23-3-2017 TROGON LODGE, COSTA RICA - ACORN WOODPECKER (Melanerpes formicivorus) COURTESY MRS VALERIE FISHER

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).

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. In most subspecies, 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.

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, as their name implies, depend heavily on acorns for food. Acorns are such an important resource to the California populations that acorn woodpeckers may nest in the fall to take advantage of the fall acorn crop, a rare behavior in birds. Acorns are stored in small holes drilled especially for this purpose in "granaries" or "storage trees"--usually snags, dead branches, utility poles, or wooden buildings. Storage holes--always in dead tissue such as bark or dead limbs--are used year after year, and granaries can consist of thousands of holes, each of which may be filled by an acorn in the autumn. Access to acorn crops influences the composition of acorn woodpecker communities. In one study in New Mexico, there were about 90% of non-breeding adults per social unit in 1976, a year of a poor acorn crop. The following year, 1977, there was a significant increase in acorn production and a correlating decrease in non-breeding adults per unit. 

Although acorns are an important back-up food resource, acorn woodpeckers primarily feed on insects, sap, and fruit. They can be seen sallying from tree limbs to catch insects, eating fruit and seeds, and drilling holes to drink sap.

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.

In some more tropical parts of its range the acorn woodpecker does not construct a "granary tree", but instead stores acorns in natural holes and cracks in bark. If the acorn crop is poor and birds cannot find enough to store, the woodpeckers will move to other areas over the winter.

23-3-2017 TROGON LODGE, COSTA RICA - ACORN WOODPECKER (Melanerpes formicivorus)

The Acorn Woodpecker (Melanerpes formicivorus) is a medium-sized bird, notable for its striking appearance and unique behaviors. It measures approximately 21 cm (8.3 inches) in length and weighs around 85 g (3.0 oz). The species exhibits sexual dimorphism, with males sporting a vibrant red cap starting at the forehead, while females have a black area separating the forehead from the cap. Both sexes have a brownish-black head, back, wings, and tail, contrasted by a white forehead, throat, belly, and rump. A distinctive feature is the white patches on their neck, throat, and forehead, as well as white circles on their wings, which are particularly visible during flight.

When observing Acorn Woodpeckers, look for the white irises that develop a few months after fledging, the greenish feathers on the small of their backs, and the characteristic white wing patches in flight. Their undulating flight pattern, consisting of a series of flaps followed by a drop, is also a key identification marker. The males' red cap and the females' black forehead area are useful for sex differentiation.

This species thrives in forested areas rich in oak trees, which are essential for their diet and nesting habits. They are commonly found in the coastal regions and foothills of Oregon, California, and the southwestern United States, extending south through Central America to Colombia.

The Acorn Woodpecker's range spans from Oregon in the United States to north Baja California in Mexico, and from southwest USA to southeast Mexico, with populations also found in Central America down to Colombia.

Acorn Woodpeckers are known for their complex social structures and cooperative breeding systems. They live and breed in family groups that can include up to 15 individuals. These groups may consist of monogamous pairs or polygynandrous collectives with multiple breeding males and females, along with non-breeding helpers that are typically offspring from previous seasons.

The species is renowned for its distinctive call, which resembles laughter, adding a touch of whimsy to their forested habitats.

Breeding in Acorn Woodpeckers is a communal affair, with multiple females laying eggs in a single nest cavity. The group's breeding males, often related as brothers or fathers and sons, compete for mating opportunities with all breeding females within the group. These females, usually sisters or a mother and her daughter, lay their eggs together, creating a shared brood.

True to their name, Acorn Woodpeckers rely heavily on acorns as a food source. They meticulously store these acorns in specially drilled holes in granaries, which can be trees, poles, or wooden structures. The granaries are fiercely defended from potential thieves. In addition to acorns, they also feed on insects, sap, and fruit, showcasing their adaptability.

The Acorn Woodpecker is currently listed as Least Concern by the IUCN. However, habitat loss and competition for nesting cavities, particularly in urbanized areas, pose threats to their populations. Conservation efforts focus on preserving mature oak forests and providing suitable nesting and granary sites.

While the Acorn Woodpecker has a unique appearance and behavior, it could potentially be confused with other woodpecker species in its range. Observers should note the specific patterns of white and the red cap in males to distinguish it from its relatives.