Minecraft wiki entry for Spawner, associated text: The spawn conditions do not include biomes for most mobs. As such, spawners can place mobs where they normally wouldn't generate. For example, a mooshroom spawner can operate in a plains biome as long as there are mycelium blocks within the spawn area because the mooshroom's spawning code checks only for mycelium. The reason mooshrooms are not actually spawned elsewhere is that the game does not normally try to spawn them in other biomes: only the mushroom field biome has mooshroom on the list of things to spawn.