![]() That is, when there are more species of oaks in an area, both the overall acorn crop and the population size of acorn woodpeckers are more stable and fluctuate less from year to year. Second, the annual variability of acorn woodpecker population size at a site is less when the number of species of oaks present is greater. Most strikingly, the range of acorn woodpeckers generally is restricted to sites containing two or more species of oaks, presumably because the probability of total crop failures in areas with only a single species of oak is too high to maintain populations over the long term. These features of the acorn crop have dramatic consequences on the geographical ecology of acorn woodpeckers along the Pacific coast. Because acorn woodpeckers, like other birds and mammals that eat acorns, tend not to be picky about which species they use, this means that the probability of a total crop failure in a locality is low, despite the large geographic synchrony exhibited by individual species. Consequently, when it’s a bad year for blue oaks, it’s unlikely to concurrently be a bad year for California black oaks or canyon live oaks present in the same area. This is especially true between “1-year” species (such as blue and valley oaks) that require only a single season to mature acorns, and “2-year” species (such as California black oak and canyon live oak) that require two years to mature a crop of acorns. ![]() Fortunately for acorn woodpeckers, as well as other acorn-dependent species, most areas in California contain several species of oaks and synchrony among species of oaks is generally not great. The flip side, however, is that in a bad acorn year, few or no blue oak acorns are likely to be found almost anywhere in the state. If we assume an average of 1,000 acorns per tree in a good year, we’re up to 100 billion acorns- enough to support a lot of woodpeckers, as well as anything else that can handle the bitterness of raw acorns in its diet. Blue oaks cover approximately 3 million acres of woodland in California at densities (based on vegetation surveys at Hastings Reservation in Monterey County) of about 173 trees per acre, which means we’re talking on the order of over 100 million trees, each of which produces a crop of acorns synchronously with the others. For example, blue oaks, which we survey at 10 localities over a 500-mile transect both in the Sierra and coast ranges, are synchronous throughout the state (i.e., when it’s a good year for blue oaks in Shasta County, it’s also a good year for blue oaks in Santa Barbara County). With only six years of data the results are preliminary, but the general conclusion is clear: acorn production is highly synchronous over large geographic areas for at least several of the species. Currently encompassing 16 sites from Shasta to San Diego County, the survey involves visually counting acorns of over 950 trees of 7 species of oaks. In order to address this issue, Jean Knops of the University of Nebraska and I initiated the California Acorn Survey in 1994. This brings us to the question of geographic synchrony in acorn production within and between species of California oaks. Whereas, if the acorn crop is not synchronous within and between species, they may only have to travel a few miles before coming to an area with lots of acorns where they could survive through the winter. If the acorn crop was poor and synchronous both among individuals of the same species and across species over a large geographic area, there would be few acorns anywhere nearby and the birds might have to travel a very long distance before finding an area where there were enough acorns to survive. How far they have to go presumably depends on how geographically synchronous the acorn crop is within and between species of oaks. ![]() However, in poor years, the birds are forced to abandon their territories and wander off in search of acorns elsewhere. In good acorn years, acorn woodpeckers not only reproduce extremely well the following spring but may even successfully breed in the fall, fledging young as late as early November. Indeed, such “storage trees” or “granaries” have been reported to contain up to 50,000 acorns or more! Holes are used over and over each year and accumulate with time. More dramatically, they harvest acorns in large numbers and store them in special trees in their territories in which the birds have drilled holes, each of which can hold an individual acorn. Acorn woodpeckers eat acorns directly off trees in the fall as acorns mature. No species is more intimately associated with oaks than the acorn woodpecker, a common resident of oak woodlands throughout California. Oaks ‘n Folks – Volume 15, Issue 1 – March 2000
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