Effects of human settlement on bird communities in lowland riparian areas of Colorado (USA)

Riparian areas in western North America have been characterized as centers of avian diversity, yet little is known about the ways that native species in streamside habitats are affected by development nearby. To address this issue, we examined patterns of habitat use by birds during the 1995-1997 breeding seasons at 16 lowland riparian sites representing an urban-to-rural gradient. As development increased, riparian woodlands tended to have fewer native trees and shrubs, less ground and shrub cover, higher tree densities, and greater canopy closure. Bird species richness also declined as urbanization increased in the surrounding landscape. Canonical correspondence analysis (CCA) revealed that measures of settlement intensity best explained variation in habitat use by riparian birds, although some residual variation was accounted for by differences in woodland understory features. Migrant and low-nesting species were associated with lower-than-average levels of development, whereas resident and cavity-nesting species tended to increase with urbanization. In partial CCA analyses, however, local habitat variables explained twice the variation that measures of settlement did; nearly half of all explained variation could be attributed to local and landscape variables simultaneously. For avian guilds based on migratory, nesting, and foraging behavior, regression analyses showed that the best variables for explaining patterns of habitat use were usually those that reflected levels of urbanization, particularly at broad scales. When the effects of local habitat variation were removed, however, the best variables for explaining residual variation in habitat use tended to describe development at relatively fine scales, especially for species that nested or foraged low for insects or seeds. These species were also the most sensitive to human trail use. Our analyses indicated that bird communities and local habitat conditions in riparian areas were both affected by development in the surrounding landscape. It may be possible to mitigate the negative impacts of human settlement on native birds in streamside woodlands by maintaining or restoring vegetation structure and composition, and by imposing limits on human recreational activity in these habitats.