Acorn Woodpeckers live year-round in oak and pine-oak woodlands of western Oregon, California, and the Southwest through Mexico and Central America. They also live in other habitats with oaks present or nearby, including streamside forests, Douglas-fir forests, redwood forests, tropical hardwood forests, suburban areas, and urban parks. Though found as low as sea level, they are more common in mountains, ranging up to the elevation limit of oak trees. Back to top
Acorn Woodpeckers eat acorns and insects (and other arthropods). The woodpeckers harvest acorns directly from oak trees and are famous for their habit of storing nuts—primarily acorns, but also almonds, walnuts, hazelnuts, pecans, and pinyon pine nuts—in individually drilled holes in one or more storage trees. These are known as granaries and can have upwards of 50,000 nuts stored in them. The birds drill the holes primarily in the winter, in the thick bark of dead limbs where the drilling does no harm to a living tree. Each year they reuse old holes and add some new ones. The acorns are wedged so tightly in their holes that they’re very difficult for other animals to remove. After they’ve been stored for a while, the fit becomes looser as the acorn dries out—group members periodically check their stored acorns and move the loose ones to smaller holes. Besides converting many kinds of live and dead trees into granaries, Acorn Woodpeckers often store acorns in structures like utility poles, fenceposts, and wood-sided buildings—a practice that has brought them into conflict with more than a few protective homeowners. Despite their association with acorns, Acorn Woodpeckers prefer to catch flying insects when those are available. They hunt for ants, beetles, and other insects by flying out from high perches. They may hunt insects at any time of year, often storing them in cracks or crevices. Besides nuts and insects, Acorn Woodpeckers also eat fruit, sap, oak catkins, and flower nectar, along with occasional grass seeds, lizards, and even eggs of their own species. In the spring they gather in groups to suck sap from small, shallow holes in tree bark, often using the same sets of sap holes for several years.Back to top
Acorn Woodpeckers excavate multiple cavities, any one of which may be used for nesting (the rest are used for nocturnal roosting). They dig cavities in dead or living limbs, large or small, either in the granary (storage) tree or any other large tree. The woodpeckers reuse nest holes for many years.
The cavity is usually about 6 inches in diameter, and it may be 8 inches to more than 2 feet deep. Acorn Woodpeckers do not build a nest within the cavity, but during the digging process a layer of fresh wood chips usually accumulates on the bottom. They replenish the chips throughout the nesting period by pecking away at the cavity walls.
|Clutch Size:||3-6 eggs|
|Number of Broods:||1-2 broods|
|Egg Length:||0.9-1.1 in (2.3-2.7 cm)|
|Egg Width:||0.7-0.8 in (1.8-2 cm)|
|Incubation Period:||11 days|
|Nestling Period:||30-32 days|
|Condition at Hatching:||Blind, featherless, and helpless.|
Acorn Woodpeckers are such unusual birds with such complicated social behavior that they have given rise to one of the longest-running behavioral studies of birds. They live in family groups of up to a dozen or more individuals, and they cooperate in raising young and in gathering, storing, and guarding food. Even their approach to cooperative breeding is unusually complex. Some groups have multiple breeding males and females, and all of a group’s breeding females lay their eggs in a single nest. Each female destroys any eggs that are present before she begins laying, resulting in the demise of more than one-third of the total eggs laid in joint nests. Once all the females have started laying their own eggs, their destructive behavior stops and they remove the debris to a nearby tree. There each egg is gradually eaten by several individuals—often including the female who laid it. Throughout the year, Acorn Woodpeckers collect acorns and wedge them tightly into holes they’ve made in tree bark. Acorn Woodpeckers fiercely defend these acorn granaries against other groups and any other species that might rob the stores. They also defend 15-acre territories around the granary. They occasionally wander outside the territory in pursuit of acorns and water. Birds that help at nests but don’t get to breed often go out looking for breeding vacancies in other groups, up to about 10 miles away.Back to top
Acorn Woodpeckers are numerous, and Partners in Flight estimates the global breeding population at 7.5 million and rates them 9 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. While Acorn Woodpeckers are common in oak woodlands, their numbers have probably declined since historic times because of development and habitat degradation, including overgrazing and loss of oaks due to disease and habitat conversion. Oak, pine-oak, and streamside forest has been converted to other uses throughout the Acorn Woodpecker’s range. In the Southwest and parts of Mexico, overgrazing has damaged mountain pine-oak habitats and streamside forests, probably reducing the Acorn Woodpecker population substantially. California populations, though not currently declining, have an uncertain future because of slow oak forest regeneration. Other threats include having nest holes taken over by European Starlings, an aggressive introduced species. Occasionally, people shoot Acorn Woodpeckers to keep them from eating nut and fruit crops. However, Acorn Woodpeckers have also shown the ability to colonize new habitats such as suburban neighborhoods, using human-made structures for roosting and acorn storage.Back to top
Koenig, Walter D., Peter B. Stacey, Mark T. Stanback and Ronald L. Mumme. (1995). Acorn Woodpecker (Melanerpes formicivorus), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.
Lutmerding, J. A. and A. S. Love. (2020). Longevity records of North American birds. Version 2020. Patuxent Wildlife Research Center, Bird Banding Laboratory 2020.
Partners in Flight. (2020). Avian Conservation Assessment Database, version 2020.
Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.