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.