Thesis

Foraging Behavior and Functional Morphology of the Bark-foraging Birds of the Bouverie Audubon Preserve

A study was conducted on five bark-foraging birds at the Bouverie Audubon Preserve in Sonoma County, California. This study was divided into two parts. I. An ecoloqical study of the foraging behavior and mechanics of scansorial, bark-foraging birds. This study consisted of field observations of five species: Nuttall's Woodpecker, Downy Woodpecker, Pileated Woodpecker, Brown Creeper, and White-breasted Nuthatch. Observations (totaling 127 hours) were conducted from February through June 1985. Observational data were recorded and analyzed for substrate condition use (live vs. Dead), substrate size utilization, foraging behavior, and foraging orientation. Each substrate and foraging category was analyzed as to percent utilization by each species by sex where detectable. These data were then tested for significant intersexual or interspecific differences. Significant differences were found between species in one or more resource categories. Intersexual differences were found for the Pileated Woodpecker only. Niche breadth and overlap were also quantified for each of the resource dimensions. Results show that even though these birds superficially appear to be using the same resources, they are actually foraging in significantly different ways which allow for coexistence. II. Functional morphology and adaptations to a bark-foraging habit. This study targeted selected anatomical and morphological characters to determine their relationship to the observed behaviors in part one. Gravitational forces acting on a scansorial bird were analyzed first. Anatomical and morphological measurements of some characters were then compared with the percentage use of the foraging behaviors. Characters measured which appear to be correlated with the quantified behaviors are; 1. the length of the tarsometatarsus, 2. bill dimensions and morphology, 3. weight of the bird, and 4. length of the bird. Each of these variables likely works in unique combination with the others to effect a certain degree of ecological separation, as expressed by their behaviors, with coexisting guild members. This results in reduced competition through resource partitioning and minimum niche overlap.

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