Minecraft wiki entry for Shulker_farming, associated text: There is a chance that a new shulker of the same color will spawn when one shulker hits another shulker (or itself) with a shulker bullet. The hit shulker must have its lid open to spawn a new shulker. The shulker spawns where the old shulker was before it teleported. The chance of this happening decreases with the amount of shulkers in an 8-block radius; when more than 5 shulkers are present in the area, no shulkers will spawn. The probability is reduced for shulkers with less than half their health remaining.