Body weight provides insight into the feeding strategy of swarming bats

Temperate bat species that hibernate in underground sites are known to visit hibernacula in late summer where they perform an activity known as ''swarming'. We analysed trends in body mass-to-forearm ratio - the body condition index (BCI) - to investigate whether bats arrive at swarming sites following intensive nocturnal feeding. In a two-year study, Myotis daubentonii, M. brandtii and M. dasycneme were captured by mist-net at one swarming site from late July until early October. In late summer, the BCIs of captured males, females and subadult bats were positively correlated to the time of their capture during the night. In September, adult bats had higher BCIs than in late summer and the BCI did not increase during the night. For subadult M. daubentonii, the BCI was positively correlated to the time of capture in early autumn. Our results indicate that in late summer bats feed intensively during the first hours of the night before visiting swarming sites. In early autumn, subadult bats may go on feeding to complete fat accumulation.