Minecraft wiki entry for Shulker, associated text: Shulkers generally remain stationary, attached to an adjacent solid block. However, if the block the shulker occupies becomes something other than air or the shulker finds itself not adjacent to any solid block, or the shell cannot open, it attempts to teleport (5 attempts per tick) to an air block within a 17×17×17 cube centered on the shulker that is adjacent to a solid block. If it finds no solid blocks to teleport onto (like when in the Void) it does not move.