Minecraft wiki entry for Spawn, associated text: The pack is spawned within a 41×1×41 (that's a 41×41 square that is one block high) area centered at the initial block. Mobs spawn with the lowest part of their body inside this area. For each spawn attempt, from the location of the previous attempt, a location up to 4 blocks away from the previous attempt is chosen at random. Thus, the spawns are heavily skewed toward the center of the pack. Approximately 85% of spawns are within 5 blocks of the pack center, and 99% within 10 blocks of the center. If the pack spawn enters a biome different from the starting biome, the rest of the pack and that spawn are canceled.