Positive associations among riparian bird species correspond to elevational changes in plant communities

Abstract

Bird count data were used to characterize patterns of abundance and distribution among 20 bird species occupying streamside habitats of the central Rocky Mountains. Cluster analysis classified bird assemblages from 10 study plots into three elevational zones that varied in bird species diversity. Monotonic declines in total bird densities over the elevational gradient corresponded to spatial fluctuations in population levels of a few numerically dominant species. Of 190 correlations in counts of species pairs, 48 were significant, a much greater proportion than that expected by chance. Only 12 of the 48 associations were negative, suggesting that current competition may be less important than other processes in structuring these communities. Five suites of the positively associating species were detected using cluster, correlational, and variance analyses. Aggregated species responded to habitat ecotones by simultaneously increasing or decreasing in abundance. Group composition was dependent on patterns of species distribution among elevational zones, and on whether species were specialists or generalists in habitat use. Abundances of 19 species were related to five habitat gradients created by principal components analysis of habitat structure. A reasonable explanation for positive covariance in bird abundance is that species responded similarly to limiting resources that were associated with elevational zones.