Diverse but declining? Population genetic structure and genetic diversity of Nathusius’ pipistrelle along the Dutch coastline during the autumn migration period

Migratory bats are experiencing substantial increases in mortality risk from wind energy developments, but data on their migratory behavior and population dynamics are often lacking. Here, we develop a novel microsatellite panel for one such migratory bat species, the Nathusius’ pipistrelle ( Pipistrellus nathusii ), and apply it to 448 samples collected at stopover sites along the Dutch coast during autumn migration over four consecutive years. With this dataset, we assessed whether the population is genetically sub-structured, characterize its current genetic diversity, and evaluate whether mothers guide their offspring during migration. We found that the population is panmictic and diverse, with an effective population size estimate that cannot be distinguished from infinite. However, we also observed a consistent decline in allelic richness across the sampling period, as well as a heterozygote excess in individuals sampled as juveniles, both suggesting an ongoing population decline. We did not find any parent-offspring pairs in our dataset, which included 30 box captures where adult female and juvenile bats were found roosting together, suggesting that juvenile bats do not follow their mothers during their first migration. Our findings provide an initial characterization and baseline measure of genetic diversity for the Nathusius’ pipistrelle that can be used as a reference for subsequent studies and systematic efforts to monitor the genetic diversity of the species. Given that monitoring population trends of migratory bat species with traditional methods remains challenging, such tracking of genetic diversity may offer a valuable proxy by which to observe substantial population declines if they occur.