Why Are Acorn Woodpeckers Appearing in Berkeley's Lowlands?

They are the clowns of the oak savannah — Acorn Woodpeckers — with their harlequin faces, gregarious habits, and off-kilter laughing calls that inspired Woody Woodpecker.

Here in the Bay Area, Acorn Woodpecker colonies are fairly common in the East Bay hills and the western slopes of Mount Diablo, particularly where there are concentrations of valley oaks. South of Livermore, they can be locally abundant in the Diablo range. They are rare in Tilden and Redwood Regional Parks, however, and practically unheard of west of the Hayward Fault.

So what’s up with the recent spate of Acorn Woodpecker sightings in urban San Francisco and the East Bay lowlands?

Beginning around Sept. 1, Acorn Woodpecker reports started rolling in from multiple East Bay sites from Oakland to Richmond. In Berkeley, they have been reported near McKinley and Bancroft streets, at the UC Botanical Gardens, in Claremont Canyon, and behind the Seabreeze restaurant near the Berkeley Waterfront. They’ve also been sighted across the Bay in San Francisco.

This irruption – the term for a sudden upsurge in a bird population — is particularly noteworthy because Acorn Woodpeckers are not migratory. What’s driving it? The most likely answer is acorn crop failure.

True to their name, Acorn Woodpeckers are acorn specialists. They develop communal granaries that may consist of tens of thousands of holes drilled in tree trunks and limbs, each stuffed with an acorn. Only about half their diet actually consists of acorns — the other half is made up of fruit, insects, and other vegetable matter — but the acorn granaries are the staple food source that gets them through lean times. According to Birds of North America:

[i]n areas where there are large seasonal fluctuations in insects and other foods, year-round residency is dependent on the birds’ ability to store sufficient acorn mast to provide food throughout the winter. Groups that exhaust their stores often abandon their territories and wander off in search of alternative food.

This year appears to be one of those years. According to the Oct. 2014 edition of the California Acorn Report, this is a medium-to-poor acorn year. Results are particularly poor for live oaks: it is either the worst or next-to-worst year ever for coast live oaks, canyon live oaks, and interior live oaks. Blue oaks are generally fair to poor and it’s a fairly poor year for tanoaks. The valley oak crop is reasonably good but, overall, acorn production is the worst it has been since 2003, and less than half the crop production of 2012 and 2013.

So what does the future hold for our clown-faced friends? Is crop failure due to drought or just cyclical variation in acorn production? University of California’s Oak Woodland Conservation Work Group sheds some light on the drivers of crop size:

For valley and blue oaks, the most important single factor is weather in April, the peak month for pollination, with crops being heavier in years when mean April temperatures are warmer. … For coast and canyon live oaks, mean acorn production is positively correlated with rainfall occurring one and (for canyon live oak) two years earlier. … Interestingly, winter rainfall in the same year as acorn production-the factor most commonly thought to determine crop size-does not correlate positively with mean annual crop size of any of the species we studied.

So it’s quite possible this year’s poor crop is attributable to the rain we didn’t get a couple years ago. In that case, it may be a while before acorn crops return to their 2012-2013 levels. In the meantime, here’s hoping our feathered friends find some good food sources in their new urban digs.

Bruce Mast writes the monthly Observations column on rare and unusual bird sightings that appears on Golden Gate Audubon Society’s website. You can email him at observe@goldengateaudubon.org. This post originally appeared in Golden Gate Birder, the Golden Gate Audubon blog.