The common pipistrelle bat, Pipistrellus pipistrellus (Schreber, 1774), is one of the most widespread and abundant of Western Palaearctic vespertilionid bat species, occurring over almost all of Europe, with a northern limit at 56°N in southern Scandinavia. It also ranges into North Africa, Asia Minor and through the Middle East up to Afghanistan (Horáček et al. 2000; Benda et al. 2004; Hulva et al. 2004). The species was recognized as being a sibling of a very similar cryptic species, Pipistrellus pygmaeus (Leach, 1825), only two decades ago (Barratt et al. 1997; Mayer and von Helversen 2001) and there are still gaps in the knowledge about its specific ecology and behaviour. In general, P. pipistrellus is a very flexible species occurring in habitats within a gradient from highly urbanised city centres to settlements in rural landscapes, where they often prefer woodlands and waters (Barlow 1997; Dietz et al. 2009; Todd and Williamson 2019). The species can even use habitats associated with human-made linear infrastructures, like railways, mostly in an intensively managed agricultural landscape (Vandevelde et al. 2014). During seasons of activity these bats occupy variable shelters but with tendency to prefer synanthropic spaces. For example, crevices in buildings can be maternity roosts, which can be switched very frequently, although bats can sometimes be found in rock or tree fissures or in bat boxes (Feyerabend and Simon 2000; Dietz et al. 2009). Similarly, individuals and clusters of bats hibernate mainly in above-ground roosts, such as tree holes, the attics of churches and houses, behind cladding boards and under roof coverings, but also in cellars, tunnels and natural caves (Kretzschmar and Heinz 1995; Uhrin 1995; Park et al. 1996; Kaňuch et al. 2010). This species also forages over a wide range of habitat types, e.g., calm rivers or woodland edges and short isolated tree lines along roads (Vaughan et al. 1997; Warren et al. 2000; Davidson-Watts et al. 2006; Nicholls and Racey 2006; Sattler et al. 2007; Abbott et al. 2009; Todd and Waters 2017). Foraging sites of the species are often very close to the colony roost (Davidson-Watts et al. 2006).
The so-called invasion behaviour of common pipistrelle bats during the swarming time in late summer and in autumn (note that the original term ‘autumn invasion’ in not used here as in the context of biological invasions, possibly one may rename it as ‘incursion’) has been described as a remarkable species-specific ecological trait (Sachteleben 1991). These invasions represent the flight of numerous groups of individuals into unoccupied or inhabited buildings and similar human-made spaces, which are often unsuitable as roosts in general and can act negatively as ecological traps (e.g., Palášthy and Gaisler 1965; Sachteleben and von Helversen 2006). As revealed from a recent review, such an unusual behaviour of this species is limited within its range to several cities of Central Europe (mainly Germany, Czechia, Slovakia; Nusová et al. 2019a). It has been hypothesised that autumn invasions are related ecologically to the close proximity of a mass swarming site, which also act as a mass underground hibernaculum. For instance, invasions in the Košice urban agglomeration (eastern Slovakia) were positively correlated with the number of bats hibernating in the largest know hibernaculum of the species, the Erňa cave (Nusová et al. 2019a); however, the causality of this relationship is not yet understood.
It seems to be a rule that most swarming bats originate from the related hibernacula (Fenton 1969; Glover and Altringham 2008; van Schaik et al. 2015), and mass hibernacula also play a role as important swarming sites (Sendor and Simon 2003; Nusová et al. 2020); however, in the case of mass winter aggregations of common pipistrelles in the caves of the Carpathian Mountains (e.g. Dumitrescu and Orghidan 1963; Nagy and Szántó 2003; Horáček and Jahelková 2005; Nagy and Postawa 2011; Nusová et al. 2017), the question of the geographic origin of these bats arises. Ringing observations have suggested mostly short-distance seasonal movements for this species (Palášthy 1988; Avery 1991; Gaisler et al. 2003). In contrast, molecular data has revealed a substantial level of gene flow among summer colonies, which indicates some migratory status, that is, either mate-conditioned long-distance movements or sex-biased long-distance dispersal (Bryja et al. 2009; Nusová et al. 2017). Nevertheless, an analysis of stable hydrogen isotope ratios in the fur of bats swarming during the autumn at the Erňa cave revealed that members of this mass hibernacula aggregation are both local bats and occasional facultative migrants, which may undertake long-distance seasonal movements (Nusová et al. 2020).
Thus, a study of the activity of common pipistrelle bats in the commuting area between their natural hibernaculum and nearby large urban agglomeration could bring new insight into understanding the causality behind the specific late summer or autumn invasion behaviour. Therefore, we compared the activity of P. pipistrellus and other bat species in the city of Košice and surrounding zones (different directions, mostly in the Košická kotlina Basin, monitored by car-based transects), including the direction towards the Erňa cave, from the end of the breeding season through the late summer invasions and autumn swarming, prior to the onset of hibernation. We used additive modelling to test the hypothesis that the relative activity (number of echolocation records) of common pipistrelles, unlike other bats, will be higher in the city and in the area between the city and the mass hibernaculum. Similarly, we hypothesised that the activity in such a commuting area will increase during the course of autumn season.