Frontiers in Ecology and the EnvironmentVolume 5, Issue 6 (August 2007), 315–324
Ecological impacts of wind energy development on bats: questions, research needs, and hypotheses
Thomas H Kunz, Edward B Arnett, Wallace P Erickson, Alexander R Hoar, Gregory D Johnson,
Ronald P Larkin, M Dale Strickland, Robert W Thresher, Merlin D Tuttle
Relatively small numbers of bat fatalities were reported at wind energy facilities in the US before 2001 (Johnson 2005), largely because most monitoring studies were designed to assess bird fatalities (Anderson et al. 1999). Thus, it is quite likely that bat fatalities were underestimated in previous research. Recent monitoring studies indicate that some utility-scale wind energy facilities have killed large numbers of bats (Kerns and Kerlinger 2004; Arnett 2005; Johnson 2005). Of the 45 species of bats found in North America, 11 have been identified in ground searches at wind energy facilities (Table 1). Of these, nearly 75% were foliage-roosting, eastern red bats (Lasiurus borealis), hoary bats (Lasiurus cinereus), and tree cavity-dwelling silver-haired bats (Lasionycteris noctivagans), each of which migrate long distances (Figure 2). Other bat species killed by wind turbines in theUS include the western red bat (Lasiurus blossivilli),
Seminole bat (Lasiurus seminolus), eastern pipistrelle (Perimyotis [=Pipistrellus] subflavus), little brown myotis (Myotis lucifugus), northern long-eared myotis (Myotis septentrionalis), long-eared myotis (Myotis evotis), big brown bat (Eptesicus fuscus), and Brazilian free-tailed bat (Tadarida brasiliensis).
How and why are bats being killed?
It is clear that bats are being struck and killed by the turning rotor blades of wind turbines (Horn et al. in press). It
is unclear, however, why wind turbines are killing bats, although existing studies offer some clues. Are bats in some way attracted to wind turbines? Some migratory species are known to seek the nearest available trees as daylight approaches (Cryan and Brown in press), and thus could mistake large monopoles for roost trees (Ahlén 2003; Hensen 2004). Tree-roosting bats, in particular, often seek refuge in tall trees (Pierson 1998; Kunz and Lumsden 2003; Barclay and Kurta 2007). As wind turbines continue to increase in height, bats that migrate or forage at higher altitudes may be at increased risk (Barclay et al. 2007).
Are bats attracted to sites that provide rich foraging habitats?
Modifications of landscapes during installation of wind energy facilities, including the construction of roads and power-line corridors, and removal of trees to create clearings (usually 0.5–2.0 ha) around each turbine site may create favorable conditions for the aerial insects upon which most insectivorous bats feed (Grindal and Brigham 1998; Hensen 2004). Thus, bats that migrate, commute, or forage along linear landscapes (Limpens and Kapteyn 1991; Verboom and Spoelstra 1999; Hensen 2004; Menzel et al. 2005) may be at increased risk of encountering and being killed by wind turbines.
Are bats attracted to the sounds produced by wind turbines?
Some bat species are known to orient toward distant audible sounds (Buchler and Childs 1981), so it is possible that they are attracted to the swishing sounds produced by the rotating blades. Alternatively, bats may become acoustically disoriented upon encountering these structures during migration or feeding. Bats may also be attracted to the ultrasonic noise produced by turbines (Schmidt and Jermann 1986). Observations using thermal infrared imaging of flight activity of bats at wind energy facilities suggest that they do fly (and feed) in close proximity to wind turbines (Ahlén 2003; Horn et al. 2007; Figure 3).
What other factors might contribute to bat fatalities?
Wind turbines are also known to produce complex electromagnetic fields in the vicinity of nacelles. Given that some bats have receptors that are sensitive to magnetic fields (Buchler and Wasilewski 1985; Holland et al. 2006), interference with perception in these receptors may increase the risk of being killed by rotating turbine blades. Bats flying in the vicinity of turbines may also become trapped in blade-tip vortices (Figure 4) and experience rapid decompression due to changes in atmospheric pressure as the turbine blades rotate downward. Some bats killed at wind turbines have shown no sign of external injury, but evidence of internal tissue damage is consistent with decompression (Dürr and Bach 2004; Hensen 2004). Additionally, some flying insects are reportedly attracted to the heat produced by nacelles (Ahlén 2003; Hensen 2004). Preliminary evidence suggests that bats are not attracted to the lighting attached to wind turbines (Arnett 2005; Kerlinger et al. 2006; Horn et al. in press).
Do some weather conditions place bats at increased risk of being killed by wind turbines?
Preliminary observations suggest an association between bat fatalities and thermal inversions following storm fronts or during low cloud cover that force the animals to fly at low altitudes (Durr and Bach 2004; Arnett 2005). Thermal inversions create cool, foggy conditions in valleys, with warmer air masses rising to ridgetops. If both insects and bats respond to these conditions by concentrating their activities along ridgetops instead of at lower altitudes, their risk of being struck by the moving turbine blades would increase (Dürr and Bach 2004). Interestingly, the highest bat fatalities occur on nights when wind speed is low (< 6 m s–1), which is when aerial insects are most active (Ahlén 2003; Fiedler 2004; Hensen 2004; Arnett 2005).
Are bats at risk because they are unable to acoustically detect the moving rotor blades?
Current evidence is inconclusive as to whether bats echolocate during migration, independent of time spent searching for and capturing insects. Bats less likely to make long-distant migrations in North America (eg members of the genera Myotis, Eptesicus, Perimyotis) and others that engage in longdistance migrations (eg Lasiurus, Lasionycteris, Tadarida) typically rely on echolocation to capture aerial insects and to avoid objects in their flight paths. However, for most bat species, echolocation is ineffective at distances greater than 10 m (Fenton 2004), so bats foraging in the vicinity of wind turbines may miscalculate rotor velocity or fail to detect the large, rapidly moving turbine blades (Ahlén 2003; Bach and Rachmel 2004; Dürr and Bach 2004). Given the speed at which the tips of turbine blades rotate, even in relatively low-wind conditions, some bats may not be able to detect blades soon enough to avoid being struck as they navigate.
Volume 18, Issue 16, 26 August 2008, Pages R695–R696
Barotrauma is a significant cause of bat fatalities at wind turbines
Erin F. Baerwald , Genevieve H. D'Amours, Brandon J. Klug, Robert M.R. Barclay
Bird fatalities at some wind energy facilities around the world have been documented for decades, but the issue of bat fatalities at such facilities — primarily involving migratory species during autumn migration — has been raised relatively recently [1,2] . Given that echolocating bats detect moving objects better than stationary ones  , their relatively high fatality rate is perplexing, and numerous explanations have been proposed  . The decompression hypothesis proposes that bats are killed by barotrauma caused by rapid air-pressure reduction near moving turbine blades [1,4,5] . Barotrauma involves tissue damage to air-containing structures caused by rapid or excessive pressure change; pulmonary barotrauma is lung damage due to expansion of air in the lungs that is not accommodated by exhalation. We report here the first evidence that barotrauma is the cause of death in a high proportion of bats found at wind energy facilities. We found that 90% of bat fatalities involved internal haemorrhaging consistent with barotrauma, and that direct contact with turbine blades only accounted for about half of the fatalities. Air pressure change at turbine blades is an undetectable hazard and helps explain high bat fatality rates. We suggest that one reason why there are fewer bird than bat fatalities is that the unique respiratory anatomy of birds is less susceptible to barotrauma than that of mammals.
As with any airfoil, moving wind-turbine blades create zones of low pressure as the air flows over them. Animals entering these low pressure areas may suffer barotrauma. To test the decompression hypothesis, we collected hoary (Lasiurus cinereus) and silver-haired bats (Lasionycteris noctivagans) killed at a wind energy facility in south-western Alberta, Canada, and examined them for external and internal injuries.
Of 188 bats killed at turbines the previous night, 87 had no external injury that would have been fatal, for example broken wings or lacerations (Table 1). Of 75 fresh bats we necropsied in the field, 32 had obvious external injuries, but 69 had haemorrhaging in the thoracic and/or abdominal cavities (Table 1). Twenty-six (34%) individuals had internal haemorrhaging and external injuries, whereas 43 (57%) had internal haemorrhaging but no external injuries. Only six (8%) bats had an external injury but no internal haemorrhaging. (...)
Among 18 carcasses examined with a dissecting microscope, ten had traumatic injuries. Eleven bats had a haemothorax, seven of which could not be explained by a traumatic event. Ten bats had small bullae — air-filled bubbles caused by rupture of alveolar walls — visible on the lung surface (Figure 1A). All 17 bats examined histologically had lesions in the lungs consistent with barotrauma (Table 1), with pulmonary haemorrhage, congestion, edema, lung collapse and bullae being present in various proportions (Figure 1). In 15 (88%), the main lesion was pulmonary haemorrhage, which in most cases was most severe around the bronchi and large vessels.
Pulmonary barotrauma in bats killed at wind turbines.
(A) Formalin-fixed L. noctivagans lung with multifocal hemorrhages and a ruptured bulla with hemorrhagic border (arrow). Histological sections of bat lungs stained with hematoxylin and eosin (100X). (B) Normal lung of an L. noctivagans. (C) Lung of Eptesicus fuscus found dead at a wind turbine with no traumatic injury. There is extensive pulmonary hemorrhage (H), congestion, and bullae (b). (D) Lung of L. cinereus found dead at a wind turbine with a fracture of the distal ulna and radius. 90% of the alveoli and airways are filled with edema. Bar = 100 μm.
Barotrauma helps explain the high fatality rates of bats at some wind energy facilities. Even if echolocation allows bats to detect and avoid turbine blades, they may be incapacitated or killed by internal injuries caused by rapid pressure reductions they can not detect.
Birds are also killed at wind turbines, but at most wind energy facilities fewer birds than bats are killed , and barotrauma has not been suggested as a cause of bird fatalities. This may be explained partly by differences in the respiratory anatomy and susceptibility to barotrauma of birds and bats. Bats have large lungs and hearts, high blood oxygen-carrying capacity, and blood-gas barriers thinner than those of terrestrial mammals . These flight adaptations suggest that bats are particularly susceptible to barotrauma. Although birds have even thinner blood-gas barriers, they have compact, rigid lungs with unidirectional ventilation and a cross-current blood-gas relationship, as opposed to mammals which have large pliable lungs with the blood-gas relationship in a uniform pool in the pulmonary alveoli 9 and 10. In addition, the pulmonary capillaries of birds are exceptionally strong compared to those of mammals, and do not change as much in diameter when exposed to extreme pressure changes . Bats’ large pliable lungs expand when exposed to a sudden drop in pressure, causing tissue damage, whereas birds’ compact, rigid lungs do not.