We have the same problem with our hens who live in one end of the orchard but are allowed out to free range part of the rest of it during the day. I bought some plastic mesh fencing from Omlet, about 4' high, and we move that around as needed so that we can control the area that they access. This gives us a chance to fill in the hoels where they've been dust-bathing and reseed bare patches. Occasionally one of the hens will get ambitious and manage to get over it, but that's the exception rather than the rule.