Over the past dozen years, the population of bison (Bison bison) on the northern winter range of Yellowstone National Park has expanded from the traditional core along the lower Lamar River, occupied new foraging areas within the park, and moved outside YNP. The winter of 1975-1976 was exceptionally severe and apparently provided the initial impetus that led to the westward movement or stress dispersal of bison on the northern winter range. However, winter conditions subsequent to 1975 to 1976 did not appear to be sufficiently severe to contribute to the continued movements and increase in numbers. The largest movements recorded to date, spanning the entire winter, occurred during exceptionally mild conditions of 1986-1987. The movements of 1976-1987 were not significantly correlated with population increases on the northern range. Availability of adequate forage for increased numbers of bison did not appear to be a factor in the movements during recent mild winters. In 1986-1987 available forage for the increased population appeared more than ample, as a result of a wet summer in 1986 and lack of snow in winter. The northern range was pock-marked throughout the winter with the feeding of elk (Cervus elaphus) and bison, indicating that both species could travel and forage at will. The acquired knowledge of areas having less snow appears to have added impetus to the bison movements. Bison appear to be generalist foragers, requiring large quantities of forage. Bison distribution on the northern winter range appeared to be unstable geographically by 1987. The resent northern Yellowstone situation may represent an intermediate stage between a migratory and a nomadic pattern of land use.
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