Minecraft wiki entry for Spawner, associated text: The spawner performs a relaxed version of the ordinary spawning check: the general solid block requirement is removed, but the volume (hitbox of the mob), the light level, and other checks are kept. As a result, for some types of mobs to spawn in the outer planes of the spawning volume, some planes outside the volume may also need to be free of opaque blocks to conform with the mobs' height, width, or other rules governing their individual spawn volumes. For example, for mobs that are two or more blocks tall such as zombies, skeletons, or blazes to spawn in the top y-layer, the layer above that must contain only air.