Acorn Woodpeckers

An Unusual Bird of California

Acorn woodpeckers exhibit some of the most bizarre social behavior on earth. Mate-sharing, group sex, infanticide, and acorn storing on a monumental scale—it’s all in a day’s work for these clown-faced denizens of the West.

These unusual birds have been studied at Hastings since 1968, with a population continuously monitored since 1974! This study is one of the longest-running bird projects, spanning many generations of woodpeckers and providing an unparalleled dataset for researchers around the world.

About Acorn Woodpeckers

Acorn woodpeckers (Melanerpes formicivorus) are a common sight in the oak woodlands of western North America. They live in extended family groups and engage in many communal activities, including territory defense, feeding of the group’s chicks at the cavity nest, and specialized acorn storage structures termed “granaries.” Acorns are an energetically rich food resource, but are available only in the fall, when the woodpeckers collect and store thousands of them. Even though the majority of their diet consists of insects, these stored acorns provide them sustenance during the winter and during reproduction the following spring. If the acorn crop is particularly good, acorn woodpeckers may even breed in the fall, trusting in their stored food to keep their young alive through the winter.

Breeding Habits

Acorn woodpeckers exhibit one of the world’s few polygynandrous mating systems, where multiple males and females breed together cooperatively. The group is composed of up to 7 cobreeding males, 4 joint-nesting females, and nonbreeding helpers of both sexes. The breeding woodpeckers breed promiscuously within the group, but never mate outside it. Cobreeding males are closely related – usually brothers or a father and his sons – and compete for matings with the joint-nesting females. These females are also closely related to each other, and lay their eggs in the same cavity. When one female starts laying, the other joint-nesting females will remove and eat her eggs, until they, too are ready to lay, ensuring that the chicks in the nest will be equally divided among the mothers.

Cobreeding males are closely related – usually brothers or a father and his sons – and compete for matings with the joint-nesting females.

Offspring produced from this communal nest may remain in the group for many years as nonbreeding helpers. During this time they will help feed their younger siblings at the nests, but they themselves will never breed. Instead, they contribute to the group via territory defense and acorn management, while spending most of their time off the territory, hunting for reproductive vacancies at other groups. Helpers can only inherit their natal territory if all cobreeders of the opposite sex die and are replaced by unrelated woodpeckers, to avoid incestuous relationships.

When a breeding vacancy occurs at a group, large same-sex coalitions of relatives will compete against other coalitions in spectacular events called power struggles. In these all-out fights, the largest coalition wins, and the losers return home and resume helping their parents. Members of the winning coalition remain at the group as cobreeders, and may even found a new dynasty at the territory.

The Research

At Hastings, researchers monitor a color-banded population of acorn woodpeckers, keeping track of the reproduction and movement of individuals across the landscape. Using genetics, all woodpeckers hatched at Hastings have their parents identified, and the combination of behavioral observations, group censuses, and a genetic family tree allows for the examination of mating success, trait inheritance, familial recognition, and more. Current research at Hastings uses state-of-the-art nanotag and automated telemetry systems paired with behavioral observation to examine hypotheses about how and why helpers help.