COMPOSITION, GROUP-SIZE, AND SPATIAL-DISTRIBUTION OF EUROPEAN BISON BULLS IN BIALOWIEZA FOREST

European bison Bison bonasus (Linnaeus, 1758) bulls were observed in t he Bialo-wieza Primeval Forest between 1976 and 1990. During snow-free periods mean (+/- SD) group size was 1.63 +/- 1.05. Mean group size i n pre-rut was 1.83 +/- 1.19 and was significantly (p < 0.001) larger t han mean bull group size in rut 1.27 +/- 0.63. Most often bulls were s olitary (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 observation s. In pre-rut period bulls active in reproduction (6-11 years old) sec luded themselves from mixed groups; they lived solitary or in bull gro ups. 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 +/- 5 10.6 ha). In the snow-free season bulls mostly used fresh and moist de ciduous forests (48.8% of encounters), and secondly mixed coniferous a nd mixed deciduous forests (28.9 and 19.1%). Bulls used tree stands mo re 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 (alltoget her 37.3% of observations). In winter, on an average 35% of adult bull s entered large mixed aggregations gathering near two supplementary fe eding 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.