This does not directly answer your question but it's worth mentioning.

Animals rarely "re-spawn" in the normal Minecraft. Most animals you see spawn with newly generated chunks. That is, when you visit an area for the first time, animals are likely to spawn. However, re-spawning in an already visited area is rare.

> Approximately one in ten newly generated chunks will contain mobs,
> usually in packs of up to four of the same species. They will always
> spawn on the highest available block in a column i.e. the one that can
> see the sky. For an animal to spawn on it, this block must be opaque and...

> Very rarely, new animals can spawn in already generated chunks, just
> like monsters do. When animals spawn in this way, they do so only on
> grass blocks with light level 9 or greater above them. This is also a
> requirement for animals spawning from monster spawners. Unlike
> monsters, animals do not spontaneously despawn, except for wild
> ocelots and wolves (which can despawn only when they are hostile).

This leaves us with the question of whether the spawning behavior in Pocket Edition is any different.


Reference: [Minecraft Wiki][1]

  [1]: http://minecraft.gamepedia.com/Spawning#Animal_spawning