Acorn woodpeckers and their hoarding habits

 

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. Acorn woodpecker colonies are common in Three Rivers and throughout the western Sierra foothills. Since acorn woodpeckers are not migratory, they can be seen here year-round, but are most often seen (and heard) in the fall during their acorn harvest.

True to their name, these birds are acorn specialists. They develop communal granaries that may consist of thousands of holes — self-drilled into tree trunks and limbs and, if convenient, fence posts, wooden utility poles, and even the siding of houses — each stuffed with an acorn. Only about half of the acorn woodpecker’s diet actually consists of acorns; the other half is made up of fruit, insects, and other vegetable matter. But their acorn granaries are the staple food source that gets them through lean times. 

According to Birds of North America:

In areas where there are large seasonal fluctuations in insects and other foods, year-round residency is dependent on the [acorn woodpecker’s] 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 October 2014 edition of the California Acorn Report (yes, there is such a thing!), this was a medium-to-poor acorn year. Results were particularly poor for live oaks: it was either the worst or next-to-worst year ever for coast live oaks, canyon live oaks, and interior live oaks. 

Blue oaks were generally fair to poor, and it was a fairly poor year for tanoaks. The valley oak crop was reasonably good but, overall, acorn production was the worst it has been since 2003, and less than half the crop production of 2012 and 2013.

So what conditions make for a stellar acorn year? University of California’s Oak Woodland Conservation Workgroup 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 studied.

So it is quite possible this year’s poor crop is attributable to the rain that didn’t happen a couple years ago. In that case, it may be a while before acorn crops return to 2012-2013 levels.

On Thursday, Jan. 15, Three Rivers resident Carole Clum will present a program on acorn woodpeckers. See the Kaweah Kalendar for details.

In addition to the sources noted in the article, information was received from the Golden Gate Audubon Society’s “Observations” blog.

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