Wind turbines have been linked to high numbers of bat fatalities worldwide (Voigt et al. 2012; Hayes 2013; Lehnert et al. 2014), with estimates of over 600,000 bats killed in the US alone for the year of 2012 (Hayes 2013). Within the US, the majority of bats killed at wind turbines are echolocating, insectivorous, migratory species that roost in trees (the top 3 species collected in carcass searches at wind turbines are the hoary (Lasiurus cinereus), eastern red (Lasiurus borealis), and silver-haired (Lasionycteris noctivagans) bat; reviewed in Arnett and Baerwald 2013; Schuster et al. 2015). Many of these fatalities correlate with nights with lower wind speeds, as bats will often restrict their movement during periods of strong wind velocities, and between the mid-summer to early autumn months, a time in which migratory bats will relocate to their winter habitats (Horn 2008; Arnett et al. 2011). A possible strategy to reduce fatalities has been to increase the cut-in speed (the speed at which a wind turbine begins to produce electricity); however, this mitigation measure results in a decrease in the amount of electricity produced by the wind turbines. Furthermore, the ultimate causes, why the bats are coming close enough to be hit by the wind turbine blades, remain largely unknown (Kunz et al. 2007; Arnett et al. 2008).
McAlexander (2013) also demonstrated that wind turbine towers are acoustically similar to smooth surfaces such as water. Based on this result, we hypothesized that the smooth surface of a wind turbine tower could act as an acoustic mirror, and any invertebrates aggregating on the light-colored surface may represent a readily accessible foraging resource for bats. In order to take advantage of prey resting on the turbine towers, however, bats would have to switch from an aerial hawking strategy to a gleaning foraging strategy. If bats that are commonly killed at wind turbines can switch foraging strategies to take advantage of conspicuous and abundant prey on turbine towers, then this foraging activity (i.e., bats entering the rotor swept zone, defined as the area occupied by the blades of the wind turbine when they are in motion, to glean prey from the tower surfaces) may be contributing to bat-wind turbine mortality. We further predict that if the turbine tower surfaces were altered so that they were no longer smooth, then prey items would not be as conspicuous to echolocating bats because of the background clutter generated by returning echoes from the textured surface itself (Clare and Holderied 2015; Fig. 2). Thus, we hypothesize that texturizing wind turbine towers would eliminate the acoustic mirror effect, thereby reducing a bat’s ability to efficiently locate prey items on the towers, which in turn would make wind turbines a less suitable foraging resource.
We therefore designed a two-part experiment using wild-caught bats (including species commonly killed at wind turbines) in a flight facility to 1) determine if aerialhawking bats could switch to a gleaning foraging strategy, and 2) estimate foraging success at a range of smooth and textured surfaces. Ultimately, the results from these experiments will provide important insights into bat foraging behavior. In addition, this research may be used to inform the development of a texture coating that could be used as a novel mitigation strategy to reduce bat foraging activity at turbine towers, especially within the rotor swept zone, and thus reduce bat fatalities at wind facilities worldwide.