Acorn Woodpecker (Male) Cave Creek Canyon Ranch January 16-18, 2021 WJ Beaver
Years ago I had an old cottonwood tree near the back of my house that had died and I left the snag for a bit until I got worried about it falling so I took it down. There was a second-floor section on the outside of the house that was wood paneling and almost immediately after the tree fell it was attacked by woodpeckers. I struggled for years with them, the paneling covered by round sheet metal patches. I seriously considered aluminum paneling but then they stopped and moved on. Woodpeckers are not liked in the city because of their destruction and because of how the males like to use sheet metal roofs for their drumming. In October of this year, we stayed at Cave Creek Ranch at the mouth of Cave Creek Canyon near Portal, AZ in the Chiricahua Mts. The ranch is a birding and naturalist paradise. I am not a birder but “when in Rome” so I took some pictures of the birds. One picture that stood out was one of a colorful woodpecker. I later found out that this is an Acorn Woodpecker and that they are unique for the complexity of their cooperative behavior. Hmm. We liked the place so much we returned in January. Seems that there is a major flock literally right over where we were staying including a nearby “granary tree.” I had wondered why our “cabin” was made of slump block instead of wood and now I know. No wood structure could survive in such a place. So I’ve been doing some research. The birds have been studied extensively and are quite remarkable in their behavior. I will drop the words “communism” (1) and “group sex” (3) right now in hope of getting extra clicks. Eh?
Acorn Woodpeckers are associated with regions containing multiple species of oak tree (18). They range throughout the California Floral Province, the low mountain Southwest US and south into Mexico, Central America, and an isolated subspecies in the Andes of Columbia. The species M. formicivorus is part of the genus Melanerpes which has 22 species, 8 of which are known cooperative breeders (17). This is unusual because cooperative breeding is rare in birds, especially in temperate climates. These birds have been studied in Columbia (2), Honduras (6, 7), Belize (4), Mexico (8), Appleton-Whittell Research Ranch in Arizona (9), and Water Canyon in New Mexico (9). The longest-running and continuous study has been at Hastings Natural History Reservation in central California. This has been going on for over forty-five years (12).
Acorn Woodpecker (Female) Cave Creek Canyon Ranch January 16-18, 2021 WJ Beaver
Unlike other birds, there is little difference between males and females. Males have red patches on their heads that extend all the way to the white on their faces while females have red “caps” and a band of black on top of the white. Fledglings all look like males but have black eyes while adults have white eyes.
Acorn Woodpecker on top of granary tree.
Cave Creek Canyon Ranch January 16-18, 2021 WJ Beaver
Cooperative breeding takes place in about 3% of the world’s birds. Most birds breed in monogamous pairs and most cooperative breeding starts when relatives of one or both sexes become helpers. So breeding pairs can have no helpers or one or helper either of one sex or two or more helpers of either sex. Male helpers are related to the father, brothers, or sons, and female helpers are related to the mother, sisters, or daughters. If this isn’t complicated enough, some nests practice polygynandry with multiple males and females breeding together. Polygynandry is even rarer in birds, around 1% but it is widespread across many orders of animals. Like helpers, each of the sexes in the breeding complex is related with there usually being more males (1-6) than females (1-3). With helpers, this can mean as many as 16 birds sharing a nest. Co-female breeders will destroy another female breeder’s eggs if she produces before everyone else. This continues until all the females are laying eggs together. Female breeders also lay many runt eggs, tiny sterile eggs containing no yolk. Other bird species also lay runt eggs but it is unusually prevalent in these birds. The reason these eggs are laid is unknown in Acorn Woodpeckers although in other species it seems to be random. (15, 16) One problem with cooperative breeding is incest and the genetic depression caused by it. What happens when a breeder dies? The sex ratio in Acorn Woodpeckers is slightly skewed towards males, this is because for some reason females die earlier. Incest does exist among these birds but the rate is very low. (14) When a breeder bird dies the dynamics of the replacement depends on the sex of the deceased and the sex of the helpers. Both helpers and breeders forage into other territories with helpers looking for breeding opportunities both within a territory and in other territories. This can lead to massive breeding wars between alliances of helpers which attract non-warring spectators and can lead to deaths. (13) A polygynandrous nest can even halt breeding for several years due to the wait for a new breeder to appear. Since almost all mating goes on in a nest evacuated deep into a tree, the dynamics between breeder sexes is unknown. Paternity testing, however, has shown that most of the fledglings belong to the older male bird. (11)
Sap holes in a branch. Cave Creek Canyon Ranch January 16-18, 2021 WJ Beaver
Acorn in a hole in a granary tree. Cave Creek Canyon Ranch January 16-18, 2021 WJ Beaver
Looking up a granary tree. Cave Creek Canyon Ranch January 16-18, 2021 WJ Beaver
Acorn Woodpeckers are named of course because of their relationship with acorns and oak trees. They eat insects and other foods but they store acorns in holes they drill in a dead tree. These holes protect the acorns from seed predation and supply the birds with food over the winter so they don’t have to migrate. This “granary” tree can have tens of thousands of holes in it! Birds take about an hour to drill each hole. Nests have their own territory on a part of a tree or whole tree or trees and guard these territories as a nesting group. As far as I can tell there is no cooperative dynamic beyond the family groups, no flock as such. (10) The birds are very attuned to the oak trees and the variability of the acorn crop. In fact, the birds don’t exist where there are only one species of oak. It takes at least two species of oak to bring Acorn Woodpeckers and the more diverse the oak forest is the more the birds thrive. There seems to be a relationship between the amount and stability of the acorn crop and the amount of cooperative breeding. Flocks at Research Ranch in Arizona are in a region of sparse oak woodland and don’t have granary trees. They migrate in the winter. (9) Cooperative breeding is non-existent although flocks in better woodlands close by have cooperative breeding. Flocks in Columbia, however, don’t have granary trees, yet there is cooperative breeding. (2) Research at the Hastings Preserve has shown that some behaviors within the nest can show a strong signal for acorn variability while others show a signal for temperature or season. During an exceptionally good crop, the birds can have a second breeding season in the fall. Studies have shown that it is the availability of granary trees that constrain and encourage cooperative groups from forming. (11) The birds use flat stretches of wood as “anvils” to crack open acorns. They also use these to beat flying insects to soften them up before eating them. They have also been seen storing these insects temporarily in cracks to eat later. They also make smaller holes in branches to eat sap. (10)
Walter D. Koenig has been researching Acorn Woodpeckers at Hastings Preserve since 1974. He has also researched the oak trees that the woodpeckers associate with and has written in some 257 publications. (19) Hastings is a long-term study doing mostly data gathering with few actual experiments. Controlled experiments are hard to do with this complex society so most of Koenig’s work has been developing an exhaustive population model of the study area and testing hypotheses against the data, a painstaking process as the birds tend to defy many theoretical expectations. One good thing about long-term studies is that new technologies and statistical methods can be applied to the data. For instance, a project between universities in China and Denmark at sequencing the genomes of 10,000 species of birds will have an eventual massive effect on the genetics of cooperation. (20) Baby birds learn the calls and songs of their parents and neighborhoods in the nest. This is how they learn to recognize relatives. Recent studies of Acorn Woodpeckers show that there are no group calls but each individual's call is unique. The birds have a long-term memory for these calls as they can remember returning helpers who have migrated to other nests to breed. (5, 21)
Biology has been for the most part a descriptive science with mathematics only recently coming into general use. I have barely scratched the surface of the current literature on these remarkable birds. What I have found is that no matter where I look in any of the original documents I find more information and insights. Because biology has been mainly descriptive, the meaning of certain words has caused constant battles. This has been especially prevalent in the biology of behavior and its relationship to modern social and political culture. This is a project for a future post but to get a flavor of this look below at an early article about our favorite bird published in 1925. (1)
(1)
Leach, Frank A. “Communism in the California Woodpecker.” The Condor XXVII (1925).
Kattan, Gustavo. “Food Habits and Social Organization of Acorn Woodpeckers in Colombia.” The Condor 90, no. 1 (February 1988): 100–106. https://doi.org/10.2307/1368438.
Koenig, Walter, and Joey Haydock. “Group Sex in the Acorn Woodpecker:” 2002, 5.
Stacey, Peter B. “Foraging Behavior of the Acorn Woodpecker in Belize, Central America.” The Condor 83, no. 4 (November 1981): 336. https://doi.org/10.2307/1367502.
Yao, Yuan. “Studies of Vocal Communications in Cooperatively Breeding Acorn Woodpeckers (Melanerpes Formicivore).” Accessed February 5, 2021. https://search.proquest.com/openview/f391d840781df64d1e37c591f4838b1d/1?pq-origsite=gscholar&cbl=18750&diss=y.
Flores, Stefany L, and Diego A Ardón. “Social Organization And Food Habits of the Acorn Woodpecker (Melanerpes Formicivorus) in the Neotropics Including Observations in Central Honduras.” Ornitología Neotropical, 2018, 6.
Cantarero, Karla J. “El presente proyecto de Tesis ha sido revisado y aprobado por los siguientes Asesores:,” n.d., 77
Rosas-Espinoza, Veronica Carolina, Elisa Maya-Elizarraras, Oscar Francisco Reyna Bustos, and Francisco Martin Huerta-Martinez. “Diet of Acorn Woodpeckers at La Primavera Forest, Jalisco, Mexico.” The Wilson Journal of Ornithology 120, no. 3 (September 2008): 494–98. https://doi.org/10.1676/06-102.1.
Stacey, Peter B. “Habitat Saturation and Communal Breeding in the Acorn Woodpecker.” Animal Behaviour 27 (November 1, 1979): 1153–66. https://doi.org/10.1016/0003-3472(79)90063-0.
MacRoberts, Michael H. “Notes on the Food Habits and Food Defense of the Acorn Woodpecker.” The Condor 72, no. 2 (April 1970): 196–204. https://doi.org/10.2307/1366631.
Koenig, Walter D. and Ronald L. Mumme. Population Ecology of the Cooperatively Breeding Acorn Woodpecker., 1987. https://press.princeton.edu/books/paperback/9780691084640/population-ecology-of-the-cooperatively-breeding-acorn-woodpecker-mpb.
Days Edge Production. “The Acorn Woodpecker.” BioGraphic (blog), October 10, 2017. https://www.biographic.com/the-anomalies-the-acorn-woodpecker/.
Barve, Sahas, Ally S. Lahey, Rebecca M. Brunner, Walter D. Koenig, and Eric L. Walters. “Tracking the Warriors and Spectators of Acorn Woodpecker Wars.” Current Biology 30, no. 17 (September 7, 2020): R982–83. https://doi.org/10.1016/j.cub.2020.07.073.
Koenig, Walter D., Mark T. Stanback, and Joseph Haydock. “Demographic Consequences of Incest Avoidance in the Cooperatively Breeding Acorn Woodpecker.” Animal Behaviour 57, no. 6 (June 1999): 1287–93. https://doi.org/10.1006/anbe.1999.1093.
Koenig, Walter D. “The Incidence of Runt Eggs in Woodpeckers.” Wilson Ornithological Society 92, no. 2 (1980): 169–76.
Mulvihill, Robert S. “Runt Eggs: A Discovery, a Synopsis and a Proposal for Future Study.” North American Bird Bander 12, no. 3 (1987): 94-96.
Koenig, Walter D., and Eric L. Walters. “What We Don’t Know, and What Needs to Be Known, about the Cooperatively Breeding Acorn Woodpecker Melanerpes Formicivorus.” Acta Ornithologica 49, no. 2 (December 2014): 221–23. https://doi.org/10.3161/173484714X687091.
Koenig, Walter D., and Joseph Haydock. “Oaks, Acorns, and the Geographical Ecology of Acorn Woodpeckers.” Journal of Biogeography 26, no. 1 (1999): 159–65. https://doi.org/10.1046/j.1365-2699.1999.00256.x.
Koenig, Walter D. “Walter KOENIG | Research Zoologist Emeritus | Ph.D. | Cited by 11,410 | University of California, Berkeley, CA | UCB | Museum of Vertebrate Zoology.” Accessed January 29, 2021. https://www.researchgate.net/profile/Walter_Koenig.
Chinese National Genebank. “B10K Database,” 2016. https://b10k.genomics.cn/.
Pardo, Michael, Eric Walters, and Walter Koenig. “Experimental Evidence That Acorn Woodpeckers Recognize Relationships among Third Parties No Longer Living Together.” Behavioral Ecology 31 (October 12, 2020): 1257–65. https://doi.org/10.1093/beheco/araa079.
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