The acorn woodpecker (Melanerpes formicivorus ) is a medium-sized woodpecker, 21 cm (8.3 in) long, with an average weight of 85 g (3.0 oz).
Acorn woodpeckers are medium-sized birds famous for their unique behavior of storing acorns in holes of trees. Adult birds have a brownish-black head, back, wings and tail, white forehead, throat, belly, and rump. Their eyes are white. There is a small part on the small of their backs where there are some green feathers. Adult males have 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. When flying, Acorn woodpeckers take a few flaps of their wings and drop a foot or so. White circles on their wings are visible when in flight.
Acorn woodpeckers are found from Oregon, California, and the southwestern United States, south through Central America to Colombia. They don't migrate and live year-round in forested areas with oaks, open oak groves near the coast, mixed forests, oak-pine canyons, and foothills.
Acorn woodpeckers have a complicated social structure; they live in family groups which raise young, gather food, store and guard it all together. Acorn woodpeckers, as their name implies, depend heavily on acorns for food. In some parts of their range, such as California, they create granaries or "acorn trees" by drilling holes in dead trees, dead branches, telephone poles, and wooden buildings. They also drill holes in the thick bark of mature living trees. These holes, always above the snow line so that the acorns can be retrieved in winter, can be observed in the hundreds on large trees. They do not harm the tree. 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 and granary maintenance requires a significant amount of the bird's time. The acorns are visible, and a group defends its granary against potential cache robbers like Steller's jays and Western scrub jays. Acorn woodpeckers will also spend their daytime sallying from tree limbs to catch insects, eating fruit and seeds, and drilling holes to drink the sap. These master hoaders communicate with each other vocally and their call sounds almost like they are laughing.
Acorn woodpeckers range from monogamous pairs to breeding collectives, sometimes called "coalitions" - a social system when offspring receive care not only from their parents but also from additional group members, often called helpers. Cooperative collectives may also contain groups with multiple breeding males and females (polygynandry). With Acorn woodpeckers, cooperative breeding occurs in two ways: coalitions and family groups. Coalitions of adult Acorn woodpeckers nest together, localizing to storage granaries. Additionally, adult offspring often stay in their parents' nest and help raise the next generation of woodpeckers. Breeding coalitions consist of up to seven co-breeding males and up to three joint-nesting females. However, most nests are made up of only three males and two females. Nesting groups can also contain up to ten offspring helpers. Acorn woodpeckers nest in large cavities excavated in a dead tree or a dead part of a tree. Each female lays about 5 eggs. In groups with more than one breeding female, the females put their eggs into a single nest cavity. A female usually destroys any eggs in the nest before she starts to lay. Once all the females start to lay, they stop removing eggs. Incubation lasts about 11 days. The chicks are altricial; they hatch blind, naked, and helpless. The young fledge at 30-32 days of age and become independent when they are 2 months old.
Acorn woodpeckers, like many other species, are threatened by habitat loss and degradation. Competition for nest cavities by non-native species is an ongoing threat in urbanized areas. These birds are also sometimes killed because they cause damage to fruit crops.
According to Partners in Flight resource, the total population size of the Acorn woodpecker is 7,500,000 breeding birds. Currently, this species is classified as Least Concern (LC) on the IUCN Red List and its numbers today are increasing.