The distribution of body sizes within a community can lend insight into how resources are partitioned among co-existing species, particularly when evaluated at the individual rather than the species level. Individual size distributions (ISDs) have been studied extensively in aquatic and tree communities, but rarely so in terrestrial animal communities. Here we use a macroecological approach to characterize the shapes of ISDs from thousands of bird communities, using recent data from the Breeding Bird Survey and Christmas Bird Count. We also aggregate these local data at increasingly large scales to compare the shapes of ISDs across scales, using bird conservation regions, biomes, and the entire continent.
All local distributions were found to be highly multimodal, with most characterized by 6 to 11 modes, in marked contrast to the simpler distributions found previously for tree communities or for bird communities analyzed at the species-level at large spatial scales. Multimodality remains a consistent feature of the distributions across all spatial scales, including the continental scale. The number of modes identified in the distributions is not systematically related to the number of individuals or the number of species found at each site. Species richness varies widely across modes, with a given mode comprised of anywhere from one to more than 30 species. Similarly, modes typically contain individuals from multiple foraging guilds, with most modes comprised of three distinct guilds. Finally, preliminary analyses suggest remarkable consistency in the position of modes along the body size axis across sites. Future efforts will include detailed study of taxonomic and geographic patterns in mode positions and magnitudes, as well as modeling, to elucidate the key mechanisms underlying the interactions among body size, abundance, and diversity.