Changes in bird distributions in Illinois, USA, over the 20th century were driven by use of alternative rather than primary habitats

Most species are distributed such that their density and occupancy is greatest in one habitat, although they are found in other habitats. For example, a species with a high affinity for forests (its primary habitat) may also use urban areas and shrublands (its alternative habitat), although occupancy of these habitats would be lower. While habitat loss is the main conservation threat for most species, less is known about how changes in primary and alternative habitats impact populations. We used a systematic bird survey of the state of Illinois that spanned the past century to investigate how use of specific habitats was related to population changes. Specifically, we used a hierarchical Bayesian model to investigate the relationship between changes in statewide occupancy (probability a species would occur in a study site) and use of specific habitats (probability a species would be in a specific habitat within our study sites) for 66 species sampled in 1906-1909 and 2006-2008. Changes in the use of alternative habitats, and not primary habitats, was related to overall changes in statewide occupancy. Many species that increased over the past century did so by increasing their use of urban areas, while declining species declined the most in agricultural and grassland areas. Although primary habitats form the core of a species' distribution, alternative habitats may provide opportunities for a species to expand its distribution; conversely, declining species may abandon alternative habitats and contract into primary habitats. Consequently, alternative habitats may play an important role in the future of many species. Understanding this role could be crucial for successful conservation.