Creature Feature: Acorn Woodpecker

“It takes a village to raise a child”, goes the old proverb. The acorn woodpecker (Melanerpes formicivorus) seemingly takes this advice to heart.  This California native species fits well into the state’s ecological, and political, landscape.  These birds lead secret lives of free love and communal values, à la hippie communes of the 1960s and 1970s.

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An acorn woodpecker proudly guards the acorn “granary”. [Source]

With their silly red heads that resemble clown faces and a call reminiscent of a squeeze toy,  acorn woodpeckers seem an unlikely poster child for polyamory. Like other, less sexually-liberated woodpeckers of North America, acorn woodpeckers are primary cavity nesters, meaning they live in holes of their own making, crafted out of trunks, telephone poles, and other structures.  When the woodpeckers move out, secondary cavity nesting birds, like bluebirds, swallows, and starlings, can move in to the newly-vacant properties.  In this way, woodpeckers serve an important ecological role, by providing housing for other species.

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An acorn woodpecker clearing out a nesting cavity. [Source]
However, acorn woodpeckers do more than constructing real estate for other birds. They live off of the mast, or acorn crops, of live oak forests across the California coast. They harvest these acorns in “granaries”, or trees drilled full of holes that act as storage units for acorns harvested during the mast season. Ever in the spirit of community, acorn woodpeckers living in a territory cooperate to store their resources in the “granary” trees, and work together to protect it from pilfering squirrels or other birds. This differentiates them from other “caching” species, like the scrub jays, which hide their stash from other birds and competitors, even going to such lengths as to feign collecting to avoid getting their goodies stolen.

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Carmel Valley, near Monterrey, CA, provides prime oak forest habitat for the acorn woodpecker.  [Source]
With these collective resources at the heart of their communities, acorn woodpeckers have built their hippie communes around their granaries and territories. This species is an opportunistic polygynandrous breeder, which translates from jargon to “free love”.  Multiple (two to four) males will vie for the attention of multiple females in a nesting territory. In addition to this love polygon, non-breeding helpers, usually the male offspring from last years’ nests, stick around the nest to help collect acorns for the group.  The breeding females jointly nest, creating a neighborhood “daycare” where they, their partners, and their teenage sons all help to raise offspring. Cooperative breeding like this occurs in other bird species, despite the fact that most birds are socially monogamous. For example, in western bluebirds, male offspring may forgo breeding and stick around their parents’ nest to help raise their siblings. However, this is more akin to a teenage brother babysitting his toddler siblings while the parents go on date night, and fits well into the idea of extended, but monogamous, family units. Observations and genetic studies reaffirm that acorn woodpeckers are definitely not monogamous!


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Peace and free love: for hippies and acorn woodpeckers. [Source],[Source]

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Interestingly, despite this love fest at the nest, woodpecker “orgies” are surprisingly difficult to observe. After 1,400 hours of observations, researchers only witnessed 26 copulations, most of them at dusk when they could hardly identify the birds and thus assumed the birds could barely do the same of each other. The researchers posited these anonymous “one-branch stands” may maintain the parental uncertainty of males, keeping them engaged in the care of nestlings.

However, things are not always all love and peace at the acorn woodpecker nest “commune”.  Males participate in parental care and contribute to the nest only when they think some of the nestlings might be their offspring.  In an experiment, scientists took male acorn woodpeckers away from their nests and social group during the main breeding period, so that they were forced to have no potential paternity of the new eggs. These males, when returned to the group, destroyed nests and eggs, forcing the females to restart the breeding period1. This infanticide tactic is seen in other group-living species, such as in lion prides and baboon troops.  In order to avoid this risk of violent males, female acorn woodpeckers most likely mate equally with all of the males so that each has a relatively even probability of being the father and thus an incentive to take care of young nestlings.

Most of the research of acorn woodpeckers comes from work by Dr. Walter Koenig, a researcher at UC Berkeley and later the Cornell University lab of Ornithology, who studied models of cooperative breeding in birds at the Hastings Natural History Reserve, a great resource of the University of California system.  In more ways than one, this unique species embodies the diversity and liberal spirit present in California’s ecology, intellectual resources, and history.

[By: Victoria Farrar]


References:

1Koenig WD, Stacey PB. (1990) Acorn Woodpeckers: group-living and food storage under contrasting ecological conditions. In Cooperative breeding in birds: Long term studies of ecology and behavior. Stacey PB and Koenig WD, Eds. Cambridge University Press, Cambridge, UK.

2Koenig WD, Stacey PB, Stanback MT, Mumme RL. (1995) Acorn woodpecker (Melanerpes formicivorus) version 2.0. In The Birds of North America,  Rodewald PG, editor. Cornell Lab of Ornithology, Ithaca, New York, USA.

Main featured image [Source]

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