Yes, bison are migratory animals. When and where they migrate depends on a complex relationship between abundance of bison, quality and quantity of summer forage, and winter snowpack. In the Yellowstone area, they move from their summer ranges to lower winter ranges as snow accumulates and dense snowpack develops. The central herd moves both west and north toward park boundaries in winter, and may remain along the west boundary well into birthing season. The northern herd migrates within the park most winters, but will move beyond the north boundary in severe winters. How many bison used to live in what is now the United States? From 30 to 60 million bison may have roamed North America before the mid-1800s. They ranged across the continent, but the majority lived on the Great Plains.