Wild Neighbors: Communards in the Oak Trees

By Joe Eaton
Tuesday March 08, 2011 - 10:19:00 PM
Nature's file clerk, the acorn woodpecker.
Kevin Cole (via Wikimedia Commons)
Nature's file clerk, the acorn woodpecker.

I’ve been watching acorn woodpeckers in the Bay Area for years, from the Stanford campus to Point Reyes, and have always found these noisy, conspicuous birds engaging. “This sociable woodpecker impresses one as an exceptionally jolly bird”, writes ornithologist Alexander Skutch, “and certainly it is one of the most amusing to watch.” 

Native groups, particularly in the Klamath region, valued acorn woodpecker scalps—along with those of the larger pileated woodpecker—as wealth objects. Among the Shastan peoples, a bride price typically included 20-30 woodpecker scalps, along with deerskins and dentalia shells. The scalps were also used as personal adornment, in feather capes and headbands, and to represent the eyes and ears of the albino deerskins displayed in the White Deerskin Dance. Spanish explorers encountered the bird in Chiapas in the 16th century, dubbing it the carpintero. Father Pedro Font, 200 years later, appears to have been the first to describe its acorn-storing habits in what is now California. 

More is known about its behavior than that of other woodpecker species. In addition to studies in Arizona by Pepper Trail, Peter Stacey and others, Walter Koenig has monitored the birds at the University of California’s Hastings Reservation in the Carmel Valley for more than 30 years. Remarkable findings have emerged from this research. 

An acorn woodpecker’s world revolves around its granary: usually a tree, sometimes a series of fence posts, a telephone pole, or a building. One group even used the radiator of a car. Pines, or sycamores where they occur, are preferred to oaks as storage trees. (Sycamores are also favored for nesting; their smoother trunks deter gopher snakes, which can be significant nest predators.) The quantity of storage space can be mind-boggling: W. Leon Dawson counted 50,000 acorns in one ponderosa pine near Santa Barbara. At the Hastings site, Koenig calculated an average storage rate of 325 acorns per bird per year. 

Few of the stored acorns go to waste. They constitute more than half the woodpecker’s diet for most of the year, supplemented by flying insects, ants, and tree sap. Hatchlings are fed a mix of insects and broken-up acorns, with older chicks receiving proportionately more acorns. 

Each granary is controlled by a breeding group. At its most complex, the family unit may include up to 7 co-breeding males, either brothers or a father and his sons; up to 3 joint-nesting females, sisters or mother and daughters; and up to 10 non-breeding helpers, hatched in previous years, who incubate the eggs, feed the nestlings, and aid in territorial defense. A group may contain as many as 13 individuals. Group nesting is not uncommon in woodpeckers, recorded in the red-cockaded woodpecker of the Southeast and a number of tropical American species. But the acorn’s combination of polygynandrous mating with nest helpers has been seen in only a few other species, including a New Zealand rail, the African wild dog, and, oddly, a cichlid fish from Lake Tanganyika. 

Helping rear your close relatives’ offspring makes evolutionary sense in kin-selection terms. But it’s not just one big happy family. Breeding males compete for mating opportunities, and females vie to have their own eggs incubated in the nest they share. The first egg laid may be tossed out or eaten by a sibling; perhaps as a result, females often begin with a nonviable “runt” egg. DNA fingerprinting studies suggest inbreeding is extremely rare. If a group loses all its breeders of one sex, their place is taken by a coalition of siblings who had been non-breeding helpers in another family, usually after a prolonged “power struggle” among candidates for the vacancy. 

The origins of this system are not fully understood. Although group size would be advantageous in defending the granary, acorn woodpeckers in Central and South America live in groups but—perhaps because resources are more dependable—do not store food. 

The full spectrum of group-nesting behavior may emerge only in the most densely populated parts of the acorn woodpecker’s range. 

The woodpeckers occur only where two or more oak species grow; in the Klamath-Siskiyou region, they approach their northern boundary. Koenig’s California study site has 5 common and 2 less common oaks, while Trail’s Arizona site had 3. The more species of oaks in an area, the more stable the woodpecker population from year to year. There’s a kind of insurance at work here: acorn production, in a one- or two-year cycle depending on the species, is synchronous over a wide area. If canyon live oaks have a bad year, California black oaks may take up the slack. 

A general failure of the acorn crop can have devastating effects. As last season’s food stores dwindle, conflict increases within the group and the birds begin to disperse, the least dominant leaving first. In the worst years, the granary and its surrounding territory may be abandoned. When I interviewed Koenig a few years ago, he told me that the woodpeckers could be seriously impacted if Sudden Oak Death Syndrome wiped out coast live oaks, black oaks, and tanbark oaks. 

The acorn woodpeckers, of course, are only one of the hundred-plus bird species (not to mention other vertebrates) that nest or forage in oaks. But it would be a particular tragedy if the new scourge put an end to the ancient and intimate association of oaks and woodpeckers.