Abundance and nesting success of acadian flycatchers (Empidonax virescens) at different scales of fragmentation in Illinois, United States of America.

摘要

I examined the effects of three spatial scales of habitat fragmentation on nesting success and abundance of a migratory songbird, the Acadian Flycatcher, in the Midwestern United States. At the local scale, I compared nesting success at varying distances from and amounts of different edges (e.g., rivers, oxbow lakes, backwater swamps, roads, agricultural, and residential lands). At the landscape scale, I compared abundance and nesting success among tracts of different sizes, shapes, and degrees of isolation. Finally, at the regional scale, I related nesting success to percent forest cover, average tract size, and percent forest interior.; Abundances were higher in bottomland than in upland forests, but were unrelated to forest fragmentation. Nesting success was most influenced by local and landscape fragmentation. Predation and parasitism were generally higher in association with anthropogenic edges (agriculture and internal openings) and some natural edges (wide rivers with large canopy gaps), whereas successful nests were surrounded by more natural edges (lakes and ponds). Edge effects were more pronounced in forested landscapes. At the regional level, nesting success was lowest at intermediate levels of forest cover.; I also tested the alternative hypothesis that vegetation structure and composition rather than fragmentation may influence nesting success and populations. I measured nest concealment, complexity and heterogeneity of vegetation in the vicinity of the nest, and the plant communities in which the nests were located. Acadian Flycatcher nesting success increased with increasing nest concealment. Although Acadian Flycatchers preferred to build their nests in certain tree species, tree species was unrelated to nest concealment and nesting success. Most of the other vegetation variables that were measured at the nest patch and habitat type did not influence nesting success. Local and landscape fragmentation effects appear to be the strongest correlates of nesting success of Acadian Flycatchers in Illinois, but other variables such as vegetation structure may account for the poor predictive power of these correlations.