Community Organization of Arboreal Birds in Some Oak Woodlands of Western North America

Community organization of the arboreal birds in two oak woodlands of western North America was examined during the course of two breeding seasons. A "natural experiment" comparing the arboreal avifaunas inhabitating oak woodlands of similar tree structure in northeastern Sonora, Mexico (referred to as Purica), and in the central coastal mountains of California, USA (referred to as Carmel), was used to address questions concerning (1) variables that account for variation in foraging behavior, (2) overall similarity in foraging behavior of the two avifaunas, (3) partitioning of resources within each avifauna, (4) guild structure, (5) foraging heights, 6) changes in foraging behavior between sites, and (7) complementarity in resource use during foraging. At Purica there were 15 breeding species, and at Carmel 10 species. Foraging technique, portion of the tree, perch substrate and height, and the substrate from which food was derived during foraging were quantified with the aid of an audio metronome. The first three axes in a Principal Components Analysis (PCA) accounted for °75% of the variation in foraging behavior at both sites, and were associated with technique, food site, and height. The rank order of the PCA loadings for all foraging variables on each PCA axis was highly correlated between Purica and Carmel, indicating that the overall pattern of foraging at both sites was very similar despite differences in avifaunal composition, density, and consuming biomass. Analysis of species breadth and overlap during foraging indicated that at both sites foraging specialization does not connote a decreased potential for species interactions. In addition, estimates of community breadth and overlap indicated that technique and food site were the most important foraging parameters in community organization at both sites. Three methods were used to develop the guild classification used in this paper: reciprocal averaging ordination, cluster analysis, and discriminant function analysis. At Purica five guilds were recognized: foliage gleaners, bark gleaners, bark probers, air salliers, and ground salliers. At Carmel four guilds were recognized: foliage gleaners, bark probers, air salliers, and ground salliers. In terms of relative guild consuming biomass (CB), foliage gleaners and bark gleaners at Purica together constitute 34% of the community. Interestingly, at Carmel a distinct bark gleaning guild is absent, and foliage gleaners constitute 32% of the community's CB. The proportion of community CB contributed by the other guilds was significantly different between sites. Although the possible functional roles performed within each community were similar (based on the kinds of guilds present), their guild structures were significantly different (based on the distribution of guild CB within each avifauna). The foraging space used by each of the guilds and the entire community was examined in detail. Foraging height of the birds was generally not related to the tree although both sites park probers derived most of their food from upper heights, foliage gleaners mostly from intermediate to upper heights, and ground salliers perched at intermediate heights. In contrast, air salliers at Purica usually perched at lower heights, while at Carmel they used upper heights most frequently. At Purica 50% of the compensatory shifts in foraging were consistent with predictions of competition—based hypotheses, whereas at Carmel none of the predictions was verified. Interspecific complementarity of resource use was demonstrated only marginally in air salliers at Purica. It is concluded that (1) the foraging behavior of some species at Purica, especially air salliers, is consistent with predictions from competition theory, although a lack of interspecific complementarity indicates that the competitive forces are rather weak, (2) at Carmel, opportunism appears to be more important in affecting the pattern of foraging behaviors, (3) other factors, especially ground cover, shrub density, and the local weather, have influenced foraging behavior and community structure at both sites, (4) similarity between sites in avian community structure is a consequence of the similarity in tree structure, whereas differences between sites probably result from short—term behavioral flexibility in microhabitat use in response to differences in prey distribution and abundance, and (5) interspecific competition is not a prerequisite for resource partitioning.