A method for actively surveying mass hibernation sites of the common pipistrelle (Pipistrellus pipistrellus) in the urban environment

Abstract: Common pipistrelles (Pipistrellus pipistrellus) congregate in mass-hibernacula, sometimes with hundreds to thousands of individuals. In the urban environment, these hibernacula are often situated in crevices and other spaces in buildings with poor insulation. Efforts to reduce carbon emissions are a threat to these hibernation sites when roost entrances are sealed to reduce heat loss, which may even lead to the total loss of roosts in buildings. We describe a survey method, based on our experiences in cities in the Netherlands, for localizing mass-hibernation sites of common pipistrelles by locating autumnal swarming behaviour. To pre-select potential hibernation roosts we used freely accessible data on buildings and carried out visual daytime checks. This method combines information on bat behaviour in specific parts of the urban landscape, including the characteristics of buildings that contain these mass-hibernation roosts and their place in the landscape. The timing and performance of fieldwork targeting the autumnal swarming behaviour is also described. We assess the status of buildings as a mass-hibernation site for common pipistrelles with a three step filter-surveying approach: A. Daytime visual site inspections of pre-selected buildings. B. Night time checks on autumnal swarming of the selected buildings. C. Checks during cold winter spells at buildings with autumnal swarming, either visually or by combined visual and acoustic observations or by long-term acoustic recordings. We invite bat researchers and conservationists to build on our approach and adapt it to their local circumstances. We also challenge bat researchers to try this method for other pipistrelle species.