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.
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