Acorn Woodpeckers (Melanerpes formicivorus) are distributed in foothill and montane habitats in Oregon, California, the American south-west, western Mexico, and southward through Central America to Colombia. Throughout their range, they are closely associated with oaks (genus Quercus), most commonly being found in pine–oak woodlands. They are generally quite common and conspicuous, and are well known for their unique habit of storing acorns, often by the thousands, in specialized trees known as storage trees or granaries. Acorn storage is characteristic of many, although not all, populations of Acorn Woodpeckers throughout their range.

Acorn Woodpeckers are also cooperative breeders. Within the family Picidae, they share this habit with the Red-cockaded Woodpecker (Chapter 3) and several tropical and Caribbean forms which, like the Acorn Woodpecker, belong to the melanerpine line. Birds in social units store and defend acorns and other mast communally. Although acorns constitute a major portion of their diet, particularly during the winter, Acorn Woodpeckers also engage in a wide variety of other foraging techniques, including sapsucking, flycatching, bark gleaning and seed eating.

We studied Acorn Woodpeckers at three sites (Fig. 14.1): Hastings Reservation (HR) in California (W.D.K.; this study, begun by MacRoberts and MacRoberts (1976), has been ongoing since 1971), Water Canyon (WC), New Mexico (P.B.S.; primarily between 1975 and 1984), and the Research Ranch (RR), Arizona (P.B.S. and C. Bock; conducted from 1975 to 1978).