Demand Protection of Bats Around Wind Farms in North America

    Bat Fatalities from Interaction with Wind Turbines

    Wind Energy has undergone rapid growth in the past decade, and it is predicted to increase in the future. Wind energy may not contribute to global warming, but it does pose several environmental impacts where they are built, primarily to raptors. According to the American Wind Energy Association there are 57, 700 wind turbines across 41 of the United States. 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 2019 Wind Power Capacity Worldwide Reaches 597 GW, which is equivalent to 597, 000 Megawatts. Wind turbines are causing unprecedented numbers of bat fatalities, in the U.S. and around the world (Cryan et al. 2014). Roughly 600,000 bats die on U.S. wind farms per year according to Mother Nature Network. Bats play an important role in many environments around the world which raises concerns for their dramatic population decrease. As wind farms continue to be built, it is important to understand their effects on the environment where they are located in order to decrease fatality rate while increasing production capacity.
    The anthropocene era has negatively affected bat populations through landscape change, agricultural development, housing developments, and habitat fragmentation. Bats belong to the order Chiroptera, and are said to be the second most diverse order of mammals who provide ecological roles as prey and predator (Kasso and Balakrishnan 2013). The majority of bat species eat night flying insects. Thus they play a significant role in insect population control, which benefits the agricultural industry. Bats are also important pollinators. The ecosystem services bats provide is valued at 200 billion U.S. dollars annually. Bats occupy a wide range of habitats, such as wetlands, woodlands, farmland, as well as urban areas. Since they cover a large array of landscapes, bats can be studied as indicators of biodiversity. During their travels while feeding on fruits and flowers, they act as pollinators and seed dispersers. Over five hundred species of plants rely on bats to pollinate their flowers; some of these are agave, guava, mangos, and bananas. Several plant species also rely on bats for seed dispersal, according to (Kasso and Balakrishnan 2013) seeds spit or defecated by bats have up to a 95% chance of germination. Their seed dispersal is crucial to the survival of world’s tropical forests and dispersing the seeds for pioneer species. In North America there are a multitude of bat species that are threatened or endangered. Almost 6 million bats in North America have been killed by white nose syndrome. Of the 45 bat species in North America more than 30% of the species in the study qualified as vulnerable, imperiled, or critically imperiled (Hamerson et al. 2017). According to their study 18% of N. A. Bats are at risk of extinction, and 13% potentially at risk. Identified by the IUCN Red List there are three critically endangered species, and an additional five species that are identified as endangered under the US Endangered Species Act.
    Bird and bat fatalities have been noted around wind energy facilities for decades, however the underlying causes of bat fatalities at wind turbines for the most part remains unknown. There are several proposed hypotheses as to why bats have high fatality rates. Baerwald et al (2018) stated that the decompression hypotheses which concerns the idea of rapid air pressure change as a leading cause. When a bat flies into a patch of air immediately after a blade tip has passed by, the sudden drop in pressure can induce a condition known as barotrauma. Cryan et al. (2014) proposes that bats are attracted to wind turbines due to resource based attraction. Bats may find turbines provide shelter, food resources, or social opportunities.

    Collisions at wind energy facilities are considered to be one of the greatest threats to North American bat populations. The bats most commonly affected by wind turbines are species which migrate long distances or roost on trees (Cryan and Barclay 2009). There are two main hypotheses for the causes of turbine related deaths in bats: collisions and barotrauma. Collisions with turbines occur at the turbine tower, nacelle (engine), or blades (National Wind Technology Center). Air pressure change induced by turbines is a nearly undetectable hazard to bat species. Moving wind turbine blades create areas of unequal pressure , as air flows through them zones of low pressure are created. Barotrauma in humans is commonly referred to as decompression sickness. Humans experience this during flights or scuba diving. Baerwald et al. (2008) described the deceased bats around wind turbines had no sign of physical injury, but upon necropsy they found the bats had ruptured ears and blood in the lungs consistent with injury due to barotrauma. Baerwald et al. (2008) found that 90% of bat fatalities were caused by barotrauma: injuries were consistent with internal hemorrhaging caused by excessive pressure change, in effect they explode (p.1). Bats may become incapacitated when trying to avoid turbines, and die from sudden pressure changes. Injuries consistent with pressure change cause damage to tympanic membranes (eardrum) due to over pressure, and damage to lungs due to under pressure (Taber A. D. 2018). Pulmonary barotrauma is damage to the lungs from expansion of air in the lungs. Bats have evolved to have large lungs and heart capable of high blood oxygen carrying capacity and have thinner blood-gas barriers compared to birds. Baerwald et al. (2008) states that mammals have more pliable lungs than of birds, and that birds have stronger pulmonary capillaries than of mammals. Baerwald et al. (2008) continues to explain that bats and birds lungs respond differently to sudden changes in pressure. Bats lungs are more flexible and expand when exposed to pressure change which damages tissue, whereas birds denser inflexible lungs do not expand. One reason why there are fewer bird fatalities compared to bats is that birds have an unusual respiratory anatomy making them less susceptible to barotrauma than that of mammals.

    Wind turbines have the potential to detrimentally effect entire bat populations. Hypothesized estimates for bat fatalities in some regions range from tens to hundreds of thousands of bats per year (Cryan et al. 2014). Many are not aware that bats have low reproductive rates, and are considered to be one of the slowest reproducing animals in the world. Bats mate in the fall and give birth in the spring. Cryan et al. (2014) found that bats more actively approach turbines during the summer and fall on the leeward side of windbreaks caused by the turbine structure. Bats likely favor leeward activity due to lower risk of predation, favorable conditions for flight, and greater availability of insects (p. 15128). It is possible bats may not be able to discern wind turbines form trees and follow the perceptual cues of the down wind air flows of the turbine pole. Migratory bats may mistake turbine poles as trees to roost in or an area to find a mate. Insects often accumulate on the leeward side of poles and behind windbreaks, this resource availability may attract bats. Baerwald et al. (2008) found that the color of the turbines influences the insects they attract, white and gray turbines were second to yellow turbines in attracting insects. In the study they found that purple is the least attractive color to insects, but purple turbines would not be as visually inconspicuous as white or gray. Many bats have been observed feeding near wind turbines, and are found dead at the base with full stomachs (Cryan et al. 2014). Bats may be acting upon the expectation of resources around the “trees” rather than the actual presence of them, and when they come in close enough proximity to an operating turbine they are likely to experience barotrauma.

    Fatalities of bats may be reducible by mitigating the impacts of wind energy on bats in accord with the U.S. Fish and Wildlife Service in the Land-Based Wind Energy Guidelines. The guidelines state that a project developer should first avoid, then minimize project impacts, and compensate for any impacts that can’t be minimized (Taber A. D 2018). Before developing a wind energy facility proper citing and accurate predictions for collision risks should be noted for the development site and local landscape. Evaluating the area pre and post construction. Another important mitigating factor is to avoid development near a known bat maternity roost. A large number of bat fatalities occur low wind speeds. Curtailment is an effective strategy at reducing fatalities by restricting turbine operation at low wind speeds. When wind is not favorable, wind turbine blades turn slowly in the wind until they reach a “cut-in speed” point. This point marks the speed at which the wind is spinning fast enough to begin generating power, which averages 7.5 miles per hour. If industry guidelines mandated that turbines must idle at low wind speeds during peak bat migration season, bat fatalities could reduce by one third. Another form of curtailment science are exploring is ultrasonic acoustic transmitter (UADs) technologies (p.16). The goal of UADs is to deter bats from approaching rotating wind turbines, however a challenge to deploying UADs is ensuring that the sounds cover the entirety of the wind turbines area. Taber A. D (2018) describes that illuminating turbines with a UV light may reduce bat activity around turbines. Cryan et al. (2014) observed that fewer bat fatalities occurred at a wind farm in Texas which employs flashing red aviation lights atop its turbines, supporting the idea that supplemental lighting of turbines discourages bats to come close. The cumulative risk wind energy poses to bats is influenced by the location of the facility as well as the the mitigation strategies implemented to reduce risk.

    Discovering the underlying reasons why bats are especially susceptible to wind turbines is important to improving the the efficiency and efficacy of existing strategies in reducing fatalities. The amount of impact on bat population varies greatly according to density and distribution of bat species in the area of interest. North American bat populations are seriously jeopardized by a combination of threats: white-nose Syndrome, wind energy facilities, habitat fragmentation, and lack of federal protection. In the span of fifteen years a third of North American bat species dropped an entire category, from Vulnerable to Imperiled. It is crucial to the North American bat populations that we understand how bats are affected by wind farms, and effectively implement strategies to deter them in order to find a balance between saving bats and generating power.


    Baerwald E. F., D’Amours G. H., Klug B. J., Barclay R. M. R., (2008) Barotrauma is a significant cause of bat fatalities at wind turbines. Current Biology, 18, 695-696, doi:10.1016.2008.06.029
    Campedelli, T., Londi, G., Cutini, S., Sorace, A., Tellini Florenzano, G. (September 29, 2014).Raptor displacement due to the construction of a wind farm: preliminary results after the first 2 years since the construction. Ethology Ecology & Evolution, 26, 376-391, doi: 10.1080.03949370.2013.862305
    Carron M. and Jenny G. (2018) Bat Acoustical Surveys at the National Renewable Energy Laboratory, National Wind Technology Center. National Renewable Energy Laboratory.
    Cryan P. M., Gorresen M. P., Hein C. D., Schirmacher M. R., Diehl R. H., Huso M. M., Hayman D. T. S., Fricker P. D., Bonaccorso F. J., Johnson D. H., Heist K., and Dalton D. C. (October, 21, 2014) Behavior of bats at wind turbines. Proceedings of the National Academy of Sciences of the United States of America, 15126-1513, doi: 10.1073.1406672111
    Frick W.F., Baerwald E.F., Pollock J.F., Barclay R.M.R., Szymanski J.A., Weller T.J., Russell A.L., Loeb S.C., Medellin R.A., McGuire L.P. (2017). Fatalities at wind turbines may threaten population viability of a migratory bat. Biological Conservation
    Hammerson G. A., Kling M., Harkness M., Ormes M., Young B. E. (2017). Strong geographic and temporal patterns in conservation status of North American bats. Biological Conservation, 212 Part A, 144-152, doi: 10.1016.2017.05.025

    Kasso M., Balakrishnan M. (2013) Ecological and Economic Importance of Bats (Order Chiroptera). International Scholarly Research Notices Biodiversity, Volume 2013, doi: 10.1155.2013.187415

    Lawson M., Jenne S., Thresher R. R. (2008) Estimating the Likelihood of Bat Barotrauma using Computational Simulations and Analytical Calculations B. Current Biology, 18, R695–R696
    (National Wind Technology Center) Reducing bat fatalities from interactions with operating wind turbines. National Renewable Energy Laboratory.
    Taber A. D., (November 15, 2018) Bats and Wind Energy: Impacts, Mitigation, and Tradeoffs. American Wind Wildlife Institute.
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