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It Takes a Village: San Diego’s Acorn Woodpecker

Acorn Woodpecker Cuyamuca State Park 2012 10 20 (1 of 1)-X2

In the 1920s, an American ornithologist called this bird “our native aristocrat—unruffled by the operations of the human plebs…”   In that same decade, an avian researcher proclaimed that these birds practiced communism. More recently, their social behavior has been likened to a noisy avian Keystone Cops routine, featuring loud calling, bobbing, wing displaying, and jockeying for position.  Or a wacky troop of wide-eyed clowns found only in western stands of oaks, perpetually seeking acorns. That last clue should clinch the identity—Acorn Woodpeckers!

A medium-sized, somewhat stocky woodpecker, an Acorn Woodpecker’s plumage, animation, and raucous vocalizations demand attention.  The conservatively patterned black back, white rump and wing patches, darkly streaked breast, and white belly provide a contrastingly staid foundation for the bird’s outlandishly patterned face.  From the golf-tee-shaped bill to the white face, crazy yellow eye, and red crown, no other woodpecker looks even vaguely similar. An adult male has a solid red crown stretching to his white forehead; an adult female shows a wide black band separating her red crown and white forehead. Juveniles look similar to adult males, except for their dark irises and duller plumage.

These lively birds take their name from their dependence on acorns. Since the eponymous acorns appear only in the fall, Acorn Woodpeckers ensure a steady supply throughout the year by drilling storage holes for acorns in trees (or utility poles, fences, wood-sided houses, or pretty much any wood structure). These so-called “granaries” are typically enormous — often consisting of thousands or even tens of thousands of individual holes, each typically filled with an acorn in autumn. Drilled primarily in the winter in dead limbs or in thick bark, the holes don’t penetrate to the lower bark layers. Consequently, these storage spaces won’t hurt living trees (although they mess with your wooden structures).  Birds store primarily acorns, but the woodpeckers will also store almonds, walnuts, hazelnuts, and pecans, if available. Generally groups of these birds share a primary and one or more secondary granaries.

Acorns and other nuts are not the only food these woodpeckers eat, though.  In fact acorns serve just as supplemental food during breeding season. Across a year, nuts constitute 53% of the diet, followed by fruit (23%); insects (e.g., ants, beetles, 23%); and other vegetable matter (1%).  

Not surprisingly, the Acorn Woodpecker’s distribution in San Diego County maps onto oak woodlands: groves of coast live oak only; riparian mixed woodland containing at least some oaks; or montane conifer forests with black oaks as well as canyon live and coast live oaks. Montane forests may actually offer the most preferable habitat, since the softer bark of pine trees makes a better granary than oaks do:  In the harder oaks, the woodpeckers tend to drill only in dead snags. Also, the tannin content of canyon live oak acorns falls below than that of coast live oak acorns, making the former a tastier food.

The genus name of Melanerpes formicivorus (the Acorn Woodpecker’s scientific name) means “black creeper,” from Greek melas (“black”) and erpein (“to creep”):  These woodpeckers move vertically up and down tree trunks, exposing their mostly solid-black upper backs.  The species name, formicivorus, translates from Latin to “anteater” (formica, “ant;” voro, “to swallow ravenously”)—a bit of a misnomer, since less than one quarter of its diet contains insects of any kind.  The origins of its common name offer less mystery, since acorns are a favorite food for surviving the winter and “woodpecker” describes the bird’s feeding and nesting behavior.

This woodpecker is a cooperative breeder with socially complex behaviors, living in a family group of up to a dozen or more individuals. Acorn Woodpeckers are polygynandrous, an extremely rare communal behavior in birds where multiple males and multiple females breed and attend the nest.  In San Diego County, Acorn Woodpeckers usually begin egg-laying in early April and can continue through mid-June.  In one family group, the breeding males (as many as seven) are all related to one another; the breeding females (as many as three) are related to one another.  But the breeding males are not related to the breeding females, so the birds avoid inbreeding. Acorn Woodpeckers excavate their own nest holes in whatever large trees are available.  Unlike many other woodpeckers, though, Acorns may reuse nest cavities for many years. With a clutch size of four or five eggs, incubation lasts 11 days. Both male and female breeders incubate; non-breeding helpers rarely do.  All group members help feed the chicks as soon as the first chick hatches. Nestlings leave the nest 30–32 days after hatching.

Evolutionarily, individual birds “succeed” only if they pass their genes on to the next generation. So why would a young woodpecker sacrifice its own reproductive success by helping its parents raise more offspring?  For Acorn Woodpeckers, the answer lies in the granaries themselves—a limited resource essential to Acorn Woodpeckers’ survival but one demanding extraordinary time and effort to construct, maintain, and defend. Starting a new family group means starting a new granary; leaving the parental group has serious consequences.

Comparing an Acorn Woodpecker to an aristocrat, a Keystone Cop, or even a wild-eyed clown is a stretch of any imagination.  But their practicing communism (small “c”)—a social organization based on the holding of all property in common—rings true. As you walk through the county’s oak woodlands, realize that you perhaps move through a small-c communist society.  But capitalism has nothing to fear from Acorn Woodpeckers—unless, of course, your capital is invested in acorns.

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