Macroecology of avian size distributions

Distributions of individual body size within communities have attracted substantial attention in aquatic ecosystems, and forest biologists have long been interested in distributions of tree size. In contrast, individual size distributions have been largely ignored in terrestrial animal communities. We use data from the Breeding Bird Survey of North America to explore local scale and macroecological patterns in avian size distributions. In contrast to aquatic and tree size distributions, avian size distributions tend to be complex, exhibiting multimodal distributions within sites and substantial variation in distribution form across sites. Despite this complexity, regular patterns emerge when looking at changes in the shape of the distribution across North America. Simple moments of the distribution (e.g., mean, variance, etc.) are correlated with environmental variables related to habitat structure. In addition, distributional overlap exhibits distance decay with a lower bound that is well characterized by a logarithmic function. This decay exists for geographic, environmental, and compositional distance between sites. These preliminary results suggest that avian size distributions result from a combination of environmental determinants (related to habitat structure) and the specifics of community assembly.