We characterized past avian cholera outbreaks in waterbirds at Hayward Marsh, California, US. In 2013, we surveyed populations and determined the presence of disease using several diagnostic methods, including behavioral and physical observations, field necropsy, and bacterial culture. We compiled this information with data from previous outbreaks from 1990–2012 to compare waterbird abundance to various measures of mortality, including percentage of mortality and percentage of difference between abundance and mortality by species. We suggest that Ruddy Duck (Oxyura jamaicensis) have consistently suffered greater mortality from this disease than have other species at this site.