| Summary: | Kaunas Fortress is one of the largest bat hibernation sites known in northeastern Europe. Bat counts have been carried out there during two periods (1978–1995 and 2007). In 2007, 52 underground and semi-underground roosts belonging to seven forts and two bunkers were inspected. In all, 958 bats of 8 species were found. Slightly more bats were apparently present as some parts of the roosts were inaccessible for study. Comparison of the counting data collected from 15 selected roosts during two periods shows that Myotis daubentonii and M. nattereri had increased significantly while Plecotus auritus and Barbastella barbastellus had decreased. Comparison of bat counting data collected from the whole Kaunas Fortress shows that bat numbers (all species together) had not changed much during three decades, but differences existed between species. Thus, in three bat species (Myotis dasycneme, Plecotus auritus, and Barbastella barbastellus) the numbers had decreased, while in Myotis brandtii the numbers had increased threefold. The reasons for this increase are not known. Bat hibernation conditions prevailing in Kaunas Fortress have remained largely the same over decades, but some changes caused by humans have occurred.
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