Use of sewage treatment works as foraging sites by insectivorous bats

Sewage treatment works (STWs) with percolating filter beds are known to provide profitable foraging areas for insectivorous birds because of their association with high macroinvertebrate densities. Fly larvae developing on filter beds at STWs may similarly provide a valuable resource for foraging bats. Over the last two decades, however, there has been a decline in filter beds towards a system of 'activated sludge'. Insects and bat activity were surveyed at 30 sites in Scotland using these two different types of sewage treatment in order to assess the possible implications of these changes for foraging bats. Bat activity (number of passes) recorded from broad-band bat detectors was quantified at three points within each site. The biomass of aerial insects, sampled over the same period as the detector surveys, was measured using a suction trap. The biomass of insects and activity of Pipistrellus spp. were significantly higher at filter beds than at activated sludge sites. In addition, although foraging activity of Pipistrellus spp. at filter beds was comparable to that of adjacent 'good' foraging habitat, foraging at activated sludge sites was considerably lower. This study indicates the high potential value of an anthropogenic process to foraging bats, particularly in a landscape where their insect prey has undergone a marked decline, and suggests that the current preference for activated sludge systems is likely to reduce the value of treatment works as foraging sites for bats.