Drought‐Induced Changes in Avian Community Structure Along a Montane Sere

Abstract

Avain community structure was studied along a meadow—aspen—fir—spruce subalpine sere in northern Utah and southern Idaho during the breeding seasons of 1976, 1977, and 1978. During 1977 one of the worst droughts of this century occurred in the western United States, and comparative changes in avian community structure were documented during and after drought in the four seral stages. The number of breeding species declining most markedly during drought in aspen. Breeding communities on the forested plots during drought were least similar to predrought communities; pre and postdrought communities were consistently most similar. Densities of singing males declined more markedly on all plots during drought than did number of species, and remained below predrought levels in 1978. Changes were also noted in avian trophic structure during drought: insectivores generally declined in number, nectivorous hummingbirds completely disappeared, and the number of granivorous cardueline finch species increased. These changes can be related to changes in food resources. During drought, insect species composition and densities changed, flower phenology was 5 wk early, and one of the best coniferous cone crops in 30 yr occurred in northern Utah. Direct effects of lack of water were observed, and many bird species probably experienced water stress. Although difficult to document, indirect effects of drought on food resources are suggested for insectivores, nectivores, and possibly carnivores. The result indicate that a drought can represent an "ecological crunch" to and affect the structure of, temperature avain communities. However, separating the variation due to drought from inherent variation is difficult without comparative long—term studies. Avian communities in deciduous vegetation may be greatly affected by drought than those in coniferous vegetation.