Melanerpes formicivorus
Year-round resident
If you hear a laughing waka-waka-waka echoing through the oak woodlands, you’ve likely found an acorn woodpecker. These charismatic birds are one of the most social woodpeckers at Edgewood. Unlike their solitary cousins, they often live in family groups and collectively build winter granaries.
Appearance
Acorn woodpeckers are often described as having a clown face due to their bold pattern of black, white, and red. They are the only woodpeckers at Edgewood with a solid black back and striking white eyes. When in flight, look for obvious white patches on their wings and a white rump. Both sexes sport a bright red crown, unlike Nuttall’s (Dryobates nuttallii), hairy (Leuconotopicus villosus), and downy (D. pubescens) woodpeckers.
For all populations except one in Colombia, you can tell the sex of adults by looking at the forehead: a male’s red crown touches the white forehead, while a female’s red crown and white forehead are separated by a black bar. Juveniles have less glossy feathers and a dark eye, and their crowns look similar to a male’s, but the red does not extend as far back on the head.
ID Tips
Acorn woodpeckers are unmistakable with their solid black back, white eyes, and white rump with wing patches.
If you see a woodpecker in a noisy group rather than alone, it’s almost certainly an acorn woodpecker.
At Edgewood
You’ll often hear these birds before you see them. Their waka call—reminiscent of Fozzie Bear from the Muppets!—is used to greet family members or settle neighborhood disputes. Look for acorn woodpeckers in large trees near the Education Center. Keep an eye out for limbs riddled with small holes, the signs of a possible family granary. See iNaturalist and eBird for observations of this bird at Edgewood Park and Natural Preserve.
Habitat and Range
Acorn woodpeckers are closely associated with oaks (Quercus spp.) throughout their range. In California and Oregon, their presence is tied to the balance of oak abundance and diversity. Because oaks are masting trees—producing large, erratic crops in some years and almost nothing in others—relying on a single species is a high-risk strategy. For our California-Oregon acorn woodpeckers, as the number of oak species in a habitat increases, the woodpeckers’ population variability decreases (Bock 1974; Koenig 1999). More oak species lead to more stable woodpecker populations.
At Edgewood, this oak diversity is exactly what ensures long-term survival. By having multiple species—such as coast live oak (Quercus agrifolia), valley oak (Q. lobata), and blue oak (Q. douglasii)—the ecosystem prevents boom and bust cycles. If the blue oaks have a poor crop one year, the coast live oaks may still produce a bounty, providing a consistent resource that allows the local woodpecker population to remain steady year after year.
Diet
While their name suggests a one-track mind, acorn woodpeckers adapt their diet based on availability, feeding on insects, acorns, and sap. Look for them hunting for their preferred meal of insects by either launching from high perches to snag bugs mid-air or gleaning them directly from branches. They may even stash insects into crevices in the tree to save for later.
California populations create granaries—specialized storage trees that hold thousands of acorns—a behavior unique to this species. These acorn pantries represent a living legacy rather than the work of a single season. A typical family group drills fewer than 500 holes per year, meaning a 50,000-hole granary (Dawson 1923) requires over a century of multi-generational labor! To protect the host tree, they typically drill into dead limbs or thick bark, avoiding the living cambium and phloem. The birds also adapt to human landscapes, building granaries in utility poles and fence posts; in 2023, a Santa Rosa homeowner famously discovered 700 pounds of acorns stashed inside a wall.
Acorn woodpeckers also create communal sap wells they maintain and reuse for years. Their construction looks similar to the scattered pattern of a granary, which contrasts with the neat, linear sap rows made by true sapsuckers (Sphyrapicus spp.).
Behavior
Acorn woodpeckers have one of the most intricate family structures ever documented. These birds live in groups of up to a dozen members, and in California, they practice a lifestyle known as polygynandry. This means multiple males and females within the same social group share mating partners, creating a complex web of relationships. These colonies are often rounded out by nonbreeding helpers—typically offspring from previous seasons who stay with the social group to assist the breeding adults.
Acorn woodpeckers’ domestic life is centered around tree cavities used for both roosting and nesting. The group maintains several of these throughout their territory, and nest cavities are often reused for several years.
With polygynandry comes intense reproductive competition. Rather than each female having her own nest, all the breeding females lay their eggs in a single communal cavity (Barve 2019). This leads to a fascinating, if slightly cutthroat, behavior. During the egg-laying period, females will remove other females’ eggs until they are ready to lay their own. These eggs are carried to a nearby tree, stored temporarily, and then consumed. While this might seem counterproductive, it actually serves a synchronizing purpose: it ensures the remaining eggs hatch in unison, giving every chick an equal start (Koenig 1995; Mumme 1983).
Males also compete by disrupting the mating attempts of other males and engaging in mate-guarding. This involves a male closely shadowing breeding females until the clutch is nearly complete, ensuring his own paternity is protected (Brownson 2015).
Once egg-laying is complete, the group shifts into a highly cooperative mode. All adult members—breeders and helpers—participate in incubating the eggs, brooding the chicks, and feeding the young, though breeding females contribute the most, followed by breeding males, and then nonbreeding helpers (Mumme 1990; Koenig 2012).
The path to becoming a breeder is a difficult one. Few females or males inherit a position in their home group, and when seeking new territories, they tend to pair up with same-sex siblings (Hooge 1995).
When a breeding vacancy finally opens up—usually because all breeders of one sex have died or disappeared—the spots are not typically filled by a helper in the group. Instead, unrelated birds from outside the social group fill the spots. This often triggers massive power struggles where 20 or more nonbreeding helpers from other social groups fight for the opportunity to fill the vacancy. These outsiders, often in their sibling coalition, battle for days or weeks by fighting, chasing, and lots of vocal chatter, with the competition reaching its most fierce for female vacancies.
Fun Facts
- Acorn woodpeckers are obsessive inventory managers. To protect their food from hungry squirrels and jays, they wedge each acorn into a hole so perfectly that it can’t be pried out by other animals. However, nature throws them a curveball: as acorns dry out, they can shrink (MacRoberts 1976). To prevent these shrinking snacks from becoming easy to steal, the birds perform a constant granary audit. They tap and shake the acorns with their bills to check for any wiggle room; if a nut has shriveled even slightly, they’ll relocate it to a smaller, tighter-fitting hole to ensure it stays locked down (Koenig 1987).
- Most animals can’t live on acorns alone because the nuts are packed with tannins. These bitter, astringent compounds are a tree’s primary defense; they prevent animals from absorbing protein, essentially starving an animal even though its stomach is full. Acorn woodpeckers, however, can survive for over two weeks on a 100% acorn diet with little to no loss in body weight (Koenig 1988).
- Woodpeckers have specialized anatomy that allows them to easily climb on vertical surfaces. See Woodpecker family to learn more about the amazing biology our local woodpeckers share.
Learn More
Learn more about acorn woodpeckers: All About Birds.
Cornell Lab of Ornithology. 2011, Apr. 23. Through the Lens: Acorn Woodpecker [Video]. YouTube.
Dionne, D. 2009, Feb. 20. Acorn Woodpecker. Macaulay Library.
Hernandez, J. 2023, Feb. 8. Woodpeckers went nuts, stashing 700 pounds of acorns in the wall of a California home. National Public Radio.
McGuire, B. 2012, Apr. 7. Acorn woodpecker. Macaulay Library. Cornell Lab of Ornithology.
References
Barve, S., et al. 2019. Habitat saturation results in joint-nesting female coalitions in a social bird. American Naturalist 193: 830–840.
Bock, C.E., and J.H. Bock. 1974. Geographical ecology of the acorn woodpecker: Diversity versus abundance of resources. American Naturalist 108: 694–698.
Brownson, A.C. 2015. The Behavioral Causes of Reproductive Skew in Cooperatively Polygynandrous Acorn Woodpeckers (Melanerpes formicivorus). Master’s Thesis. Old Dominion University, Norfolk, Virginia.
Dawson, W.L. 1923. The Birds of California: A Complete, Scientific and Popular Account of the 580 Species and Subspecies of Birds Found in the State. Volume 2. South Moulton, San Diego, California.
Hooge, P.N. 1995. Dispersal Dynamics of the Cooperatively Breeding Acorn Woodpecker. Doctoral Thesis. University of California, Berkeley, California.
Koenig, W.D., and J. Haydock. 1999. Oaks, acorns, and the geographical ecology of Acorn Woodpeckers. Journal of Biogeography 26: 159–165.
Koenig, W.D., and M.K. Heck. 1988. Ability of two species of oak woodland birds to subsist on acorns. Condor 90: 705–708.
Koenig, W.D., and R.L. Mumme. 1987. Population Ecology of the Cooperatively Breeding Acorn Woodpecker. Princeton University Press, Princeton, New Jersey.
Koenig, W.D., and E.L. Walters. 2012. Brooding, provisioning, and compensatory care in the cooperatively breeding acorn woodpecker. Behavioral Ecology 23: 181–190.
Koenig, W.D., R.L. Mumme, M.T. Stanback, and F.A. Pitelka. 1995. Patterns and consequences of egg destruction among joint-nesting acorn woodpeckers. Animal Behaviour 50: 607–621.
Koenig, W.D., E.L. Walters, P.B. Stacey, M.T. Stanback, and R.L. Mumme. 2020, Mar. 4. Acorn woodpecker (Melanerpes formicivorus), version 1.0. P.G. Rodewald and B.K. Keeney (Eds.). Birds of the World. Cornell Lab of Ornithology, Ithaca, New York.
Leonard, P. 2017, Jun. 12. The Beauty and Biology of Egg Color. Living Bird.
MacRoberts, M.H., and B. R. MacRoberts. 1976. Social organization and behavior of the acorn woodpecker in central coastal California. Ornithological Monographs, No. 21.
Mumme, R.L., W.D. Koenig, and F.A. Pitelka. 1983. Reproductive competition in the communal acorn woodpecker: Sisters destroy each other’s eggs. Nature 306: 583–584.
Mumme, R.L., W.D. Koenig, and F.A. Pitelka. 1990. Individual contributions to cooperative nest care in the acorn woodpecker. Condor 92: 360–368.
Reed, C.A. 1901, Jan. Californian woodpecker [Illustration]. American Ornithology. 6(1): 81. Worcester, Massachusetts.