Light-averse behaviour of attic-dwelling bats when commuting through urban areas

Abstract

Light pollution is spreading around the world, causing a variety of problems, particularly for nocturnal animals such as bats. As urban areas expand, attic-dwelling bats face the problem of being exposed to more and more artificial light as they move to their dark foraging areas. Avoiding street lights could force bats to make energetically costly detours, which could eventually lead to the collapse of urban colonies. We therefore investigated the response of attic-dwelling bats to street lighting, using the brown long-eared bat (Plecotus auritus) as an example. Using radio-tracking, we followed 17 bats during their commutes through two urban areas: a small town and a village in rural Germany. After a conservative filtering process to ensure a high spatial accuracy of the tracking data, we analysed the data of nine bats in detail, using a use-versus-availability approach for the small town and a descriptive approach for the smaller dataset obtained from the village. The best-fitting model confirmed that the bats responded to artificial light at night. Close to the roost (∼10 m), bats preferred areas with low light intensities, but seemed to tolerate some light levels. We speculate that this pattern was caused by bats being forced to leave the roost into an illuminated area. At medium distances from roosts (∼400 m), most bats avoided street lights. In fact, all bats avoided being in close proximity to street lights, even when the lanterns produced only low levels of light. We conclude that brown long-eared bats avoid street lights when moving through urban areas. We recommend the establishment of dark corridors in urban areas to support high bat diversity, including all species along the continuum of light-opportunistic and light-sensitive species.