Composition, group size, and spatial distribution of European bison bulls in Bialowieza Forest

European bison Bison bonasus (Linnaeus, 1758) bulls were observed in the Biało­wieża Primeval Forest between 1976 and 1990. During snow-free periods mean (± SD) group size was 1.63 ± 1.05. Mean group size in pre-rut was 1.83 ± 1.19 and was significantly (p < 0.001) larger than mean bull group size in rut 1.27 ± 0.63. Most often bulls were solitary (62.0%), small groups of up to 3 bulls were accounted in 32.2% of observations, and groups of 4-9 individuals in 5.8% of observations. In pre-rut period bulls active in reproduction (6-11 years old) secluded themselves from mixed groups; they lived solitary or in bull groups. Mean summer home range size of bulls over 4 yrs old was 2288.0 ± 696 ha. Mean monthly home range in pre-rut period was 963.3 ± 406.4 ha, i.e. significantly (p < 0.001) smaller than in rut (1662.0 ± 510.6 ha). In the snow-free season bulls mostly used fresh and moist deciduous forests (48.8% of encounters), and secondly mixed coniferous and mixed deciduous forests (28.9 and 19.1%). Bulls used tree stands more than 20 yrs old (54.3% of observations), but also open areas, i.e. young plantations up to 10 yrs of age, clearcuts and meadows (allto- gether 37.3% of observations). In winter, on an average 35% of adult bulls entered large mixed aggregations gathering near two supplementary feeding sites. The re­maining bulls took advantage of other feeding racks or enter farmland. Mean winter home range size of bulls was 426.1 ± 307 ha, and is significantly positively correlated to the animal age.