The annual life cycle of temperate bats is typically accompanied by seasonal movements. In autumn, individuals of several species
display very active flying behaviour around preferred locations, often hibernacula, a behaviour called swarming. This behaviour is
usually characterized by a strong male-biased sex ratio and is often considered to be related to mating. Alternatively, these groupings
may be a social behaviour related to the location and quality assessment of hibernacula and may enable this information to be
transferred to juveniles, the maternal guidance hypothesis. Our study reports the results of a weekly survey of a bat community in
an abandoned mine in Switzerland from April to October 2009, a period longer than the swarming period. The site is characterized
by numerous visiting species as well as a high number of common pipistrelle bats displaying autumnal swarming like behaviour at
the entrance of the mines. Thirteen species were present at the site with a total of 1,589 individuals of which 1,404 (88%) were
common pipistrelle bats allowing us to focus on intersexual and age variations in their arrival phenology. Although the recapture
rate was too low to obtain a reliable population size estimator, our data suggest that the site attracts individuals from a large area,
possibly from several different nursing colonies and therefore requires a national conservation status. Adult males were present at
the site throughout the study period with a peak from mid-July to the end of August. Females arrived later and both sexes displayed
a swarming like behaviour in July and August. During this period, the sex ratio was slightly female biased, an unexpected pattern
when compared with other swarming species. The absence of a male-biased sex ratio suggests that swarming behaviour in pipistrelle
bats may have functions other than mating. Juveniles of both sexes joined the site about two weeks later suggesting that the maternal
guidance hypothesis also did not explain this phenology pattern. The number of days between recaptures was greater for males than
females indicating that males stay longer or return more often to the sites than females. Further studies may bring insights to the
understanding of the function of swarming in pipistrelle bats.