Avian and Vegetation Community Structure and Their Seasonal Relationships in the Lower Colorado River Valley

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Abstract
Records of vegetation density and seasonal bird species richness, densities and diversities for 78 line transects along the lower Colorado River were examined to: isolate seasonal and spatial patterns of avian community variation; determine relationships between patterns of attributes in avian communities and vegetation characteristics; and evaluate effects of different spatial levels of investigation. Principal components analysis of foliage density and diversity measures revealed 2 important, independent patterns of variation among transects. Avian density was usually independent of, and more regularly arranged than, diversity within each season. Regression of principal components, derived from principal components analysis of avian community attributes on vegetation components, showed that avian density was more closely related to variation in the vegetation than was avian diversity. Avian community attributes outside the breeding season were more highly correlated with the vegetation measures than were those for communities during the breeding season. Vegetation density and diversity were both important predictors of avian community measures at the habitat level but accounted for little of the variation in bird communities at the transect level. Additional principal components analyses using the combined avian and vegetation data evaluated at the habitat level produced results similar to those found at the transect level. When vegetation and avian community variables were analyzed together, results were similar to previous findings. The additional studies reinforced the findings of the 1st analyses, although when evaluating habitat data, avian and vegetation variables were more closely associated than when the analysis was conducted using a more local spatial scale. Several generalizations were possible. For example, common summer visitors in mature cottonwood-willow (Populus fremontii-Salix gooddingi) habitats differed ecologically from species visiting structurally simpler cottonwood-willow habitats in summer. The general similarities among habitats when considering resident avian species contrasted with the differences when species present only part of the year were considered.