Tracing the geographic origin of common pipistrelles (Pipistrellus pipistrellus) swarming at a mass hibernaculum

Abstract

Mass swarming of tens of thousands of common pipistrelles in front of hibernacula of the Carpathian Mountains suggests that bats may originate from a large catchment area. However, until now neither banding nor molecular data have resolved the geographic origin of common pipistrelles at these sites. Here, we measured the acoustic activity of bats and the stable hydrogen isotope ratios (δ2H) in metabolically inert fur keratin of bats to infer the relative swarming activity and the putative summer origin of bats, respectively, observed in autumn at Erňa cave, one of the largest bat hibernacula in Europe. Swarming activity declined with decreasing ambient temperature during the early season, while it increased during colder days towards the onset of hibernation. Based on δ2H values, we deduced that about 50% of the animals did not have a local origin. Provenance of all but one of these migrants was identified as the Pannonian Basin, while a single long-distance migrant may have originated from the northern margin of the species’ European distribution range. Modelling the variation in δ2H values of bats in response to sex, body condition and season suggested that, towards the onset of hibernation, males of low body condition were likely to be of distant geographic origin. Throughout the swarming season, females were mostly of local origin, yet towards the onset of hibernation, their body condition was not as variable as in males. We conclude that common pipistrelles observed at this mass hibernacula site are facultative migrants, which may undertake long-distance seasonal movements occasionally. At our study site, common pipistrelles are more likely to include long-distance migrants because of the mass occurrence of this species at this large hibernaculum.