During the winter season, bison head to lower ground in the north area of Yellowstone. The snow and temperatures are milder there, and it's easier to feed on grass under the snow. This can mean that hundreds of bison migrate to Lamar Valley, Mammoth Hot Springs, Old Faithful area and sometimes cross Yellowstone's border into private ranches in Montana. To avoid contact with livestock, states work with Yellowstone to keep the herds near the border and direct livestock to other grazing grounds. This also helps the bison have a shorter return to Yellowstone in warmer weather.
Watch the video below to see how bison shovel snow with their heads to get to the grass underneath. This is why bison often have their heads, which can weigh around 250 pounds, covered in snow. It can make quite a magical sight to see a frosty-looking bison. In deep snow, bison have been known to dig as far as four feet into the snow to get to their meals. While the grass is an important food source, it's fairly nutrient deficient in the winter time. Some Yellowstone staffers compare its winter nutritional content to eating cardboard.