Wild Neighbors: Rossmoor, Spare Those Woodpeckers!

By Joe Eaton
Thursday November 20, 2008 - 10:54:00 AM
An acorn woodpecker and a colony’s granary tree.
Ron Sullivan
An acorn woodpecker and a colony’s granary tree.

I have no idea how what kind of readership the Daily Planet has in Rossmoor. For whatever it’s worth, though, here’s my two cents on the acorn woodpecker controversy. You may recall that the Rossmoor homeowner’s association has obtained a U.S. Fish and Wildlife Service permit to execute 50 of the local woodpeckers for malicious destruction of property, namely drilling acorn-storage holes in human residences. 

I’ve been observing 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.”  

More is known about the acorn’s behavior than that of other woodpecker species. UC biologist Walter Koenig, author of Population Ecology of the Cooperatively Breeding Acorn Woodpecker, has monitored the birds at the Hastings Reservation in the Carmel Valley for over 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 used the radiator of a car. Pines or sycamores are preferred to oaks as storage trees. 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 seven co-breeding males, either brothers or a father and his sons; up to three 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.  

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—don’t store food.  

The full spectrum of group-nesting behavior may emerge only in the most densely populated parts of the acorn woodpecker’s range, like central California. The woodpeckers occur only where two or more oak species grow. Koenig’s study site has five common and two less common oaks. 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: 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. 

A few years back I asked Walter Koenig how Sudden Oak Death Syndrome could impact the acorn woodpecker. “I don’t really know what it could mean for the woodpeckers,” he said, “but as a worst case scenario, if it wiped out coast live oaks, black oaks, and tanbark oaks, we’re talking a considerable decrease in oak species diversity throughout the state (and potentially beyond), which, give that their populations are highly dependent on oak species diversity, would most likely restrict their range significantly.” 

The acorn woodpecker is not on the endangered species list, or any of the less formal watch lists, yet. But that could change if the SOD pathogen spreads. It’s not a good time for indiscriminate killing. Let’s hope whoever is in charge at Rossmoor gives nonlethal control methods more of a chance. See the Lindsay Museum’s web site (www. for options.