Multicopter-assisted acoustic surveys complement ground-based transects in assessing bat activity and species group composition

Abstract

We evaluated multicopter-based acoustic transects as a complement to traditional ground-based bat surveys, which cannot reach the airspace in which wind turbines operate. We compared species group composition and activity levels acoustically recorded along the same transect routes, surveyed simultaneously at ground level and at 104 m height with two multicopter types, across three habitat types (forest, meadow, and waterbody). Overall bat activity was almost eight times higher at ground level than at 104 m. Myotis species were virtually absent at 104 m, Pipistrelloid activity declined strongly with height, and Nyctaloid activity did not differ significantly between the two heights. Activity at 104 m was largely unrelated to the habitat structure at ground level. The two multicopter types did not differ in recorded activity when the recording device was positioned sufficiently below the multicopter and oriented away from it. Multicopter-based acoustic transects therefore extend the vertical reach of bat surveys into the risk area around wind turbine nacelles and can inform conservation and mitigation planning.