Acorn woodpeckers: Helping at the nest, polygynandry, and dependence on a variable acorn crop

Introduction The acorn woodpecker (Melanerpes formicivorus) plays an important role in the history of cooperative breeding. It was one of the earliest species for which cooperative breeding was noted with more than two individuals feeding at a single nest as well as apparent mate-sharing (Myers 1915; Leach 1925; Michael 1927). Subsequently, in his classic Animal Dispersion in Relation to Social Behaviour, V. C. Wynne-Edwards singled out the acorn-storing habits of this species as a means by which birds assessed the food supply and adjusted their breeding so as to avoid overexploitation of resources. Indeed, Wynne-Edwards suggested that acorn storing was “the perfect example of an epideictic rite, combining as it does a sampling of the food-supply, a territorial symbol (the tree), and social competition” (Wynne-Edwards 1962: 325). Four years later, cooperative breeding in this species was predicted to be based (this time accurately) on kinship in G. C. Williams’s Adaptation and Natural Selection, written to counter the group-selectionist bent of Wynne-Edwards (Williams 1966). It was but a few years after Williams’ prediction, itself an outgrowth of W. D. Hamilton’s (1964) papers introducing inclusive fitness theory, that Michael and Barbara MacRoberts began a study of acorn woodpeckers at Hastings Reservation in central coastal California, a field station run by the Museum of Vertebrate Zoology at the University of California, Berkeley. During the years of their work from 1968 to 1974 they banded 139 birds and described many aspects of their ecology and social behavior (MacRoberts and MacRoberts 1976). It is their study that the senior author took up in July 1974 and has worked on continuously ever since along with a series of students and collaborators, most recently the two coauthors of this manuscript. The work summarized here is thus a product of over 40 years of study during which time we have banded over 5,500 individuals and followed over 1,600 nests. We also refer when appropriate to earlier parallel studies conducted by Peter Stacey at Water Canyon, New Mexico and The Research Ranch in southeastern Arizona (Stacey and Bock 1978; Stacey 1979a, 1979b), updating results summarized in Koenig and Stacey (1990). Acorn woodpeckers are a New World species found in oak woodlands along the Pacific Coast of North America between southern Washington State and Baja California, the American Southwest, and the mountains of western Mexico, Central America, and northern Colombia.