We investigated relationships between riparian bird abundance and local vegetation characteristics and habitat features across the Sacramento/San Joaquin Valley, California. Number of detections was analyzed for each of 21 species from point count surveys over a 4-year period at 22 sites from three regions (Sacramento River, Cosumnes River, and San Joaquin River) in relation to 16 measures of habitat and vegetation composition within 50 m of 184 survey points. Tree variables, including tree height and trunk diameter, were often important, as was specific composition of tree species, especially Fremont cottonwood (Populus fremontii) and valley oak (Quercus lobata). Effects of mugwort (Artemisia douglasiana) and blackberry (Rubus spp.) were generally positive. The median partial R2 due to vegetation/habitat characteristics was 16% after controlling for regional differences in abundance per species. Comparisons of model results at the local versus regional scale revealed spatial variation in bird abundance was independent of spatial variation in habitat variables. The effect of a habitat variable differed among the three regions for 11 of 16 variables. Models that used one or more of the first three principal components (extracted from the 16 vegetation and habitat variables) had substantially lower predictive ability than models built using individual variables. The results emphasize the importance of both understory vegetation and tree characteristics at different spatial scales. Local vegetation and habitat characteristics are important in explaining variation in local abundance, but there is a need to develop models specific to each subregion.
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Vol. 120 • No. 4