Spatiotemporal pattern in the autumn invasion behaviour of the common pipistrelle, Pipistrellus pipistrellus: Review with a case study

Common pipistrelle bats are known for their autumn or late-summer invasions, when temporary groups of individuals fly into inhabited buildings in urban areas. This specific display has been reported since 1862. In this review, we collected all available records and analysed their spatiotemporal pattern within the species range with regard to the numbers and structure of individuals involved in such invasions (in total 1,025 invasions from 51 publications and almost 80 unpublished or own records). We found that invasions occurred mostly in Central European towns in Slovakia, Czechia and Germany between 48 and 55° of northern latitude, mainly during the late decades of 20th century. Although the majority of invasions occurred just once or twice at a site and involved only dozens of individuals, sites that were repeatedly invaded also exhibited a higher number of bats involved (hundreds of individuals). The reviewed data suggest that predominantly young animals without sex bias took part in the invasions. A special emphasis was put on the city of Kosice, Slovakia, where invasions repeated annually since 1996, mostly in August and September, while bats altered the invasion sites during the analysed period. However, the number of invasions and invading individuals was related to the number of bats hibernating in the largest know hibernaculum in the species range (Erňa cave). This review suggests that invasions are some misconduct associated with the migration into large swarming and hibernation sites while the social memory of adult females should be a mechanism facilitating such seasonal movements in young bats. However, social calls of inexperienced juveniles which got stuck inside buildings tend to lure other individuals into this ecological trap.