Minecraft wiki entry for Beginner%27s_guide, associated text: If it is not sunny, you may have worse problems: You may need to kill zombies or even skeletons (or just retreat back in your shelter until the sun comes out). Zombies can be easy to kill by themselves, but in a pack, they can overrun you — and if attacked, they can call any other zombies in the area to join the fight. At night or during a storm, they can actually summon new zombies!