Discussion
Our results indicate that the three-dimensional structure of forests is an independent axis, along with precipitation and temperature, which help shape patterns of species richness and functional diversity for avian communities across spatial scales. In addition, forest structure varies significantly across forest ecosystems of North America, and this structural variation in forest structure is just as important, if not more important, in comparison to temperature and precipitation for organizing bird communities. The inclusion of habitat structure with temperature and precipitation also allows for the harmonization of leading ecological theories (habitat productivity, habitat heterogeneity) that aim to predict the species richness and functional diversity of communities of organisms.