Northern movements of European Nathusius’ pipistrelle bats ( Pipistrellus nathusii ) before autumn migration

Despite decades of protecting migrating animals by international treaties, conservation efforts are severely hampered by our limited understanding of large-scale seasonal movements, particularly in cryptic migratory animals like bats. Banding data from a field site on the Latvian coast of the Baltic Sea suggests that some European Nathusius’ bats (Pipistrellus nathusii) move > 2000 km to their wintering range in Spain and France. As the banded bats were trapped en route, their summer origins remain unknown. Here, we used a stable hydrogen isotope approach to track the provenance of almost 700 P. nathusii moving southwards along the Latvian coast during three consecutive years. In general, P. nathusii originated from a large catchment area, with some geographic variation between years. About a quarter of all individuals came from areas south of Latvia, including northern Ukraine and neighbouring countries. This is consistent with the stable carbon isotope data, suggesting a summer origin south of the site for some of the captured bats. Our findings are in line with recent insights into the migratory orientation of temperate zone bats, with some individuals moving northwards before continuing to their southern wintering range. In conclusion, the migration of P. nathusii, and potentially also those of other temperate zone bats, may be more complex than previously anticipated.