If you see an acorn woodpecker (Melanerpes formicivorus) there are most certainly others nearby. Living in communal groups acorn woodpeckers have a most interesting social structure. Walter Koenig from Cornell Lab of Ornithology said it well: acorn woodpeckers engage in mate sharing, group sex and infanticide.
An acorn woodpecker communal group consists of up to seven breeding males and up to three breeding females and any number of additional nonbreeding “helpers”. The group of up to fifteen birds maintains and defends a territory consisting of oak trees, one nest, and one granary tree.
The breeding males and breeding females do not have specific partners and mate with each other.
The acorn woodpecker nest is simply some wood chips in the bottom of a tree cavity. When a female lays the first egg in the nest, the birds in the group will remove the egg and eat it. The mother also will participate in eating the egg. This continues until all of the females have laid one egg in the nest, after which future eggs are not destroyed. All the eggs are laid in one cavity and all the females then cooperate in incubating and raising the young.
The young birds often stay with their parents for many years as non-breeding helpers. The helpers forage, drill holes in and defend the granary tree and help care for the babies. Eventually if an opportunity arises some of the helpers may move off to join other acorn woodpecker groups.
When all the breeding birds of one sex have died there is a “vacancy” in the flock. At that point helper birds of the same sex from other bands (often groups of siblings) arrive to fill the void. “Wars” between groups competing for admission to the flock will continue for several days with much calling and wing-flapping. Eventually several new birds win the competition and join the commune. Helpers of the opposite sex from the original colony can now also breed with the newcomers and become breeding members of the group.
Why do the young birds remain with the colony as helpers? Maintaining a granary tree takes an enormous amount of effort. In addition, the stash of acorns is like an investment or inheritance. By waiting in the family unit until new blood arrives to replace all the dead breeding birds of one sex, the helpers can then keep their “inheritance” without any inbreeding. Fascinating!!
Acorn woodpeckers are not simply a flock of noisy, busy birds. They have a very complex and interesting social structure.
The acorn woodpecker in the crotch of the oak tree belongs to the family on the Pacific Crest Trail that I mentioned the last two days. The woodpecker on the juniper tree belongs to a clan on Lower Hat Creek. Both acorn woodpecker groups are in Shasta County CA.