Bird Communities and Forest Structure in the Sierra Nevada of California

I examined the relationship of forest structure and vegetation diversity to bird communities in the mixed conifer and red fir zones of the central Sierra Nevada. Bird populations were censused in open- and closed-canopy forests in both vegetation zones from June through September 1974 to test the prediction that structurally similar forests support similar avifaunas. Comparisons of bird and vegetation data revealed that forest canopy cover was a primary factor influencing the size and composition of avian communities in the nesting and post-nesting seasons. Closed-canopy forests had lower bird densities, diversities, species richness and consuming biomasses than open-canopy forests in both vegetation zones. The composition of feeding guilds was similar in the two canopy types, but ground-understory foragers, hummingbirds and flycatchers were less abundant in the closed forests. When analyzed by dominance-diversity curves, bird communities in these conifer habitats generated geometric curves in the closed forests and lognormal curves in the open areas. Lognormal curves indicate more bird species of intermediate abundance. The open forests had well-developed understories and higher foliage-height diversities offering a greater array of foraging substrates for birds.