Bird communities and habitat as ecological indicators of forest condition in regional monitoring

Ecological indicators for long-term monitoring programs are needed to detect and assess changing environmental conditions. We developed and tested community-level environmental indicators for monitoring forest bird populations and associated habitat. We surveyed 197 sampling plots in loblolly-shortleaf pine forests, spanning an area from Georgia to Virginia (U.S.A.) and representing a gradient in levels of anthropogenic disturbance. Ninety of these plots were randomly selected from a sampling grid, permitting quantitative assessment of cumulative distribution functions for bird community and habitat parameters. Species were independently classified into habitat assemblages indicating birds typical of disturbed habitat (e.g., shrubland, forest edge) and undisturbed habitat (mature forest). Relative abundances of these assemblages were used to form a bird community index-similar to the index of biotic integrity applied to aquatic systems-showing the effects of habitat disturbance on forest bird communities. Bird communities on the majority of the sample area (52-75%, 90% confidence interval) were dominated by disturbance-tolerant species. Sites dominated by mature-forest species were comparatively uncommon. Habitat assemblages appeared to be particularly useful tools for environmental monitoring; individual species abundance was positively correlated with assemblage species richness, and assemblage members showed consistent responses to variations in disturbance level. To a lesser extent, component species of nesting guilds showed this pattern of cohesive responses, but those of foraging guilds did not. We also developed a habitat index based on habitat variables that predicted bird community index values. Habitat and bird community indices were strongly correlated in an independent validation dataset, suggesting that the habitat index can provide a reliable predictor of bird community status. The two indices may be used in combination, with the bird community index providing a direct measure of the status of the bird community and the habitat index providing a basis on which to separate changes in the bird community into local habitat effects versus other factors (e.g., landscape level effects, changes on wintering grounds).