Acorn Woodpecker: The Clown of the Oak Woodlands
Written by Clark Moore   

Part I - The Bird

When the Tehachapi Mountains Birding Club was discussing what species to adopt as a club logo and mascot bird, these harlequins came close to being chosen. What else could we do in this oak woodland country? However, Tulare County Audubon already claimed this clown as its own. The Tappings, very apropos, is the name of their newsletter. So the TMBC turned to a good representative species of these mountains, and a bird held in awe by many, as well as being thought of as wise, a virtue we needed a lot of - the Great Horned Owl. The HOOT became the name of the club's monthly newsletter. However this did not lessen our intrigue with the clown of the oak woodlands.

From the beginning of these Bear Valley Springs CUB writings (over 140 columns - approaching 12 years!) it was foreordained a column would be devoted to this ubiquitous bird. I have been reluctant though. For to write about this species risks that some readers, even those devoted to the column - if there are any - may tear the CUB to bits. I cannot help it, for these birds are pulling, preposterous and puzzling (some say dim-witted clowns).

(Click photos to
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Acorn Woodpecker

TMBC members have recorded 12 woodpecker species in our listing area. Generally Acorn Woodpeckers behave like most “Picids”. Woodpecker flight pattern is described as bounding (only the Lewis's Woodpecker flight is crow like). That is: flap flap flap, followed by a high, with wings closed, up and down arch, then resuming the flapping level flight. The central tail feathers are stiff and pointed, thus acting as a prop when clinging to or climbing trees.

Woodpecker feet are zygodactyl - digits 2 and 3 forward, with 1 and 4 to the rear. Although excellent for clinging and climbing, they are ill-equipped for ground landings. Yet they do spend time on the ground, especially in the middle of the road, looking as if they are squatting on their bellies. The female's bill is shorter than the male's.

There is no mistaking this bird's vocalization. I hear it as a sort of laughing “akka akka akka”. However literature usually describes this call as a “wheka wheka wheka”. This is not their entire repertoire. There are at least 9 or 10 calls plus a lot of nonsensical jabbering.

An Acorn Woodpecker's drumming is slow, short and accelerating - not long lasting like several woodland woodpeckers. Concentrate on their interactive vocalizations while watching a socializing family group of Acorn Woodpeckers. It is pure entertainment.

Acorn Woodpeckers range throughout the oak woodlands from southern Washington, Oregon, California, Baja, through parts of Arizona, New Mexico, extending east into Texas and as far south as Columbia.

Part II - Acorns

The Acorn Woodpecker range extends throughout the oak woodlands of southern Washington to Baja California, extending east to the Big Bend area of Texas, and on through Mexico as far south as Columbia. Although they feed in part on insects, various nuts, fruits, and sometimes bird eggs, fifty percent of their diet is acorns. Walt Koenig, who has studied and written about this bird for years, feels “no species is more intimately associated with oaks than the Acorn Woodpecker”. It certainly is fair to say this bird is an oak woodland obligate species (and, for a change, is certainly well named).

Acorn Woodpecker GranaryAcorn Woodpeckers begin to consume these nuts even as they mature on the tree. As the acorns age in ever increasing quantities these birds begin harvesting the producing trees, transferring the bounty to “granaries”. Granaries are storage trees, poles, cavities, where as many as 50,000 acorns may be put away. These granary trees and their holes are critical in their scheme of things for they are used year after year, sometimes for decades, until they fall. This larder is depended upon over the winter, into the spring nesting and fledging of the young, and until the hoped for fall crop.

What if there is a crop failure? If there is a failure, what will the birds do? Move? How far? After all, to overcome challenges birds must migrate, adapt or die. As the local Native Americans, the Nuooah, knew, there are many oak tree species here in the Tehachapis. Valley (Quercus lobata), Blue (Quercus douglasii), Black (Quercus kelloggii), Canyon (Quercus chrysollepis)(Golden), Interior Live Oak (Quercus wislizenii), plus several scrub and hybrid species. Many acorn exploiting birds are the subjects of many Nuooah stories. For our woodpecker, as well as the Nuooah, this diversity is critical in the life community scheme of things.

Geographical synchrony. For our purpose we’ll mention only the four species: Blue, Valley, Black and Golden. Generally, if a Blue Oak crop fails in the Redding oak woodlands it will also fail here in the Tehachapi oak woodlands – geographical synchrony. Seldom do both Valley and Blue (which produce crops annually) fail in the same year. Throw into the mix Black and Golden acorn crops every two years. In a diverse ecosystem, like the Tehachapis, where all four species are present, it is highly unlikely there would be a total acorn crop failure across all oak species.

In the Tehachapis the valleys and foothills show a good mix of Blue and Valley Oaks. Generally, these species give way to Black and Golden Oaks at 5000 feet. Adding Interior Live Oaks, there is little threat of crop failure for the woodland clown. Given a Blue Oak failure, a family might have to move from the Bear Valley Springs gate area to the Upper Valley where Valley Oak is the predominant species. In the event of a Black Oak failure, birds may drop off the mountain and feed off Golden Oaks and lower hill top Valley Oaks. As long as there is oak specie diversity, our woodland clown will do just fine.

Part III - A Dilemma For Darwin

Acorn WoodpeckerOur Clown of The Oak Woodlands has one of the most unusual breeding systems of any bird. In California, Acorn Woodpeckers are cooperative breeders. That is, consisting of as many as 15 males and females, they live in family groups with all members participating in the raising of the young. Cooperative breeding is not rare. For examples: Pied Kingfisher, Purple Gallinule, Florida Scrub Jay among many. However, in about one-quarter of Acorn Woodpecker family groups studied on the coastal Hastings Reservation, communal nesting was also found.

Communal nesting means two, rarely three, females lay their eggs in the same nest hole. Unlike usual cooperative breeding, where there is a single breeding pair along with their offspring from previous years who serve as nonbreeding helpers, in communal nesting, the complex group structure is made up of, along with non-breeding offspring, multiple cobreeding males and multiple cobreeding joint-nesting females laying their eggs in the same nest cavity.

Immediately images of a cavity with the chaotic activity of up to 15 clowns engaged in incestual free sex come to mind. Well, not exactly. First of all, cobreeders of the same sex are almost always close relatives, either siblings, a parent and an offspring, or a combination of the two. However, breeders of the opposite sex are nearly always unrelated, so incestuous matings are rare. In these communal nests there are usually just two cobreeding males and two cobreeding females.

These birds are cooperative. They share stored acorns. They jointly defend the granary. All members help feed the young. Males will guard only when two or more males from the group are present. In other words the threat is posed not by strangers, but by other males in this extended family. In addition, the females are also competitive, which takes form in a strange way, reported in only one other bird species (the Grooved-billed Ani).

It took two years for William Koenig and Ronald Mumme to definitively conclude as to what they had seen when they first saw a female Acorn Woodpecker removing egg shells from a cavity nest - a cavity nest otherwise vacant. Finally observing a cobreeding female Acorn Woodpecker remove a whole egg from a nest, they established that cobreeding female nest mates are competitive and remove each other's eggs! An interesting unique twist is that the removed egg is cached and consumed just as if it were a stored acorn.

Eventually the competition ends with the nest containing an average number of eggs (7 +) for a cooperative nesting clutch (perhaps 5 from one female and 2 from the other) resulting in the usual number of fledges. In those rare cases of three female cobreeders, the competition is so fierce that less than the normal number of eggs and fledges result. An important consequence of this unique behavior is both synchronized egg laying and hatching! This minimizes nestling starvation, thus helping to assure all offspring fledge.

This ovicide is a defense behavior, seemingly increasing the chances that surviving offspring will be those of, for example, female #1. But the behavior is also offensive, for as we noted above, the number of eggs in the clutch is not decreased. Further, although these birds are mitigating egg destruction by laying additional eggs and by eating the destroyed eggs (thereby recouping energy), egg destruction is a vivid example of behavior appearing to be detrimental to the species. Yet it flourishes within the population, and, we know empirically the species does well.

For what end purpose does this behavior serve? Improvement of species through some adaptation(s)? What is going on here? There must be a direct or indirect benefit to the fitness of the birds, and thus the species, resulting from this communal nesting as well as from other forms of cooperative breeding. Otherwise, it is a dilemma for Darwin.

Observing these compelling enigmatic woodland clowns is great sport. For further reading - Cooperatively Breeding Acorn Woodpecker - Koenig and Mumme, as well as the June 1997 edition of Natural History magazine.

Good birding, gone birding.

© Photos by Beryl Stark