The Distribution of an Avian Guild along a Temperate Elevational Gradient: The Importance and Expression of Competition

Abstract

A guild of ground—foraging, insectivorous birds which seasonally occupy montane temperate forests was investigated. By examining the roles of interspecific competition and specific habitat selection, I have dissected the factors controlling the distribution patterns of the guild members along extensive elevational gradients. The guild was composed of Hylocichla mustelina (Wood Thrush), Catharus fuscescens (Veery), C. guttatus (Hermit thrush), C. ustulatus (Swainson's Thrush), and C. minimus (Gray—cheeked Thrush). The five thrush species are sympatric on high mountains in the northeastern United States, but as one proceeds southward down the Appalachian Mountain chain, the species drop out one at a time. Anecdotal information indicated that the species showed elevational amptitude expansion as members of the guild dropped out with decreasing latitude. As a consequence of the opportunity for ecological release in southern latitudes, the thrush guild was ideally suited for testing the role of competition in determining distributional limits of the species when sympatric. I studied the behavioral interactions and habitat selection of the five species in sympatry on high mountains in the northeastern United States. The species have distinct distributions along these elevational gradients, but large spatial overlaps between adjacent species were observed. To test for significant differences in habitat selection, the breeding territories of these species were quantified by measuring 55 structural habitat variables. Using stepwise discriminant function analysis, I detected statistically significant differences in the species' breeding habitats. By employing a subset of 9 of the initial 55 variables, over 77% of the Northeastern habitat quantifications were assigned to the correct species group. Additional information extracted from the structural niche analysis indicated the habitat niche breadth of each species and the degree of habitat overlap between adjacent species, and provided insight into the limiting similarity allowed between these coexisting species. Song playback was used to test the contribution of behavioral interactions to the observed distribution patterns. All species showed greater response to conspecific song. The results indicated that over interspecific behavioral interactions contributed little or nothing to the observed distributional and specific habitat selection patterns of these species. To test the contribution of interspecific competition to the habitat selection patterns detected on Northeastern mountains, a subset of the five—member guild was studied on Southeastern mountains. In the Smoky Mountain range only H. mustrelina and C. fuscescens breed. By employing identical quantification techniques, I tested for expansion by these two species into habitat types not occupied when sympatric with the other three species. Evidence of extensive ecological release was obtained, implying a strong role for interspecific competition in Northeastern mountains. Thus, my results further implicate interspecific competition as an integral force contributing to the organization of temperate bird communities.