Recovery Plan: Acorn Woodpecker (Melanerpes formicivorus)
This is a conservation recommendation plan for the Acorn Woodpecker (Melanerpes formicivorus) of Oregon within the State of Jefferson-bioregion. The Oregon Department of Forestry has deemed the Acorn Woodpecker to be a species of likely concern in the near future due to its susceptibility of declining populations due to a loss of habitat from urbanization pressures and encroachment of a changing diversity of habitat type.
a. Geographic range in the state of Jefferson
The Klamath bioregion is encompassed in the State of Jefferson and is home to 4 national forests; the Siskiyou, six rivers, Klamath and Shasta-Trinity. Also in this region is the redwood national park, a network of state parks and wilderness areas, Jackson state forest, as well as forests on private land. This region receives much of its attention on redwood forests; however some areas contain extensive tanoak forests (Altman 2012).
Melanerpes formicivorus or the Acorn Woodpecker can be found mainly in oak and pine- oak woodlands; also along riparian corridors, and in Douglas-fir, redwood, and tropical hardwood forests as long as oaks are present or available nearby. They are found at sea level in southern California, but more generally in mountains up to the distributional limit of oaks (marshall 2003).
b. Numbers/density/variability in population size
The species general distribution ranges from south central Washington down along the west coast of California and as far-east to southern Utah and Texas, continuing scattered populations have been observed through the northern tip of South America.
In Oregon, of seven subspecies, only Melanerpes formicivorus bairdi can be found. It is fairly common in the Rogue foothills and locally common in the Klamath River Canyon. It is also locally common in the Southern Willamette Valley and west of Portland (Marshall 2003)
Determination of population density estimates were derived from a number of factors. Given the average Acorn Woodpecker group size in California and Benton County Oregon was between 4.4 and 4.25 birds per group, with an average territory range of about 6 Ha and vegetation maps of Oregon estimate about 400,000 Ha of inhabitable hardwood forest by tree type; it is estimated that about 293,333 Acorn woodpeckers inhabit the state of Oregon of the 5,000,000 estimated from the Christmas Bird Count total.
Populations are limited by the availability of acorns and suitable granaries. Territorial behavior sets an upper limit to the number of breeders in a group. Group size is limited by granary size and social factors, primarily inbreeding avoidance and reproductive competition. However, by the second year of life non-breeding females engage in forays or a flight by individuals to neigh boring territories searching for reproductive vacancies. This is where metapopulation and immigration exhibit exchange to avoid genetic complications (Koenig 2014).
c. Population structure (continuous population, highly fragmented with limited dispersal among sub-populations?)
Acorn Woodpeckers have complex social behaviors. The quantity and quality of stored acorns influence group size and composition, reproductive success and survivorship (Ulev 2007). Acorn woodpecker groups defend year-round territories, cooperatively store acorns at least 2 or more granaries, produce 1 nest at a time and raise their offspring together.
Groups consist of 2 to 15 individuals, with an average of 5 or 6 individuals. Within the groups, there are 1 to 4 breeding males, 1 to 2 breeding females and 0 to 10 non-breeding offspring born in previous years (Ulev 2007).
Some Acorn Woodpeckers live in groups and some live completely independently and store few acorns. Both types of behavior may be present within the same population at any given time, and may shift between strategies, depending on acorn production. During years of good acorn production, acorn