The acorn woodpecker (Melanerpes formicivorus) is a medium-sized woodpecker with a length of around 20 cm (8 in), and an average weight of 85 g (3.0 oz). It is found across Central America, as well as North into the western United States and South into parts of Colombia.

The acorn woodpecker was formally described in 1827 by the English naturalist William Swainson under the binomial name Picus formicivorus from a specimen collected in Mexico. The specific epithet combines the Latin formica meaning "ant" with -vorus meaning "eating". The type locality is Temascaltepec in Mexico. The acorn woodpecker is one of 24 species now placed in the genus Melanerpes that was introduced by Swainson in 1832. Within Melanerpes the acorn woodpecker is sister to a clade containing two South American species: the white woodpecker (Melanerpes candidus) and the white-fronted woodpecker (Melanerpes cactorum).

Seven subspecies are recognised:

The adult acorn woodpecker has a brownish-black head, back, wings and tail, white forehead, throat, belly and rump. The eyes are initially dark in fledglings, turning to white within a few months. There is a small part on the small of their backs where there are some greenish feathers.^{[citation needed]} The bird is mostly black, with 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, they 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 have a call that sounds almost like they are laughing.

Measurements:

The acorn woodpecker's habitat is forested areas with oaks in the coastal areas and foothills of Oregon, California, and the southwestern United States, south through Central America to Colombia. This species may occur at low elevations in the north of its range, but rarely below 1,000 m (3,300 ft) in Central America, and it breeds up to the timberline. Nests are excavated in a cavity in a dead tree or a dead part of a tree.

Acorn woodpeckers are cooperative breeders, living and breeding in family groups of up to 15 individuals. Field studies have shown that within the same population, groups range from monogamous pairs to polygynandrous breeding collectives consisting of coalitions of up to 8 males and 4 females, along with nonbreeding "helpers at the nest" that are offspring from prior breeding events. Regardless of composition, all breeder males (who are usually brothers or fathers and their sons) compete for matings with all breeder females (who are sisters or a mother and her daughter), the latter of which lay their eggs communally in the same nest cavity. There is considerable variability within and among populations, suggesting extraordinary social plasticity. Cooperative breeding, defined as more than two birds taking care of nestlings in the nest, is a relatively rare evolutionary trait that is thought to occur in only nine percent of bird species. Most cooperative breeding species have helpers at the nest, but acorn woodpeckers are unusual in exhibiting both helping at the nest and cooperative polygamy (polygynandry). It is generally believed that limited territories are a key driver of cooperative breeding behavior in birds, and in the case of the acorn woodpecker, the availability of acorn storage granaries (see below) is a key limited resource.

Breeding coalitions consist of up to eight cobreeding males and up to four joint-nesting females. However, most nests consist of only a single breeder female and 1 to 3 cobreeder males. Nesting groups can also contain up to ten offspring helpers. As mentioned above, the breeder males are often brothers, and the females are usually sisters. However, reproductive vacancies—formed when all the breeders of one sex die—are filled by unrelated birds from elsewhere, so inbreeding is rare, despite the high degree of relatedness among most group members.