The Minddeath

Whatever accidental process it was that caused the awakening of the animals of The Shire, it was very limited in scope to only within a hundred or so feet of the farm proper.  Within that boundary, mice and other small animals of that approximate size are gifted with human-level intelligence and dexterity.  Normal animals that cross the boundary into the farm will become awakened within a few days.
However, any awakened creature that crosses the boundary immediately reverts to being a normal variant of their species.  Mice become normal field mice, sparrows fly off to find a new nest, all driven completely by instinct instead of intellect.
Much more horrible, though, is the consequence of an animal returning over the border: they will again become awakened like any other animal, but their former memories will be completely gone.  For all intents and purposes, the original entity that inhabited that body is now gone forever.
All of the communities of the Shire have regular patrols of the borders nearby to watch for those who may have stumbled out or in and are in need of assistance. All of the communities will welcome a minddead creature in to become one of their members.
The border itself is invisible, and since it's not uniform, it's impossible to know for sure how close you really are at any time.  It's tradition for anyone nearby to mark the location where somemouse had become lost, providing a warning to proceed with caution.