Acorn Woodpeckers

Hastings is a Biological Field Station of the University of California, Museum of Vertebrate Zoology and Natural Reserve System.

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Social Behavior of the Cooperatively Breeding Acorn Woodpecker

Walt Koenig and Joey Haydock
Hastings Reservation and Museum of Vertebrate Zoology University of California Berkeley

Acorn Woodpeckers are common residents of oak woodlands in western North America.

Groups engage in a many communal activities, incuding territorial defense, feeding of young at the communal nest and acorn storage in special trees known as GRANARIES. Stored acorns are an important food resource, both during the winter and for successful reproduction the following spring. Groups can even breed in the fall when the acorn crop is particularly good.

They have one of the most bizarre mating systems of any bird in the world. They are COOPERATIVE BREEDERS and live in groups composed of up to 6 COBREEDER MALES, 3 JOINT-NESTING FEMALES, and NONBREEDING HELPERS of both sexes.

COBREEDER MALES are brothers and/or fathers and their sons competing for matings with the JOINT-NESTING FEMALES, who are sisters or a mother and her daughter who lay their eggs in the same nest cavity. Offspring produced from this communal nest may remain in their natal group for several years as NONBREEDING HELPERS, during which time they help feed younger siblings at subsequent nests.

This kind of mating system is known as POLYGYNANDRY. All individuals within the group are close relatives except that cobreeder males are not related to joint-nesting females. INCEST AVOIDANCE is maintained because helpers only inherit and become cobreeders following REPRODUCTIVE VACANCIES when the breeders of the OPPOSITE sex die and are replaced by unrelated birds from elsewhere. Reproductive vacancies are often filled by a unisexual set of siblings who compete against other sibling groups in spectacular events called POWER STRUGGLES. Winners of power struggles become cobreeders in the new group; losers return home and resume nonbreeding helper status.

Here's a nest. (And Walt Koenig about 20 years ago!) The entrance used by the birds is the upper circular hole; we cut the hole beneath it to reach inside. After checking the hole, we nail the cut piece of wood back in place. Incubation is 11 days and the nestling period is a month; this nestling is about a week old.

There is also a great deal of competition for reproduction within groups. Joint-nesting females even destroy each other's eggs, removing them from the nest during egg-laying and placing them in a tree where group members come and eat them. Since joint-nesting females are close relatives, birds are destroying eggs that are also related to themselves!

A female (head with white, black and then red) removing her sister's egg. You can tell she's a female because of the black band on her forehead; adult males and juveniles have solid red foreheads.

Our study of Acorn Woodpeckers has been going on since 1971, supported by the National Science Foundation, and the Hastings family.