Quote from Purplemonkey273
I've never really been bothered by animals spawning on mountains, but I think this would be a great idea. The only problem I see is that if you have too many of the tamed animals, you would never see any other animals anywhere, and I personally think that would make for a somewhat dull Minecraft experience. But if you only had like 6 or so of the same kind of animal, I don't think it would be much of a problem.
Another thing to add to that would be to cause an animal to despawn that cannot move in any direction more than a block. That would specifically target the "cliffhangers"
I don't mind the part you said about there'd be no animals elsewhere, because you can always breed less or kill your farm animals if it got that way (and you didn't like it). As it is now, I almost never see animals on the ground (except for wolves) anyways.
The problem with making animals that can't move specifically despawn is sometimes you want them like that in your far (ie sheep for easy shearing or cows for milking).