The Acorn Woodpecker (Melanerpes formicivorus) is a medium-sized American woodpecker. It depends heavily on acorns for food and family groups create granaries or "acorn trees" by drilling holes in dead trees, dead branches, and wooden buildings. The woodpeckers then collect acorns and find a hole that is just the right size for the acorn. As acorns dry out, they are moved to smaller holes. The acorns are visible, and the group defends the store from other acorn eating birds and mammals. Studies have shown that breeding groups range from monogamous pairs to breeding collectives of seven males and three females and up to 10 nonbreeding helpers. Young have been found with multiple paternity.