The Wanderings of Acorn Woodpeckers

In the Pacific Northwest, Acorn Woodpeckers (Melanerpes formicivorus) have been reported with increasing frequency over the last few years. The northernmost confirmed nesting population of this species occurs in Lyle, Klickitat County, Washington, many miles from the nearest regular territories of the species. Population density increases slowly southward and then markedly in the oak habitats of southwestern Oregon and into California.

Similar wanderings of many bird species are often attributed to a decreased food supply within their usual ranges. If we apply this explanation to the recent Acorn Woodpecker reports, it would indicate that the acorn crops have failed within the home ranges of these individuals. Failed acorn crops may be a factor in these recent forays, but the dynamic natural history of this woodpecker makes the situation a bit more complex. Below, I offer some alternatives.

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This female Acorn Woodpecker was photographed in Pima County, Arizona on 25 April 2009. (Photo by Scott Carpenter)

In general, Acorn Woodpeckers eat acorns from about August through May, but more so in the fall, as the crop ripens. From winter through summer, they supplement their forage with sap (usually from wells they make themselves). They will also eat insects/arthropods any time of year, with insects becoming their primary food during the breeding season. In all seasons, they prefer insects, whenever they are available. Their bill and tongue morphology indicate that they are generalists, although the bill shape may also make it easier to carry acorns. Their wing and body morphology are specially adapted for catching flying insects.

Acorn Woodpeckers are generally non-migratory, and they are only resident in regions with a reliable acorn source. In addition to acorns they also need granary, or storage, trees (although one small population in the Southwest has plenty of acorns but no storage trees, and they have adapted to this by migrating). Most Acorn Woodpeckers will remain on their home territories through the winter, as long as they have a reliable food supply.

Breeding Bird Survey data for Oregon (with the highest degree of credibility for the survey) show an increasing trend in the state since the mid-70s, with an increase of approx. 7.8% per year since 1980 for the 14 routes in the state where the species is reported.

Acorn Woodpeckers live in family groups, which at a minimum are comprised of a breeding male and female. Sometimes male, and to a lesser extent female, offspring from prior years may remain on the natal territory for up to 10 years, where they help to raise subsequent broods. These helpers may even breed, or attempt to breed, in the same nest as their parents—but this is another story! Also, males generally outnumber female fledglings each year, possibly explaining the dynamics of this highly social behavior.

The Acorn Woodpecker is considered a pioneering species. A few months after fledging, juveniles begin prospecting outside their home territories in search of their own territories and potential mates. They may stay on their natal territories for years, but they are always searching for new ones. Suitable habitat for a new territory must have three things: potential nest-cavity substrate, generally snags or dead branches of live trees; oak trees, for the fall feeding ritual; and granary trees, typically trees with soft thick bark, e.g. Douglas-fir, but also snags or artificial

trees such as power poles. Once they find suitable habitat, they may attempt to attract a mate and establish a new family group.

If they do not find suitable habitat, they probably just keep searching. After all, they are compelled to produce offspring. California is the locus, or heart of the range of “Baird’s” Acorn Woodpecker (M. f. bairdi), the subspecies found in the Pacific Northwest. California populations generally live among multiple species of oak tree. Sometimes, if one oak species experiences an acorn crop failure, others may still produce. In Oregon and Washington, we generally have one oak species, the Garry Oak or Oregon White Oak. An acorn crop failure of this single species would presumably force birds to relocate in search of suitable fall forage.

Single-oak-species habitats have another problem. The removal of these oak trees from the equation (for whatever reason) will force oak-dependent species to look elsewhere for food. And it just might force Acorn Woodpeckers to go well beyond their normal range in search of acorns.

Starting to get the picture? With recent steady growth in the Oregon population of Acorn Woodpeckers, more individuals are out scouting for new territories, and they may travel quite a distance in search of suitable habitat. A bumper acorn crop in Oregon could result in even more offspring produced, and hence an increase in the number of pioneering individuals during the fall. A failed or absent acorn supply may send them even farther to feed in that season. If they find new habitat that is suitable for breeding, they just might stay (e.g. Lyle, WA). If do not encounter optimal conditions, they may return to their natal territory for the next breeding season. Some individuals may even continue to pioneer long distances, many hundreds of miles in rare cases, never returning to where they were raised.

It is exciting to see many birders’ lives brightened by the unexpected presence of this very charismatic and fascinating species. And you can probably count on it happening as long as there are Acorn Woodpeckers.

Special thanks to Acorn Woodpecker guru Dr. Eric Walters of Cornell University for reviewing this article. Eric leads the current research on the longest known study of any woodpecker population on the continent, and perhaps the world: the Acorn Woodpeckers of the University of California’s Hastings Reservation in Carmel Valley, California.

Steve Shunk is a professional naturalist and woodpecker specialist living on the east slope of the Oregon Cascades. He is also the author of the seriously-almost-finished Peterson Reference Guide to Woodpeckers of North America.

1

Steve offers a thorough discussion of a topic that has spurred lots of comments on statewide listservs in Oregon and Washington in recent days. There are likely multiple factors causing the more frequent and widespread recent wanderings of Acorn Woodpeckers in these two states.

2

I enjoyed this article by Steve Shunk, and the mention that Acorn Woodpeckers will occasionally use “artificial trees” for acorn storage. Earlier in the day, before I read the article, I happened to see two birds doing just that, and the artificial “tree” they were using was actually a building, stuffing acorns into a crack between a window and a brick wall, and also under the edges of roof shingles. (A photo is at http://empids.blogspot.com.)


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Floyd Schrock
McMinnville, Oregon USA
http://empids.blogspot.com
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3

This business of Acorn Woodpeckers using buildings for storage is actually a serious problem. I get lots of complaints about this one. Anyone have a good solution for keeping Acorn Woodpeckers from using buildings as a grainary?

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