The dynamics of biological populations often appear quite complex, exhibiting considerable year-to-year variation in local abundances. One approach to dealing with ecological complexity is to reduce the system to one or a few species, for which meaningful equations can be written and even solved. Here we explore an alternative approach by studying statistical properties of a vastly larger assemblage comprising over 600 species. Specifically, we quantitatively analyze one of the most comprehensive data sets available: the North American Breeding Bird Survey, which records annual species abundances over a 31-year period at more than 3,000 survey routes. We analyze all the data on an equal footing, and find features common to other, inanimate systems composed of strongly interacting subunits.