I have a chicken pen outside and got about 8 in there for an egg farm and everytime I go around to it there are less and less in there, I don't think they're escaping because I even made the fence two high and filled in gaps around it, that didn't work.
This is just how the game was during 1.6. I *THINK* I read that 4J is changing this in the 1.7+ update. In fact, I'm pretty sure they are. I played on PC even before 1.6, and we could pen animals well before the adventure update.
Anyway... There's a list floating around somewhere. Just google it.
Friendly mobs will no longer respawn (for the most part) nor despawn in Beta 1.8. They only spawn in new chunks (again, for the most part). So they become a mostly limited resource. Breeding will be added in Release 1.0.