Graduate Thesis Or Dissertation
 

An analysis of guild structure of avian communities, Columbia River, Oregon

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  • Effects of habitat and season on guild structure of avian communities were examined for islands in the estuary zone of the Columbia River, Oregon. Bird species were classified into guilds according to their major food item, typical foraging substrate, and foraging behavior. Variation in community structure indices (number of guilds, guild diversity, evenness, bird density, and biomass) among habitats and seasons were analyzed with multivariate analysis of variance and discriminant function analysis. Cluster analysis was used to examine patterns of guild distribution among habitats and seasons. Four habitat-bird community associations (tree-shrub, upland, beach-low marsh, and high marsh) were described based on results from this study. High guild evenness within tree-shrub habitats during the breeding season (spring and summer) distinguished tree-shrub bird communities from communities within the 3 other habitats. Relatively stable bird density within tree-shrub habitats was attributed to high vegetation heterogeniety and later successional stage of tree-shrub habitats. Seasonal variation of community structure indices was greatest within upland habitats. Comparison of seasonal patterns in number of guilds, guild evenness, bird density, and guild composition between tree-shrub and upland habitats suggested greater stability of food resource productivity, availability, and diversity within tree-shrub habitats. Differences in avian community structure between the 2 terrestrial seral stages (tree-shrub and upland) were ascribed to differences in successional stage and vegetation complexity. High avian biomass distinguished bird communities of beach-low marsh habitats and was probably related to high productivity typical of estuary systems. Seasonal variation of community structure, primarily a winter reduction in bird density, was least definitive in beach-low marsh habitats. Lack of winter reduction in number of guilds and seasonal changes in guild composition suggested low winter density was related to reduced availability of food resources as affected by tidal inundation, low temperatures, and ice on the river. Bird communities associated with the high marsh habitat were characterized from all other bird community-habitat associations by greater number of guilds. Less extensive tidal inundation, greater vegetation complexity and more edge effect in the high marsh habitat than in beach-low marsh habitats seemed to affect greater occurrence of terrestrial associated guilds. Results of this study indicated description of avian community structure on the basis of food-resource defined guilds was useful in interpreting how environmental (habitat and season) variation may affect bird communities. Validity of inferences from this study requires more detailed analysis of the bird communities and their habitats. Thus, the guild approach as used in this research is most appropriate for preliminary examination of communities, indicating areas where more specific study should increase understanding of the structure and functioning of avian communities.
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