Yellowstone Buffalo

We saw three buffalo just outside the park. We were told that they are part of the Yellowstone herd and that they are allowed to roam wherever they please. As long as there are just a few buffalo, together, outside the park, they are left alone. When the herd size increases to 15 or so, the round them up and drive them back into the park. I guess they came down to get out of the snow!! LOL