Influence of forest vegetation structure on avian community composition in west-central Idaho

Locally derived information is needed if management guidelines intended to improve wildlife habitat are to be relevant, reliable, and applicable. Within the Intermountain West, few studies have examined habitat relationships of forest songbirds in mixed-conifer forests. During 1996-1997, we studied breeding bird communities in a mixed-conifer forest in west-central Idaho, USA, to describe how relative avian abundance was related to forest structure. Our study objectives were to 1) describe bird use of 3 distinct forest age-classes, 2) identify groups of species that expressed similar relationships with forest structural attributes, and 3) model species-specific bird-habitat relationships. We sampled 46 study sites (forest stands) classified into 1 of 3 common "vegetation growth stages" (seral stages): 1) shrub/seedling/sapling (n = 15 stands), 2) small tree (n = 16 stands), and 3) old growth (n = 15 stands). In each stand we surveyed breeding birds using fixed-radius point counts and measured forest structure at each point-count location. Of 30 species examined, the relative abundance of 10 differed among vegetation growth stages. Differences in stand structure were reflected by several strong gradients of vegetation structure and composition (e.g., canopy cover, diameter of large trees, snag density, and stump density) across vegetation growth stages. We used cluster analysis to establish novel information about how birds responded to these gradients and identified 3 distinct bird assemblages: forest generalists, early seral specialists, and mid- to late-seral specialists. Habitat models derived from multiple linear regression revealed that canopy cover was the best predictor of variation in abundance for numerous bird species. We conclude that changes in the amount of overstory cover will have the greatest influence on birds. Where canopy cover is reduced below, or increased above, a threshold level of approximately 25-40% cover, we predict that occurrence or density of some breeding birds will change. Our habitat models suggest that Townsend's warbler (Dendroica townsendi), red-breasted nuthatch (Sitta canadensis), and brown creeper (Certhia americana) would respond negatively to reductions in canopy cover; chipping sparrow (Spizella passerina), dusky flycatcher (Empidonax oberholseri), and warbling vireo (Vireo gilvus) would respond positively. This type of information will contribute to regional planning processes by allowing species "trade-offs" to be assessed under different forest management scenarios.