The Acorn Woodpecker is a common, conspicuous inhabitant of foothill and montane woodlands from southern Washington state (Klickitat County), Oregon, California, the American Southwest, and western Mexico through the highlands of Central America to the northern Andes in Colombia. Throughout its range, this species is closely associated with oaks (genus Quercus) and is most commonly found in pine–oak woodlands. It is probably best known for its highly social habits and unique method of storing acorns in specialized trees known as storage trees or granaries, although group living and acorn storage are not characteristic of all populations. This is generally a sedentary species, but at least one population migrates annually and irregular migrations occur elsewhere when local acorn crops fail.
Like several other species in the genus Melanerpes, this woodpecker is a cooperative breeder and lives in family groups of up to a dozen or more individuals. Birds in social units store acorns communally and cooperatively raise young. Although acorns constitute a major portion of the diet, particularly during the winter, this species also engages in a wide variety of other foraging techniques including flycatching, bark-gleaning, seed-eating, and sapsucking.
The Acorn Woodpecker has been the subject of numerous studies, including long-term projects at Hastings Reservation in central coastal California initiated by M. H. and B. R. MacRoberts and continued by W. D. Koenig, E. L. Walters, R. L. Mumme, M. T. Stanback, and coworkers, and at Water Canyon in central New Mexico by P. B. Stacey and coworkers. Detailed observations at Hastings Reservation began in 1971 and continue through the time of this writing (2019), whereas information from Water Canyon comes mostly from 1975 to 1984. Most of the behavioral and demographic information reported here comes from these studies. Where quantitative measurements are presented, we primarily use data from Hastings Reservation because of the larger sample sizes; additional information for Water Canyon is given when there are substantial differences among populations.