Technical Abstract: We surveyed the genetic diversity of avian influenza virus (AIV) in wild birds, comprising 167 complete viral genomes sampled from 14 bird species in four locations across North America. This revealed 29 combinations of hemagglutinin (HA) and neuraminidase (NA) subtypes, with 26% of isolates showing evidence of mixed subtype infection. Through a phylogenetic analysis of all available AIV genomes, the largest undertaken to date, we were able to document a remarkably high rate of genome reassortment, with no clear pattern of segment association. From this, we propose that AIV in wild birds forms transient ‘genome constellations’, continually reshuffled by reassortment, in contrast to the spread of a limited number of stable genome constellations that characterizes the evolution of mammalian-adapted influenza A viruses. Low pathogenic (LP) influenza A viruses are widely distributed in wild avian species, with asymptomatic infections most common in waterfowl of the orders Anseriformes and Ciconiiformes. Wild birds are thought to represent major natural reservoirs for influenza A viruses and at least 105 species of wild birds, mostly juveniles, have been identified as harboring influenza A viruses. Avian influenza A viruses (AIV) possess antigenically diverse hemagglutinin (HA) and neuraminidase (NA) subtypes, and all known HA (H1-H16) and NA (N1-N9) subtypes, in at least 103 of the possible 144 combinations, have been found in wild birds.