Map showing overlap in breeding relative abundance for mallard and gadwall species. Note that the geographic distribution of gadwall breeding locations is contained almost entirely by areas where mallard breed, with similar areas of high- and low-breeding concentrations across the contiguous United States. The mallard tested positive at some of the highest rates and the gadwall was near the lowest in proportion of AIV positive tests, suggesting geographic overlap alone does not explain variations in species prevalence patterns.
Published in Farnsworth ML , Miller RS , Pedersen K , Lutman MW , Swafford SR , et al. (2012) Environmental and Demographic Determinants of Avian Influenza Viruses in Waterfowl across the Contiguous United States. PLoS ONE 7(3): e32729. doi:10.1371/journal.pone.0032729