Automated long-term monitoring of RFID-tagged individuals reveals high hibernaculum site fidelity in Daubenton's bats and Natterer's bats

In the temperate zone, many bat species depend on underground sites during the autumn mating period and as hibernacula in winter. Anthropogenic or natural alteration or destruction of a hibernaculum may severely threaten the bats that rely on the site, especially if they are not aware of suitable alternatives. Currently, the rate at which bats visit other hibernacula, or disperse between them, is poorly understood. Here, we studied hibernation site fidelity in two European bat species, Daubenton's bat (Myotis daubentonii) and Natterer's bat (Myotis nattereri), that share hibernacula but differ in their hibernation phenology and mating systems. The activity of over 2500 RFID-tagged individuals was monitored using an automated recording system at the entrances of three hibernacula over eight full years. Hibernation site fidelity was high in both species, with only 1.6% of Daubenton's bats and 4.1% of Natterer's bats moving between the monitored sites during their lifetimes. Juvenile bats moved more frequently than adults and contrary to our expectations, in Natterer's bats, more females moved between hibernacula than males. Movements almost always occurred during the autumn mating season, and more frequently between neighboring hibernacula than between distant sites. Finally, we also found that individuals rarely explored more than one of the three entrances to a site, despite their close proximity. Taken together, our results suggest that some juvenile bats may explore multiple sites, but that they rarely disperse to other hibernacula as adults. As a result, most individuals are either not aware of alternative sites or do not regularly update this information. We therefore argue that the creation of new underground objects as a mitigation measure may not function as an immediate compensation for the disturbance or the loss of an existing hibernaculum. Instead, the proactive identification and protection of existing hibernacula should be prioritized.