How to leave the church: light avoidance by brown long-eared bats

Light pollution, light in the wrong place at the wrong time, is an emerging environmental issue with wide-ranging consequences. It interferes with the fundamental 24 h light–dark cycle, and has biological effects at all levels, from molecules to ecosystems, including human health and welfare. Here, exemplified by flood-lit churches, artificial lights compromise the survival of resident bats, as darkness provides protection from predation. We predicted that brown long-eared bats Plecotus auritus emerging from churches should (1) avoid illuminated church walls, and (2) avoid extended flights in the open. To test these predictions, bats were observed emerging from 33 country churches in Sweden. A model for light intensity at increasing distances from light sources was made. This model, in combination with known distance between church walls and surrounding lamps, was used to predict lux levels (lx) at each church wall. Higher light intensities were strictly avoided and the majority of bats emerged when light intensities at church walls were below 1.25 lx. Further, most brown long-eared bats flew into the canopy of trees less than 15 m distant from the churches. Bats and their roosts are strictly protected in Europe, but this is poorly enforced with respect to churches and other historical buildings. Nevertheless, lighting of buildings is a key issue in bat conservation and of worldwide significance. We strongly advise that installation of lights near historical buildings is regulated and subject to environmental impact assessments.