The role of urban and agricultural areas during avian migration: an assessment of within-year temporal turnover
Migration is often identified as the most vulnerable period in the annual cycle for birds, and land-use change is likely to have altered how avian populations are regulated during migration events. However, the consequences of land-use change for avian diversity are typically assessed based on annual surveys of breeding communities with little consideration given to migration or other phases of the annual cycle.
Forty-four North American ecoregions.
We use eBird avian occurrence data to estimate, at a monthly temporal resolution for the combined period 2004 to 2013, how species richness and temporal turnover in species composition is structured within years across a land-use gradient (intact vegetation, agricultural and urban).
Species richness peaked on average during spring and autumn migrations. Intact vegetation had the highest and urban areas the lowest species richness on average. Despite differences in community size, the three land-use categories had similar patterns of within-year temporal turnover, suggesting analogous effects of geographic diffusion by migrating species. Agricultural and urban areas had comparatively dampened temporal turnover across the annual cycle, suggesting more homogeneous within-year species compositions. Relative to eastern ecoregions, differences in species richness and temporal turnover among land-use categories were substantially more pronounced in western ecoregions.
Agricultural and urban areas have lower species richness and reduced within-year temporal turnover across the annual cycle relative to areas of intact vegetation, particularly in the west. Our findings suggest that avian diversity has been simplified across the annual cycle through the influence of human activities, with human-transformed landscapes maintaining a degree of relevance for migratory birds, especially in the east.