As mentioned elsewhere, animals in MC are bugged to death. Chickens will happily vanish from wherever you put them regardless of what measures you put in place. Cows seem to stay where they are but will try their hardest to jail break. Anyone know how reliable pigs and sheep are?
I have started up a Pig Farm to keep up with demand on Pig racing, so I can tell you they are reliable i've only lost one in about a week or so but that was human error i hadn't raised the roof high enough, (costing me another saddle. -__- ) So yeh I reckon there Ok. as long as there's at least one spare hanging around to breed with.
That's the thing, I want to have a realistic looking animal pen, not something that'd test the might of the men of Stalag Luft III.