Granary-Site Selection by Acorn Woodpeckers in the Willamette Valley, Oregon

The acorn woodpecker is among the most common primary cavity nesting bird of the Oregon white oak woodlands. In most of their range, acorn woodpeckers are dependent on granaries for acorn storage, yet little is known about their selection of granary sites. We compared habitat characteristics within 12 m of granary and non-granary trees at 20 acorn woodpecker colonies in Benton County, Oregon during the winter of 2001. Compared to non-granaries, granary plots consistently had greater oak basal area and shorter shrub height, and granary trees were of larger diameter. Within each of the 20 sites, oak basal area was greater near granary than non-granary trees. This, together with the selection for larger diameter granaries, suggests acorn woodpeckers are more likely to locate granaries in the immediate area of high acorn production. Increased acorn production in the vicinity of granaries is likely beneficial to the birds because minimal effort is expended in caching maximum forage. Our results shed light on granary selection at the spatial scale of the immediate area surrounding granaries and suggest factors associated with acorn woodpecker distribution at the landscape scale. 1 Current address: Bureau of Land Management, 2795 Anderson Ave #25, Klamath Falls, 97603 2 Author to whom correspondence should be addressed. E-mail: [email protected] Introduction The acorn woodpecker (Melanerpes formicivorus) is highly specialized for living in oak (Quercus spp.) communities, and its range is limited by the distribution of oak woodlands and diversity of oak species (Bock and Bock 1974). The demography of acorn woodpeckers is similarly affected by acorn mast and the ability to store it (Koenig et al. 1995). The central feature of acorn woodpecker territories is the storage tree, or granary. Acorn woodpeckers gather acorns directly from the tree and drill small holes in granaries in which to store them for the winter (MacRoberts 1970, MacRoberts and MacRoberts 1976, Koenig and Mumme 1987). Typically, each home range will contain one primary granary tree, and often one or more secondary granaries with fewer storage holes (Koenig et al. 1995). Granaries are defended from competitors and are reused each fall (Koenig et al. 1995). Why specific trees are chosen as granaries is unknown. It is probably energetically beneficial for the birds to center their activities, and consequently their granaries, on the highest quality microhabitats within their home range (Pyke et al.. 1977). Proximity to acorn production may be important to granary-site selection due to increased caching efficiency and acorn quantity. Larger diameter trees may provide many advantages including greater mast production and a greater volume of dead and decaying wood, which may be beneficial for the construction of granaries and cavities. Avoidance of predation and competitors for acorns may also play a role on granary location. Granaries in areas with low ground cover height may be indicative of a safer site with lower terrestrial competition and predation. Although many tree species and human structures may be used as granaries (Koenig et al. 1995), oaks may be more attractive as granary sites than other tree genera. As large expanses of suitable habitat decline (ODFW 2005), particularly in the northern edge of their range (e.g., Ryan and Carey 1995), more attention to granary-site selection may be necessary to ensure viable acorn woodpecker populations. Our objectives were to compare tree and surrounding habitat characteristics between granary and non-granary trees for a population of acorn woodpeckers at the northern fringe of their range. We examined the selection of granaries at the spatial scale of the immediate (<15 m) area surrounding the granary within colonies of acorn woodpeckers. Such information will aid management of oak woodlands and savannas, threatened vegetation types in Oregon (ODFW 2005).