The Acorn woodpecker is a medium-sized woodpecker (21 cm long) with a very characteristic appearance. In English it is called Acorn woodpecker because an important food is acorns which it also collects and stores in so-called "granary trees". It cuts out a hole in the tree trunk that just holds an acorn and then places an acorn there. A granary tree can contain hundreds of acorns and be a store for a group of Acorn woodpeckers. These breed, interestingly enough, in a form of group marriage with usually three males and two females, together with a group of juveniles (yearlings) that help to feed the nestlings.