Minecraft wiki entry for Spawn, associated text: One in ten newly-generated chunks attempts to generate animal mobs, usually in packs of up to 4 of the same species. The spawn attempt always starts on top of the highest available block in a randomly chosen column within the chunk. The chosen position must not be a solid block for the animals to generate. Once the starting position is chosen for a chunk, a second position is chosen in a 9×9 block area around the starting position. Blocks toward the center of the 9x9 area are more likely to be chosen than blocks toward the edge. The block does not need to be a grass block nor does it need to be illuminated as it does with mob spawning. If an animal can spawn at the second position, it does so. The second position becomes the first position, and a new second position is chosen like before in a 9x9 area. This process can repeat until each chunk has attempted to spawn 1–4 mobs.