Habitat selection and home range size of field voles Microtus agrestis in S³owiñski National Park, Poland Zbigniew BOROWSKI Borowski Z. 2003. Habitat selection and home range size of field voles Microtus agrestis in S³owiñski National Park, Poland. Acta Theriologica 48: 325–333. Habitat preference, home range size and intra-specific overlap were investigated in summer 1998 among field voles Microtus agrestis (Linnaeus, 1761) in S³owiñski National Park (N Poland). Eight individuals (2 females, 6 males) were radio-tracked for one week in August. Field voles were shown to exhibit a marked preference for meadow and the ecotone between grassland and habitats with common reed, while avoiding alder forest and proper reedbeds. No significant differences between night and day in habitat-use of voles were noted. The results suggest that, at the end of the breeding season, it was food resources, rather than the risk of predation, played an important role in the voles’ utilisation of space. The home ranges of males were larger and more diverse than those of females; their sizes being correlated with body mass, such that heavier males had larger home ranges. This further suggests that intra-sexual com- petition exists between males for females. The low number of females influenced their spatial behaviour, as females had completely exclusive home ranges. Four males (out of six) had overlapping home ranges with other males; three of the overlaps were of less than 20%. Attributes of promiscuity (such as a 3.5:1 operational sex ratio of males to females, intra-sexual competition between males and the territorial exclusivity of females) influenced the social system. However, the period of radio-tracking during this study was too short to define accurately the social system in the field vole population. Section of Wildlife Management, Forest Research Institute, Bitwy Warszawskiej 1920 r. 3, 00–973 Warsaw, Poland, e-mail: Z.Borowski@ibles.waw.pl Key words: Microtus agrestis, home range size, space use, habitat selection, radio- -telemetry Introduction The patterns of space-use by individuals are perceived as among the main factors capable of influencing the density of populations of small mammals (Charnov and Finerty 1980, Hestbeck 1982, Krebs 1985). In small rodents, the social behaviour of females is of primary importance to population dynamics, since this may regulate the density of populations of breeding females (eg Bujalska 1973, Madison 1980). Referring to arvicoline rodents, Ostfeld (1990) hypothesized that variability in social and mating systems could be explained by reference to differences in patterns to the availability of food resources. These differences are an important factor in the evolution of patterns of the space use by females. The temporal distribution of receptive females influences the defensibility of females by [325] Acta Theriologica 48 (3): 325–333, 2003. PL ISSN 0001–7051