Mamm. biol. 67 2002) 179±184 Mammalian Biology ã Urban & Fischer Verlag http://www.urbanfischer.de/journals/mammbiol Zeitschrift fu È r Sa È ugetierkunde Short communication Numbers of hibernating Barbastella barbastellus Schreber, 1774) Chiroptera, Vespertilionidae) and thermal conditions in military bunkers By K. SACHANOWICZ and K. ZUB Polish Society for the Protection of Birds, Gdan Âsk and Mammal Research Institute, Polish Academy of Sciences, Biaøowiez Ça, Poland Receipt of Ms. 28. 02. 2001 Acceptance of Ms. 18. 12. 2001 Key words: B. barbastellus, numbers, temperature, bunkers, wells Among bats wintering in military fortifica- tions in north-eastern and central Poland, B. barbastellus may be one of the dominant species Lesin  ski 1986, Fuszara et al. 1996). Mass hibernacula of this bat are known in central Europe, where it is still fairly common, whereas in western Europe, it is rare and threatened Urban  czyk 1999). Community structure, winter dy- namics, thermal and shelter preferences of B. barbastellus were studied in Nietoperek Bat Reserve and fortresses in western and central Poland, where numerous species of bats coexist in large numbers Bagrowska- Urban  czyk and Urban  czyk 1983; Bogda- nowicz and Urban  czyk 1983; Lesin  ski 1986; Urban  czyk 1991). However, many different types of bunkers, dating from World War II, are scattered in eastern Po- land. Scant data concerning community structure of bats hibernating in these bun- kers, which may be important hibernation roosts of B. barbastellus , have been pub- lished to date Fuszara et al. 1996). The aim of this study was to determine mid- winter community structure of bats hiber- nating in bunkers, the importance of water wells inside bunkers as winter roost sites of B. barbastellus and differences in utilisation of wells and bunkers under changing ther- mal conditions during the hibernation pe- riod. The studied bunkers are located on the Podlasie Lowland central-eastern Poland). They are recognised as a part of the Molo- tov Line fortified front, which was con- structed in 1940±41 by the Soviets, to secure the war-time border between the Soviet Union and Germany Ribbentrop±Molotov agreement). Four, better preserved bunkers numbered S3, S5, S7, S13), were selected for the study from the available bunkers. They are situated in agricultural landscape, among villages: Anusin, Olendry, Mac Âko- wicze and Boratyniec Ruski, near the town of Siemiatycze 22°51' E, 52°25' N) and the bunkers are a maximum of 2 km apart. Each of the studied bunkers contains sev- eral small rooms located on two elevations and wells for drinking water from 3 to 6 m in depth), situated under the bunkers. Three of the wells were free of water inside and one contained of about 30 cm of water. The wells were constructed of concrete rings 120 cm in diameter and 50 cm in height. It was necessary to count bats hiber- nating in crevices between rings, since they were not visible watching from the entrance of wells. One of us was able to climb down to inside the wells, using metal elements 1616-5047/02/67/03-179 $ 15.00/0.