Minecraft wiki entry for Spawner_traps, associated text: Zombie spawners are also a bit tricky, as the spawner may spawn baby zombies, which are a 1×1 mob, making proper containment essential, (particularly on harder difficulties). The zombies may spawn other zombies nearby to "help", making area security a priority, and the spawner may also spawn zombie villagers which makes adding in some manner of player-controlled sorting mechanism a consideration. On that note, however, zombie spawners can prove to be one of the most lucrative to trap, as the variety of items it can drop is extensive, and potentially limitless if you factor in the added option to use it to generate Villagers, which can then, in turn, be traded with, themselves "farmed", or used to make iron golem farms.