The diet of B. bonasus varies seasonally. As discussed in sections below, bison aggregate into large groups during the winter, normally around areas where humans regularly place hay for them. Supplementing the diet of the bison this way may not be necessary for their survival, but has been done since the days when Bialowieza and its animals were protected by Polish or Lithuanian royalty, and continues as tradition. During the rest of the year, the bison are primarily grazers (accounting for 95% of feeding time), though they occasionally browse (3%) or eat bark (2%). The latter activity occurs mainly in early spring, when neither graze nor browse is available in large quantities. During the summer, when food is most plentiful, adult males may consume 32 kg of food per day, and adult females 23 kg per day. Borowski et al. (1967) present a list of specific food items utilized by B. bonasus in Bialowieza. Gebczynska et al. (1991) provide both an updated estimate of the number of plant species used by bison (110-140 species) and an analysis of seasonal variation in bison diets, based on rumen contents of culled animals. Their data are as follows: in spring (April-May), bison diet consists of: 8.8% trees; 0.1% bushes; 65.5% grasses and sedges; 1.5% herbaceous plants; and 24.1% mosses, pteridophytes, fungi, and unidentifiable food items. In summer (June-August), bison diet consists of: 9.8% trees; 1.4% bushes; 68.6% grasses and sedges; 1.7% herbaceous plants; and 18.5% mosses, pteridophytes, fungi, and unidentifiable food items. In autumn (September-October), bison diet consists of: 4.3% trees; 0.6% bushes; 69.9% grasses and sedges; 6.7% herbaceous plants; and 18.1% mosses, pteridophytes, fungi, and unidentifiable food items. In winter (November-March), bison diet consists of: 7.4% trees; 0.2% bushes; 72.4% grasses and sedges; 0.9% herbaceous plants; and 19.1% mosses, pteridophytes, fungi, and unidentifiable food items. Unlike certain other large ungulates, bison feeding tends not to greatly alter the habitat, except during winter aggregations of large groups and near popular watering places. (Borowski et al., 1967; Cabon-Raczynska et al., 1983; 1987; Gebczynska et al., 1991; Krasinska et al., 1987; Pucek, 1984)
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