Near or far: revision of distribution, breeding and winter ranges of two sibling Pipistrellus species (Chiroptera, Vespertilionidae) in Eastern Europe and the Caucasus

1. Knowledge of species distribution and, for migratory species, seasonal occurrence is particularly important for vulnerable and protected animals such as bats. The former European bat species Pipistrellus pipistrellus was split into two, Pipistrelus pipistrellus s.s. and Pipistrelus pygmaeus, over 20 years ago. However, their distribution, breeding and winter ranges as well as migratory status in Eastern Europe and the Caucasus remained obscure.2. In our study, we scrutinised records of both species and assessed the sex ratio during their breeding season in this region.3. We show that Pipistrellus pipistrellus s.s. has a significantly smaller range than previously assumed, being restricted to the southern part of the study area. On the contrary, Pipistrelus pygmaeus has a broader distribution, covering the Caucasus and Eastern Europe, up to ca. 59 degrees N. Hence, all historical records and long-distance movements of Pipistrellus pipistrellus sensu lato from Eastern Europe within the inferred allopatric occurrence of Pipistrelus pygmaeus should be attributed to Pipistrelus pygmaeus.4. The high portion of adult males in breeding localities, mass wintering within the breeding range and lacking evidence of long-distance seasonal movements imply that Pipistrellus pipistrellus s.s. is a sedentary species. In Pipistrelus pygmaeus, the sex ratio among adults during the breeding season varies latitudinally. The female-biased sex ratio, absence of mass wintering and the historical long-distance recoveries of ringed individuals suggest that this species is migratory in Eastern Europe. The male-biased sex ratio during the breeding season, along with late autumn records, suggests that the Caucasus represents its winter range.