384 bat passes were recorded from five species, including eastern red bats (59.33 percent of total 

bat passes), big brown bats (3.05 percent), hoary bats (0.10 percent), tricolored bats (0.10 percent), 

and silver-haired bats (0.07 percent). 

• Bat passes generally increased beginning in April, peaked in August, and declined gradually until 

December, when relatively few passes were recorded. 

• After accounting for seasonal trends and autocorrelation, all weather variables explained some 

residual variation in bat activity. However, the degree to which weather variables accounted for bat 

activity varied among the monitoring stations in terms of significance. Mean nightly temperatures 

consistently had a significant positive effect on bat activity, and mean nightly wind speeds 

consistently had a negative effect on bat activity. 

Conclusions: This research concluded that bats migrate along the Atlantic coastline and use barrier 

islands, possibly for navigation or as stopover sites. This has implications for proposed offshore windenergy facilities, in that bats migrating along the coast may collide with wind turbines. Wind turbines 

offshore may be attractive to bats as resting places or possible mating areas; this would increase the risk 

of bat mortality. Bats were less active during periods of low temperatures and high wind speeds, 

possibly above thresholds for wind turbine cut-in speed. 
32

Johnson, J.S., Watrous, K.S., Giumarro, G.J., Peterson, T.S., Boyden, S.A., and Lacki, M.J., 

2011b, Seasonal and geographic trends in acoustic detection of tree-roosting bats: Acta 

Chiropterologica, v. 13, p. 157–168.

This study examined seasonal and geographic trends in acoustic detection of migrating treeroosting bats at 1 existing and 13 proposed wind-energy facilities in seven eastern states in the United 

States between April and November of 2007 and 2008. Hypotheses: (1) nightly detection rates of 

migratory tree-roosting species would peak earlier in the northeastern United States during fall 

migration and peak earlier in the mid-Atlantic United States during spring migration; (2) nightly 

species-specific detection rates would be correlated between surveys located in the same geographic 

region; and, (3) nightly species-specific detection rates would be correlated with nightly species 

mortality at the nearby wind energy facility. Methods: In 2007, acoustic bat surveys were conducted in 

Maine (2), New Hampshire (1), Pennsylvania (3), and West Virginia (1). In 2008, surveys were 

conducted in Maine (2), Vermont (1), New York (1), Pennsylvania (3), West Virginia (2), and Virginia 

(1). Each survey was placed into one of three geographic regions to allow for latitudinal comparisons: 

northeast (Maine, New Hampshire, and Vermont), northern Allegheny Plateau (southern New York and 

northern Pennsylvania), and mid-Atlantic (southern Pennsylvania, West Virginia, and Virginia). Anabat 

II detectors were used to record echolocating bats. Species-specific linear correlations were used to 

compare nightly silver-haired, hoary, and eastern red bat calls/detector/night between all possible pairs 

of survey locations within the same geographic regions. Nightly silver-haired, hoary, and eastern red bat 

calls/detector/night at two proposed sites in West Virginia were compared to the number of mortalities 

at Mount Storm Wind Energy Facility, also in West Virginia, using linear correlations. 

Key Findings: 

• Nightly detection rates of silver-haired and hoary bats at the two sites in West Virginia were 

positively correlated with the number of dead silver-haired and hoary bats at the Mount Storm Wind

Energy Facility. 

• No significant correlation was found between the eastern-red-bat detection rate and the mortality at 

Mount Storm Wind Energy Facility.

• Nightly detection rates of silver-haired, hoary, and eastern red bats during fall migration peaked 

earlier in the year in the two northernmost geographic regions compared to the mid-Atlantic region. 

• Nightly detection rates of the three species during the spring migration peaked earlier in the year in 

the mid-Atlantic compared to geographic regions farther north. 

• In all regions, eastern red bats were more frequently detected later in the year than were hoary bats. 

Conclusion: The seasonal detection rates of hoary bats, silver-haired bats, and eastern red bats reflect 

their different migratory patterns, and these patterns may be useful in predicting the timing of mortality 

events at wind-energy facilities.

Klug, B.J., and Baerwald, E.F., 2010, Incidence and management of live and injured bats at windenergy facilities: Journal of Wildlife Rehabilitation, v. 30, p. 11–16. 

During studies of patterns and causes of bat fatalities at wind-energy facilities, the authors found 

numerous live and injured bats. Objectives: to examine the patterns of live-bat occurrences at three 

wind-energy facilities in southwestern Alberta, Canada, and to suggest methods for conducting searches 

at wind-energy facilities to increase the likelihood of finding live bats. Methods: Carcass searches were 

conducted at 40 turbines from July 15 through September 30, 2006, and at 42 turbines from July 15 

through September 30, 2007. Two searchers used a spiral technique with a 45-m rope to search the base 
33

of turbines. If a live bat was found, it was immediately immobilized and transported to a vehicle for 

assessment and treatment. Bats were examined for obvious physical trauma such as hemorrhaging, 

broken or severed wings, and open lacerations. Bats were either euthanized if the injuries were deemed 

too severe or they were rehydrated and monitored for recovery if the injuries did not seem lifethreatening. 

Key Findings: 

• Of 1,033 bats encountered during the two years of the study, 26 (2.5 percent) were alive. 

• Significantly more live bats were found at turbines when searched daily than turbines searched 

weekly. 

• Only eight of the live bats found had visible signs of skeletal damage or considerable soft tissue 

trauma. 

• Injuries observed included wing fractures, wing amputation, leg fractures, and open abdominal 

wounds. 

• Sixteen of the 26 live bats found were treated and 13 (81.3 percent) were successfully released.

Conclusions: Searching turbines daily results in the discovery of more live bats than does searching 

each turbine once per week. Dehydration appeared to be the immediate threat to survival for bats 

without severe injuries. The authors suggest that live and injured bats can be properly managed if 

personnel involved in fatality surveys at wind-energy facilities are authorized to handle live bats and are 

prepared to rehydrate suitable individuals and euthanize those with terminal injuries. 

Klug, B.J., Turmelle, A.S., Ellison, J.A., Baerwald, E.F, and Barclay, R.M.R., 2011, Rabies 

prevalence in migratory tree-bats in Alberta and the influence of roosting ecology and sampling 

method on reported prevalence of rabies in bats: Journal of Wildlife Diseases, v. 47, p. 64–77.

This study examined rabies prevalence in hoary (Lasiurus cinereus) and silver-haired bats 

(Lasionycteris noctivagans) collected during mortality surveys at wind-energy facilities in southern 

Alberta, Canada, during 2007 and 2008. Objective: to estimate rabies prevalence in hoary and silverhaired bats obtained from carcasses collected at wind-energy facilities and compare this data to 

prevalence estimates reported in the literature. Methods: Bat carcasses were collected during mortality 

surveys conducted at three wind-energy facilities in 2007 and 2008 (see Baerwald and Barclay, 2009). 

When a bat carcass was discovered, it was identified to species, sexed, and aged. Carcasses were 

submitted for rabies diagnosis to the Centers for Disease Control and Prevention (CDC) in Atlanta, 

Georgia. A literature review was conducted on both peer-reviewed and government reports for reported 

rabies prevalence in bats in North America. Rabies rates were compiled for passive surveillance from 

public health records spanning the previous 56 yr, and were also obtained for active-surveillance 

estimates. Estimates of rabies prevalence from this study was compared to rabies prevalence in the 

literature using Chi-square or Fisher’s exact tests. 

Key Findings: 

• Rabies testing yielded a single positive result from a total of 121 hoary bats and 96 silver-haired bats 

submitted for testing. The positive bat was a subadult silver-haired bat and was typed as the silverhaired bat variant. 

• For both species, rabies prevalence was lower than the combined estimates of prevalence reported 

from passive surveillance. 

• The prevalence estimates in this study did not differ from previous estimates for these two species 

based on active-surveillance studies. 
34

• Rabies-prevalence estimates based on passive surveillance were higher than estimates from active 

surveillance across all bat species. 

• Non-synanthropic bat species had higher rabies prevalence than synanthropic species. 

Conclusions: Most previously reported rabies-prevalence estimates for hoary and silver-haired bats are 

overestimates due to a bias in sampling. Furthermore, high rabies-prevalence estimates in some bat 

species are partly due to a bias in sampling resulting from the difference in roosting ecology. The 

authors also point out that the influx of bat carcasses from wind facilities could have effects on the 

surrounding ecosystem by potentially increasing the risk of disease transmission to mammalian 

scavengers. 

Kunz, T.H., Arnett, E.B., Erickson, W.P., Hoar, A.R., Johnson, G.D., Larkin, R.P., Strickland, 

M.D., Thresher, R.W., and Tuttle, M.D., 2007a, Ecological impacts of wind energy development 

on bats—Questions, hypotheses, and research needs: Frontiers in Ecology and the Environment, v. 5, 

p. 315–324.

This paper synthesizes information on the impacts of wind-energy development on bats. The 

authors highlight ongoing development of wind-energy facilities in the United States, summarize 

evidence for bat fatalities at these sites, make projections of cumulative fatalities of bats for the MidAtlantic Highlands (Maryland, Pennsylvania, Virginia, and West Virginia), identify research needs to 

help reduce or mitigate adverse environmental impacts at these facilities, and propose hypotheses to

evaluate where, when, how, and why bats are being killed. 

Highlights:

• Since utility-scale wind turbines were first deployed in the United States in the 1980s, turbine height 

and the rotor-swept area has steadily increased with each new generation of turbines. 

• Relatively small numbers of bat fatalities were reported at wind-energy facilities in the United States

prior to 2001.

• Recent monitoring studies have indicated that some wind-energy facilities have killed large numbers 

of bats.

• Of the 45 species of bats found in North America, 11 have been identified during ground searches. 

• Nearly 75 percent of carcasses found were foliage-roosting eastern red bats, hoary bats, and tree 

cavity-dwelling silver-haired bats, each of which migrate long distances.

• Number of bats killed in the eastern United States at wind-energy facilities installed along forested 

ridgetops has ranged from 15.3 to 41.1 bats/MW/yr. Bat fatalities reported from other regions of the 

western and mid-western United States have been lower, ranging from 0.8 to 8.6 bats/MW/yr.

• Most bat fatalities in North America have been reported in late summer and early autumn.

• Highest bat fatalities occur on nights when wind speed is low (less than 6 m s-1

).

• Bats are being struck and killed by the turning rotor blades of wind turbines; however, it is unclear 

why wind turbines are killing bats.

• Questions of why bats are killed at turbines include: Are bats attracted to wind turbines? Are bats 

attracted to sites that provide rich foraging habitats? Are bats attracted to the sounds produced by 

wind turbines? Do some weather conditions place bats at increased risk of being killed by turbines? 

Are bats at risk because they are unable to acoustically detect the moving rotor blades? 

• The projected number of annual fatalities (rounded to the nearest 500) in the year 2020 range from 

33,000 to 62,000 individuals, based on the National Renewable Energy Laboratory (NREL) WinDS 

model. Using the PJM (Pennsylvania, Jersey, Maryland Power Pool) grid operator interconnection 
35

queue, the estimates of annual fatalities range from 59,000 to 111,000 bats. This breaks down to 

9,500 to 32,000 hoary bats; 11,500 to 38,000 eastern red bats; and 1,500 to 6,000 silver-haired bats.

• There may be a substantial impact on both migratory and local bat populations; however, baseline 

population estimates and demographics are unknown for migratory tree bats.

• The authors propose 12 research directions in this paper and highlight that research should focus on 

regions and at sites with the greatest potential for adverse effects. 

• The authors call for full cooperation and research support from the wind industry.

• Scientifically sound research and monitoring studies are needed to inform policy makers during the 

siting, permitting, and operation of renewable energy sources.

Kunz, T.H., Arnett, E.B., Cooper, B.M., Erickson, W.P., Larkin, R.P., Mabee, T., Morrison, M.L., 

Strickland, M.D., and Szewczak, J.M., 2007b, Assessing impacts of wind-energy development on 

nocturnally active birds and bats—A guidance document: Journal of Wildlife Management, v. 71, p. 

2449–2486.

This publication complements and extends the National Wind Coordinating Committee 

document “Studying Wind Energy/Bird Interactions: A Guidance Document” (Anderson and others, 

1999). The purpose of this document is to provide researchers, consultants, decision-makers, and other 

stakeholders with guidance to methods and metrics for investigating nocturnally active birds and bats in 

relation to utility-scale wind-energy development. The paper is organized with the following topics: (1) 

Limits of current knowledge about impacts of nocturnally active birds and bats; (2) Methods and 

metrics for observing nocturnal behavior of birds and bats; (3) Methods and metrics for collecting 

additional data on nocturnally active birds and bats; (4) Conducting pre- and post-construction 

monitoring; and, (5) Management implications. The authors emphasize that sampling protocols and 

methodology employed to quantify bird and bat mortalities must be rigorous and scientifically valid. 

Overall, these guidelines were developed to encourage efficient, cost-effective experimental designs that 

will result in comparable data. 

Kuvlevsky, W.P., Jr., Brennan, L.A., Morrison, M.L., Boydston, K.K., Ballard, B.M., and Bryant, 

F.C., 2007, Wind energy development and wildlife conservation: challenges and opportunities: 

Journal of Wildlife Management, v. 71, p. 2487–2498.

This review paper examines wind-energy development and wildlife-conservation issues focusing 

on the Lower Gulf Coast (LGC) of Texas, a region identified as having potentially negative impacts on 

migratory birds and bats, with respect to wind-farm development. The authors summarize what is 

known about how wind-energy developments influence wildlife populations and identify specific 

research opportunities and management challenges that can help mitigate the potential negative effects 

of wind-energy development. An overview of bird-collision mortality and habitat loss is presented. Bird 

collisions range from 0 to greater than 30.0 collisions/turbine/year. This variability is due to numerous 

factors, including: inconsistent experimental designs and data-collection protocols, layout design of the 

wind farm, weather conditions, topography, and bird species and the numbers of birds using the site and 

their behavior. Habitat loss associated with wind energy-developments can be an even greater threat to 

bird populations than collision fatalities. An overview of bats and the impact of wind-farm development 

on their populations is presented. Other wildlife have the potential to be impacted by wind-energy 

developments (including other birds indirectly affected, mammals, and herpetofauna), because 

developments alter wildlife habitat. These alterations include habitat disturbance, construction of 
36

additional electrical transmission lines, and the development of extensive road networks. The authors 

conclude by highlighting the LGC region in Texas and its unique vegetation communities and 

topography. Three migratory bird flyways converge north of Corpus Christi, Texas. This flyway funnels 

tens of millions of migratory birds along the LGC each year. Wind farms would severely affect these 

migrating birds. In addition to birds, wind farms located in LGC could negatively impact resident and 

migratory bat populations, notably the migratory Mexican free-tailed bat (Tadarida brasiliensis). The 

authors conclude with a list of research needs and a discussion of experimental designs for investigating 

wildlife and wind energy interactions. Several management recommendations are provided: (1) develop 

guidelines to assist wind developers in how best to site their projects; (2) provide a place to house the 

compiled data from all wind developments in each state; and, (3) provide incentives for the developers 

to avoid high-risk areas within each state, and if these high-risk areas cannot be avoided, provide ways 

to offset or mitigate the effects of developing in high-risk areas. 

Larkin, R.P., 2006, Migrating bats interacting with wind turbines—What birds can tell us: Bat 

Research News, v. 47, v. 23–32. 

Small, night-migrating birds have been used as surrogates for bats when formulating hypotheses 

and suggesting research techniques for investigating migration patterns, but the appropriateness of this 

surrogacy is unknown. This purpose of this paper is to provide bat biologists with information on 

aspects of bird migration that might be useful in designing research on bat migration and wind-energy 

interactions with bats. Techniques for studying migration include searching for killed animals beneath 

tall structures, analyzing DNA and other chemical clues, making observations at night with optical 

instruments and radar, and banding bats. The author proceeds to describe the process of bird migration 

as it might pertain to bat migration. Based on ornithological data, bats would not be expected to fly in 

narrow corridors over level terrain or large bodies of water. However, when encountering mountains, 

some nocturnal migrants alter their course to fly along valleys and ridges, where winds and topography 

interact. There is enormous temporal variability in migration patterns for birds, and this should also 

apply to bats; studies on migration should not be short-term. There is also variability in the heights at 

which birds migrate. The height of bats engaged in long-distance migratory flight is also critical to 

understanding risks to mortality from wind turbines. Migrating bats may select narrowly defined height 

bands for favorable winds, and turbine blades may cut the air progressively above, at, or below this 

band. The author concludes by stating that biologists have been studying bird migration for many years 

and much of what is known may be useful in understanding the process of migration in bats (however, 

see Willis and others, 2010).

Miller, A., 2008, Patterns of avian and bat mortality at a utility-scaled wind farm on the southern High 

Plains: Lubbock, Tex., Texas Tech University, Unpublished Master’s Thesis. 113 pp.

This thesis examines the patterns of avian and bat mortality at a utility-scaled wind farm at the 

Red Canyon Wind Energy Center in the southern portion of the Texas Panhandle. Chapter 1 provides a 

general introduction to wind-energy development and its effects on birds and bats. Chapter 2 looks at 

avian mortality patterns. Chapter 3 examines bat mortality patterns. This summary focuses on Chapter 

3. Objectives: (1) assess the incidence and frequency of bat mortality; (2) model mortality rates; and, (3) 

identify the spatial and temporal distribution of bat mortality within the wind-energy development. 

Methods: Standardized carcass searches were conducted for 52 weeks, beginning in September 2006, on 

28 of the 56 turbines. Bias corrections were made by conducting searcher efficiency and carcass-
37

removal trials. Bias-correction trials were conducted seasonally to account for differences in vegetation 

and scavenger-predator characteristics. Spatial and temporal analyses were conducted using Poisson 

models.

Key Findings: 

• During the study period, 56 bat mortalities were found. 

• Brazilian free-tailed bats composed 94 percent of fatalities, hoary bats 4 percent, and red bats 2

percent.

• Mortalities were found beneath 24 of the 28 searched turbines. 

• Peak mortality detections occurred in October.

• For Brazilian free-tailed bats, 41 percent of fatalities were adults, 43 percent were juveniles, and 16

percent unidentifiable to age. Fifty-three percent were male, 10 percent were female, and 37 percent

were unidentifiable to sex.

• For hoary bats, 40 percent were adults, 40 percent were juveniles, and 20 percent were 

unidentifiable to age. Sixty percent were male, 20 percent female, and 20 percent were

unidentifiable to sex. 

• Turbines were classified as low, medium, and high mortality. Low-mortality turbines were estimated 

to kill 15.29 bats/MW. Medium-mortality turbines were estimated to kill 45 bats/MW, and highmortality turbines were estimated to kill 106.67 bats/MW.

• Total mortality was estimated for the entire wind energy development as 36.87 bats/MW.

Conclusions: Bat mortality at this site was above the reported average bat mortality rate reported for 

new-generation wind projects in the United States (2.1 mortalities per MW per year). This is one of the 

first post-construction mortality monitoring studies conducted on wind energy development in Texas 

and the southwestern United States. 

Osborn, R.G., Higgins, K.F., Dieter, C.D., and Usgaard, R.E., 1996, Bat collisions with wind 

turbines in southwestern Minnesota: Bat Research news, v. 37, p. 105-108. 

Prior to this study, most studies on the biological effects of wind energy were designed to assess 

effects on birds. Bat collisions with turbines appeared to be infrequent and had therefore received little 

attention by natural-resource agencies or the scientific community. While conducting a study to assess 

the impacts of wind energy on birds in southwestern Minnesota, the authors found several bats. 

Objectives: (1) to document the species composition and timing of bat mortality associated with the 

Minnesota wind facility during 1994 and 1995; and, (2) to review some current theories regarding bat 

collisions with anthropogenic structures and to speculate on the possible causes of bat mortality at wind 

turbines. Methods: Buffalo Ridge Wind Resource Area (WRA) was searched for wildlife carcasses from 

April 1994 through December 1995. Search intensity varied by season: during spring and fall, all 73 

turbine plots were searched once per week; however, during summer and winter, only 30 plots were 

searched once per week. 

Key Findings: 

• During 20 months of continuous monitoring, 13 bat carcasses were found, representing 5 species. 

• Five of the carcasses found were hoary bats, four were silver-haired bats, two were eastern red bats, 

and one each of a big brown bat and a tricolored bat. 

• Carcass-recovery dates ranged from May 12 to September 22. One collision (8 percent of total) 

occurred in spring, 11 (85 percent) during the summer, and one (8 percent) on the first day of fall. 
38

• Six of the 13 bat carcasses (46 percent) were found within 15 m of turbines, and 69 percent were 

found within 20 m. 

• On September 13, 1995, a tricolored bat carcass was found, extending the known range of this bat 

approximately 160 km farther west in Minnesota. 

Conclusions: The timing of bat-carcass recovery at Buffalo Ridge WRA suggests that bats are 

susceptible to collisions with anthropogenic structures at times other than during migration. The 

findings from this study represent the first evidence of frequent bat collisions with wind turbines.

Piorkowski, M.D., and O’Connell, T.J., 2010, Spatial pattern of summer bat mortality from collisions 

with wind turbines in mixed-grass prairie: American Midland Naturalist, v. 164, p. 260–269.

This study examined the spatial pattern of summer bat mortality from collisions with wind 

turbines in the southern Great Plains, Oklahoma, during May–July, 2004 and 2005. The authors 

proposed to test the hypothesis that collision mortality was randomly distributed among the 68 wind 

turbines at the Oklahoma Wind Energy Center (OWEC). Objectives: (1) to estimate a summer-season, 

per-turbine rate of bat collision mortality; and, (2) to analyze the spatial distribution of collisions at the 

OWEC relative to vegetation cover, topography, and location of individual turbines. Methods: The 

bases of all 68 turbines were visually inspected for bird and bat carcasses in 2004 and 2005. Surveys 

occurred in late May and late June in 2004 and every 2 weeks from May 15–July 15 in 2005. Circular 

transects were walked around the base of each turbine at 5, 10, and 15 m from the base. An observerdetection rate was estimated in 2005 with blind field trials and models built to approximate free-tailed 

bats. Three estimates of mortality were calculated based on (1) raw carcass counts, (2) raw counts 

divided by observer-detection rate, and (3) raw counts modified by observer-detection rate and removal 

rate. Broad spatial distribution of mortalities were plotted using GIS, and “hotspot” analyses were 

conducted for fine-scale spatial distribution patterns.

Key Findings: 

• Eleven turbine-killed birds of six species and 111 turbine-killed bats of seven species were found. 

• First documented cave myotis (Myotis velifer) killed by a wind turbine.

• The majority of bat carcasses (94) were Brazilian free-tailed bats (Tadarida brasiliensis) 

representing 78 percent of all fatalities and 85 percent of all bat fatalities.

• Examination of 64 (58 percent) relatively fresh and intact bat carcasses indicated a female-biased 

sex ratio.

• Raw carcass counts yielded estimates of 0.53/fatalities/MW in 2004 and 0.56 fatalities/MW in 2005.

• For Brazilian free-tailed bats, rates adjusted for probability of detection and incorporating published 

removal estimates yielded 74–92 individuals (0.73–0.90/MW) in 2004 and 67–85 (0.66–0.83/MW) 

in 2005.

• Carcasses were detected up to 46 m from turbine bases; however, 77 percent were located within a 

20-m search radius.

• Brazilian free-tailed bat mortality was not associated with topography or habitat when both years of 

the study were combined.

• Three turbines emerged as significant “hot spots” of collision mortality. These three turbines were 

located near the head of a forested ravine cut by an intermittent tributary to the Cimarron River. The 

three turbines were less than 5 percent of the turbines at OWEC, but they accounted for 16 percent

of the Brazilian free-tailed bat carcasses collected during both years.
39

• Seventeen individuals were necropsied; 14 specimens had broken bones and 3 had no apparent 

skeletal injury. In the bats with no sign of injury, no obvious visceral hemorrhaging was observed 

potentially consistent with barotrauma.

Conclusions: The results from this study 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 (roughly 15 km) to both a maternity colony and a bachelor colony. Authors rejected their 

original hypothesis of random collision mortality among the 68 turbines at OWEC. The cluster of three 

turbines emerged as a hotspot of collision mortality. This suggests that bats moving through this area

were either more susceptible to collision or that the number of bats moving through the area was greater 

than at other areas of OWEC. They concluded that this provided support for the “coincidental collision” 

hypothesis of Cryan and Barclay (2009). If monitoring pointed to particular turbines as hotspots for 

mortality, then mitigating management (for example, curtailment of those specific units under specific 

conditions) could be developed. 

Reynolds, D.S., 2006, Monitoring the potential impact of a wind development site on bats in the 

northeast: Journal of Wildlife Management, v. 70, p. 1219–1227.

This paper investigated spatial and temporal patterns of bat activity across a proposed windenergy project site during the summer breeding season and the migratory season in the Tug Hill Plateau 

region of western New York. Hypotheses: (1) the physiogeography of the site would limit the species 

diversity and total bat abundance at the project site; (2) the bat community would be shifted towards 

species that are more commonly found at higher elevation (for example, hoary and silver-haired bats); 

and, (3) the climate of the project site would shift the sex ratio of the bat community towards males that 

were not as energy limited as reproductive females during the summer months. Methods: The study area 

was the Maple Ridge Wind Project, a 198-turbine project that began construction in August, 2005. 

Summer activity was sampled with mist netting and acoustical monitoring (Anabat II) in 2004. 

Migratory activity was sampled using acoustic monitoring during spring 2005, at two locations. 

Meteorological data was collected by the Maple Ridge wind-development group. Summer activity was 

analyzed using a 2-factor general linear model. Temporal patterns of bat activity during the spring 

migratory sampling period using a 3-factor general linear model. 

Key Findings:

• Mist-netting was conducted at 24 sites and 35 bats of 3 species were captured. Northern long-eared 

bat (Myotis septentrionalis, 17), little brown bat (M. lucifugus, 15), and big brown bat (Eptesicus

fuscus, 3). 

• Summer acoustic sampling found that activity levels were highly skewed across sample sites.

• Bat activity was significantly influenced by habitat, with ponds being the only habitat showing 

preferential use by bats. 

• Bats of the genus Myotis accounted for almost 95.7 percent of all bat calls and 98.8 percent of all 

feeding buzzes. 

• Acoustic monitoring during the spring migratory period revealed a median activity level of 2.0

bats/night. 

• On 20 April, 101 bat passes of the tricolored bat (Perimyotis subflavus) were recorded and on the 10 

June, 115 bat passes of the hoary bat (Lasiurus cinereus) were recorded. 

• Most of the migratory activity occurred at minimum wind speeds below 1.3 m/s.
40

• Temperature appeared to have a strong influence on migratory bat activity. Days with high 

migratory bat activity had a mean maximum temperature of 23.9 plus or minus 4.4oC compared to 

9.8 plus or minus 4.8oC.

Conclusions: The results of this study strongly suggest that the Tug Hill Plateau region does not support 

a large bat population in terms of either species diversity or total bat abundance. Most of the bat 

migratory activity occurred when daily mean wind speeds were below 5.4 m/s. This is encouragingly 

close to the cut-in speed (the lowest economically useable wind speed) of a typical commercial wind 

turbine. (This study did not sample fall migration.)

Rollins, K.E., Meyerholz, D.K., Johnson, G.D., Capparella, A.P., and Loew, S.S., 2012, A forensic 

investigation into the etiology of bat mortality at a wind farm—Barotrauma or traumatic injury?: 

Veterinary Pathology, accessed February 14, 2012,

http://vet.sagepub.com/content/2012/01/30/0300985812436745. [published online January 30, 2012] 

The authors in this paper studied the causes of bat mortality at a central Illinois wind-energy 

facility compared to bat mortality from collisions with buildings in Chicago, Illinois (Ill.). They 

designed a 2-part forensic investigation to examine two competing hypotheses for the mortality of bats 

at turbines: (1) blunt force trauma from being struck by moving turbine blades and (2) barotrauma. 

Objectives: (1) to test whether postmortem decomposition and environmental conditions would 

influence the development of morphologic artifacts of pulmonary barotrauma using laboratory mice; 

and, (2) to examine salvaged bats collected from a wind-energy facility compared with a control group 

of bats that had died following collisions with buildings. Methods: The authors conducted two

experiments. The first experiment used laboratory mice and performed a decomposition trial, an 

elevated temperature trial, and a freezing trial. After each trial, lungs, heart, and trachea were removed 

from each mouse and fixed for routine processing and staining. Lungs were examined for signs of 

vascular congestion, erythrocyte extravasation, and edema, and for each parameter, samples were scored 

for extent of damage. The second experiment examined 53 bats salvaged near buildings in downtown 

Chicago, Ill., and compared these with 262 bats salvaged from the wind-energy facility. Both bats killed 

by buildings and bats killed at the wind-energy facility were radio-graphically examined for bone 

lesions consistent with traumatic injury. Additionally, 66 of the 262 bats from the wind-energy facility

were necropsied and examined for supplemental lesions consistent with trauma. All bats killed by 

buildings were assessed for external lacerations, but were not available for full necropsies. A subset of 

each group of bats was also examined for evidence of auditory barotrauma. 

Key Findings:

• Decomposition caused changes in vascular congestion, erythrocyte extravasation, and edema in 

murine lungs. Vascular congestion scores increased 2 hr postmortem, edema scores increased after 

24 hr, and erythrocyte extravasation scores increased after 72 hr.

• Elevated temperatures (33°C) accelerated postmortem morphologic changes in mice.

• Freezing and freezing/thawing trials increased erythrocyte extravasation and edema compared to 

fresh lung tissue.

• Bone fractures were detected in 9 of 53 (17 percent) building-killed bats, whereas 152 of 262 (58 

percent) turbine-killed bats had bone fractures. 

• Among bats without fractures, turbine-killed bats had significantly more cases with external

lacerations than building-killed bats.

• Only one (1 of 42; 2 percent) building-killed bat showed evidence of tympana (ear drum) rupture,

whereas 11 of 81 (14 percent) of turbine-killed bats showed evidence of ruptured tympana. 
41

However, only 5 of 81 (6 percent) of the bats with tympana ruptures had no detectable evidence of 

traumatic injury; this suggests that a small fraction of turbine-killed bats had lesions possibly 

consistent with barotrauma.

Conclusions: Determining the causes of lung damage from salvaged bats can be confounded by 

common postmortem artifacts (for example, decomposition, temperature differences) as well as the 

competing hypothesized causes of traumatic injury (for example, blunt force trauma, barotrauma, 

laceration). Because these competing factors cannot be confidently excluded during a pathological 

examination, pulmonary barotrauma should not be diagnosed from salvaged bat carcasses unless 

assessments are also made for ruptured ear drums. The authors conclude that their results point to a need 

for improvements in the current monitoring procedures at wind-energy facilities and in the experimental 

approaches used to determine whether barotrauma or other causes contribute to bat mortality. 

Smallwood, K.S., and Karas, B., 2009, Avian and bat fatality rates at old-generation and repowered 

wind turbines in California: Journal of Wildlife Management, v. 73, p. 1062–1071.

This study examined avian- and bat-fatality rates at old-generation and repowered wind turbines 

at Altamont Pass Wind Resource Area (APWRA) in California. The APWRA began operations during 

the 1980s and until recently was the world’s largest wind farm. In 1998, the APWRA comprised about 

5,400 wind turbines of various models. In 2005, 126 turbines were replaced with 31 horizontal-axis 

wind turbines. The new turbines were more widely spaced and operated at a lower rotor speed, which 

were traits thought by some to be safer for birds. Objectives: to compare estimates of fatality rates 

between (1) the periods 1998–2003 and 2005–2007; and, (2) a repowered wind project and the 

concurrently operating old-generation wind turbines. Methods: Fatality searches were performed at 

selections of turbines. Searches were performed by walking parallel transects about 4–8 m apart. 

Fatality rates were adjusted for carcasses not found due to searcher-detection error and for scavenger 

removal. Bat–fatality rates were estimated by applying scavenger-removal and searcher-detection rates 

estimated for small-bodied bird species. Therefore, estimates of bat–fatality rates were likely biased 

low. 

Key Findings: 

• For APWRA-wide fatality rates, between 1998–2003 and 20052007, estimated mean adjusted 

fatality rate increased 23 percent for all birds combined.

• Comparing only adjusted fatality rates from old-generation turbines mutually monitored both 1998–

2003 and 2005–2007, fatality rates increased 51 percent for all birds combined.

• The first repowering project in the APWRA did not change fatality rates for any species or group of 

specie, because fatality rates did not differ between the old vertical-axis turbines and the new 

horizontal-axis turbines.

• The adjusted fatality rate of bats increased from zero at the old vertical-axis turbines to 16.4/yr at the 

new, repowered turbines, but this difference was not significant, probably due to small sample sizes. 

• Compared to the concurrently operating old-generation turbines during 2005–2007, adjusted fatality 

rates in the repowered turbines were 66 percent lower for all birds combined.

• The adjusted fatality rate of bats was nearly 800 percent greater at repowered turbines compared to 

the concurrently operated old-generation turbines, but this large difference was not significant, 

probably due to sample sizes.

Conclusions: Repowering the entire APWRA would likely reduce fatality rates a great deal for birds, 

especially if considered on a power-generation basis and if carefully done by locating new turbines 

where they pose the least hazard. A possible downside to repowering, however, may be increased bat 
42

fatalities caused by wind turbines. Extrapolating the mean adjusted bat-fatality rate from the repowered 

turbines to a completely repowered APWRA, about 454 bat fatalities/year might result, but using more 

realistic scavenger-removal and searcher-detection rates could increase this number to the thousands of 

bats. More research is needed to address bat fatalities at the APWRA.

Sovacool, B.K., 2009, Contextualizing avian mortality—A preliminary appraisal of bird and bat 

fatalities from wind, fossil-fuel, and nuclear electricity: Energy Policy, v. 37, p. 2241–2248.

This article examines the threats that wind farms pose to birds (and bats) when compared to 

threats from fossil-fuel and nuclear electricity. The authors conducted a survey of 600 studies, articles, 

and reports investigating avian deaths and wind farms published from 1998 to 2008. The authors

address three common problems with these studies: (1) they rarely compare their results with studies of 

other wind farms to contextualize their estimates; (2) most do not compare the possible avian deaths 

from wind electricity with other sources, and when they do, studies typically do not compare them to 

other energy sources; and, (3) no studies attempted to calculate the number of avian deaths per kilowatthour from energy sources, so that more meaningful comparisons might be made between different forms 

of electricity supply. To address these shortcomings, the author assessed and compared the avian deaths 

per GWh from three electricity systems: wind farms, fossil-fueled power plants (coal, natural gas, and 

oil generators), and nuclear plants. Averaged over six wind farms, 339 turbines were responsible for 

0.279 avian deaths per GWh. The assessment of avian mortality for fossil-fuel power plants was 

estimated to be 5.18 fatalities per GWh. Finally, the total avian deaths per GWh for nuclear power 

plants was estimated to be about 0.416. The author concludes that for wind turbines, the risk appears to 

be greater to birds striking towers or turbine blades and for bats suffering barotrauma. For fossil-fueled 

power stations, the most significant fatalities come from climate change, which is altering weather 

patterns and destroying habitats that birds depend on. For nuclear power plants, the risk is almost 

equally spread across hazardous pollution at uranium mine sites and collisions with draft cooling 

structures. Fossil-fueled facilities are about 17 times more dangerous to birds on a per GWh basis than 

wind and nuclear power stations, according to the author. Putting this in absolute terms, wind turbines 

may have killed about 7,000 birds in 2006 but fossil-fueled stations killed 14.5 million and nuclear 

power plants 327,000. Far more detailed, rigorous, and sophisticated analyses are called for in the 

future. This preliminary study has quite a few shortcomings that are further discussed by the author. 

[The author lumped birds and bats together for this analysis. Also note that Willis and others (2010)

disputes the methods and findings of this paper.]

Weller, T.J., and Baldwin, J.A., 2011, Using echolocation monitoring to model bat occupancy and 

inform mitigations at wind-energy facilities: Journal of Wildlife Management, v. 9999, p. 1–13, 

accessed January 23, 2012, at http://www.fs.fed.us/psw/publications/weller/psw_2011_weller001.pdf.

This research article combines results of continuous echolocation and meteorological monitoring 

at multiple stations to model the conditions that explain the presence of migratory bats at a wind-energy 

facility in southern California. Objectives: (1) to characterize year-round patterns of bat activity at a 

wind-energy facility in southern California; (2) to determine survey effort necessary to characterize batactivity levels at the facility; (3) to model bat presence on-site with respect to date and meteorological 

conditions; and, (4) to determine whether incorporation of a full suite of meteorological conditions 

could improve predictions of bat presence. Methods: This study took place at the Dillon Wind Energy 

Facility (DWEF) near Palm Springs, California. This site is within the San Gorgonio Pass Wind 
43

Resource Area, one of the oldest and largest wind-energy developments in the world. Bat activity was 

monitored using Anabat II detectors attached to four meteorological towers at three different heights (2 

m, 22 m, and 52 m above the ground). Detectors were operated nightly for 17 months from October 25, 

2007 through March 31, 2009. Bat passes were classified as either “LowF” or “HiF.” Most migratory 

species of bats use LowF echolocation calls. Site-occupancy models were used to estimate the 

probability of LowF bat presence on a given night. Presence and detection probability was estimated as 

a function of detector height, mean nightly wind speed, wind direction, temperature, and proportion of 

moon illumination.

Key Findings:

• Overall LowF bat-passes rates increased with detector height from 2 m [0.14 bat passes/detector 

night (dn)] to 22 m (0.26 bat passes/dn), and 52 m (0.43 bat passes/dn). 

• Passes of LowF bats comprised 49.2 percent, 83.6 percent, and 98.5 percent of bat passes recorded 

at 2 m, 22 m, and 98.5 m, respectively.

• Detection probabilities ranged from 0.027 (SE = 0.014) at 2 m to 0.564 (SE = 0.032) at 52 m.

• Mean wind speed had the highest relative importance for predicting LowF bat activity in all but 2 

time periods.

• Mean nightly wind speeds were lower on nights when LowF bat passes were recorded than on 

nights when they were not in all time periods.

• Proportion of moon illumination was important for predicting bat activity in 4 time periods.

Conclusions: Despite low overall bat-activity levels, the authors were able to build models that 

successfully predicted conditions when LowF bats were present. The models developed confirmed 

results from wind-energy developments in other regions and habitats which have demonstrated that bat 

activity is generally higher at lower wind speeds and higher temperatures. The results also highlight the 

importance of modeling conditions that explain bat activity on a seasonal basis. There were marked 

differences in activity levels seasonally, and the relative importance of explanatory variables also 

differed seasonally. The authors conclude that using this model-based approach could improve the 

effectiveness and efficiency of mitigations for bat fatalities at wind-energy facilities. If properly 

conducted, echolocation monitoring may provide a reliable index to expected bat fatality levels at windenergy facilities.

Willis, C.K.R., Barclay, R.M.R., Boyles, J.G., Brigham, R.M., Brack, V.B., Jr., Waldien, D.L., and 

Reichard, J., 2010, Bats are not birds and other problems with Sovacool’s (2009) analysis of animal 

fatalities due to electricity generation: Energy Policy, v. 38, p. 2067–2069.

This forum paper is a response to Sovacool’s (2009) comparison of North American bird (avian) 

and, presumably bat (chiropteran) mortality resulting from three methods of electricity generation. 

Although bats figure prominently in the title of Sovacool’s paper, the text is virtually devoid of data on

bats. The authors of this response point out that bats are not birds. Bats occupy an entirely different 

ecological niche than almost all birds, since bats are the primary consumer of night-flying insects. It has 

become increasingly clear that, in general, mortality at wind turbines is an issue that affects bats more 

than birds, and it is bats that face the most widespread and worrisome species-level conservation 

consequence from wind turbines. Bats investigate wind turbines; birds do not. Migrating bats appear to 

be attracted to wind turbines, and there is little evidence that birds are attracted. Bats are also prone to 

depressurization injuries (barotraumas) at turbines, to which birds are less susceptible. Sovacool’s 

(2009) estimate of the average number of birds killed per GWh of wind power is incorrect and omits a 

large body of easily accessible, published data. Sovacool also states that death rates of all flying animals 
44

from wind turbines have decreased in recent years; however, this is not true for bats. There is clear 

evidence that, as turbines have become taller, bat mortality has increased. Sovacool also implies that 

pre-construction monitoring and mitigation at proposed wind-energy sites reduce the risk of mortality 

for birds and bats, which is simply not true for bats. Sovacool reviewed 600 sources, but failed to 

reference the primary literature on bat fatalities at wind turbines. The authors of this forum paper 

conclude by pointing out that Sovacool advocates averaging data across many species, large areas, and 

long time periods to estimate the total number of animals killed by three different electricity systems. 

This approach fails to address the importance of spatial and temporal variation when contextualizing the 

conservation implications of different electricity systems. Sovacool’s conclusions are based on 

fundamental errors and could influence energy policy in ways that are counter-productive for the wind 

energy industry and the conservation of both groups of flying vertebrates: birds as well as bats.

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Bat Ecology: Chicago, Ill., University of Chicago Press, p. 156–208. 
45

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