Bird communities of the Colorado Rocky Mountains along a gradient of exurban development

Most insights about the effects of residential development on wildlife are based on research from urban and suburban areas. Yet exurban development is an increasingly prevalent form of human settlement in many parts of the world. Moreover because such development often occurs near the periphery of protected areas, it may have a disproportionate influence on native species. We examined bird community patterns in the Rocky Mountains of north-central Colorado across a gradient of exurban development to determine how avifauna responded to this form of settlement. Using fixed-radius point counts, we surveyed the occurrence and abundance of breeding birds in 11 developed sites and in 6 nearby undeveloped sites, all occurring in a forested matrix. We sampled ground-level habitat features at each survey point and derived digital land-cover maps from aerial photographs to characterize and quantify road development and building density. We found little evidence that land-cover varied with development intensity, yet bird abundance increased significantly with building density. Patterns of species richness were equivocal due to a highly influential observation. Principal components analysis (PCA) distinguished two axes that explained nearly 50% of the variation in the bird community and were strongly associated with road and building density. Weighted average analysis showed a marked decline in the abundance and richness of species that were insectivorous or nested in snags, suggesting that reductions in dead wood may have influenced the bird community. However, no conclusions could be drawn because dead wood was confounded with building density. Logistic regression of species occurrence and building density indicated that the incidence of some generalist species increased with building density, whereas the incidence of specialists decreased. Overall, our results suggest that development at low densities can favor habitat generalists. Similar patterns have been observed in more urbanized areas. If exurban development persists as a popular form of settlement in the Rocky Mountains, a regional plan for protecting avian habitat and limiting development dispersion will be necessary to maintain native bird communities.