The Acorn Woodpecker (Melanerpes formicivorus) is a medium-sized woodpecker, 21 cm long with an average weight of 85 g.

The adult has a black head, back, wings and tail, white forehead, throat, belly and rump. The eyes are white. The adult male has a red cap starting at the forehead, whereas females have a black area between the forehead and the cap. The white neck, throat and forehead patches are distinctive identifiers.

The breeding habitat is forested areas with oaks in the hills of coastal California and the southwestern United States south to Colombia. This species may occur at low elevations in the north of its range, but rarely below 1000m in Central America, and it breeds up to the timberline. The breeding pair excavate a nest in a large cavity in a dead tree or a dead part of a tree. A group of adults may participate in nesting activities: field studies have shown that breeding groups range from monogamous pairs to breeding collectives of seven males and three females, plus up to 10 nonbreeding helpers. Young have been found with multiple paternity.

Acorn Woodpeckers are larder hoarders. Breeding groups gather acorns and create a granary by drilling holes in a dead tree, or a dead branch on a live tree, and stuffing acorns into them. The acorns are visible, and the group defends the tree against potential cache robbers. The acorns represent a significant part of their diet; they also eat insects, picking them off tree bark or catching them in flight, and in addition fruit, seeds and sometimes tree sap.

This bird is a permanent resident throughout its range. They may relocate to another area if acorns are not readily available. It is sedentary and very sociable.