An unusually harsh winter that buried Yellowstone National Park under a heavy blanket of snow and ice this year pushed a large portion of the park’s bison herd down to lower elevations and out of the park in search of milder climes and food. The hunt is part of a strategy to prevent them from getting near livestock, because about 60% of the bison herd carries a disease, brucellosis, that could infect cattle and cause cows to abort their calves.