Wind power is a fast-growing renewable energy source with great potential to reduce pollution, water consumption, and greenhouse gas emissions (Ledec 2011, REN21 2015). Nevertheless, the large numbers of dead bats that have been discovered beneath operating wind turbines have raised concerns about its environmental sustainability (Kunz et al. 2007, Arnett et al. 2008). From 2000 to 2011, bat mortality at utility-scale wind turbines in North America was estimated to be 650,000 to 1,300,000 bats, which translates to an annual fatality rate of 54,000 to 108,000 bats per year (Arnett and Baerwald 2013). However, due to continued growth in the wind energy industry, the estimated mortality in 2012 alone was >600,000 bats (Hayes 2013, but see Huso and Dalthorp 2014). Three migratory, tree-roosting bat species (collectively known as tree bats), the hoary bat (Lasiurus cinereus), eastern red bat (Lasiurus borealis), and silver-haired bat (Lasionycteris noctivagans), make up 78% of total fatalities with peak fatality rates occurring between July and October, which coincides with fall migration (Kunz et al. 2007, Baerwald and Barclay 2011, Arnett and Baerwald 2013, Hein et al. 2013). Although reliable population estimates are lacking for these and other affected bat species, annual fatalities at wind turbines may have population-level consequences because bats are long-lived and have low reproductive rates which limits their ability to recover from declines (Barclay and Harder 2003, O'Shea et al. 2003, Brunet-Rossinni and Austad 2004, Podlutsky et al. 2005).
At present, we have a good understanding of the proximate causes of bat fatalities at wind energy facilities (collision with rotating wind turbine blades and barotrauma; Baerwald et al. 2008, Grodsky et al. 2011, Rollins et al. 2012). Nevertheless, we still do not have a good understanding of the reasons bats come in close proximity to wind turbines. Three broad hypotheses have been proposed to this end: (1) random chance, where wind turbine kills are simply a representative sample of bat abundance and activity in an area; (2) coincidence, where some aspect of bats’ ecology (e.g., migration) brings them in close proximity to wind turbines; and (3) attraction, where bats are attracted to wind turbines because wind turbines provide resources, bats perceive wind turbines to be resources, or bats are simply curious about wind turbines (Cryan and Barclay 2009). While some bat species and individual bats are likely to be at wind turbines due to random chance and coincidence, these two hypotheses alone do not appear to be able to explain the emerging patterns of bat fatalities (Kunz et al. 2007, Arnett et al. 2008, Hein et al. 2013).
The bat species commonly comprising fatalities at wind energy facilities all use echolocation rather than sight to navigate and capture prey (Barclay 1986, Obrist and Wenstrup 1998). McAlexander (2013) suggested that if the acoustic similarity between wind turbine towers and water contributes to bat-wind turbine collisions, then steps to reduce the similarity between wind turbine towers and water should reduce bat fatality rates at wind turbines. We hypothesized that incorporating texture additives (e.g., sand) as part of the wind turbine paint coating could alter the acoustic properties of wind turbine towers such that ultrasonic echoes returning from these modified towers could be differentiated from those returning from smooth surfaces. We further hypothesized that addition of texture additives with larger particle sizes would result in greater acoustic differences from smooth surfaces. As an initial step in testing these hypotheses, we conducted an ultrasonic playback experiment with the objective of determining the extent to which these modified surfaces could be differentiated from smooth surfaces using synthetic bat echolocation calls. In order to achieve our objective, we characterized the acoustic features of echoes returning from synthetic bat echolocation calls played at a range of smooth and textured surfaces.