Presented by Flor Hernandez - Email: email@example.comHybridization rates between the Mexican duck (Anas platyrhynchos diazi) and mallards (A. platyrhynchos) were previously based on plumage characteristics only. However, traits among Mallard-like ducks that appear to be indicative of a hybrid may, in fact, be ancestral. Thus, using phenotypic characters alone may be inaccurate for estimating hybridization rates. Instead, we outline steps and present the first genetically vetted phenotypic scoring key that can be used to accurately identify Mexican ducks and hybrids in the field. We documented 22 plumage characteristics for samples that are also genetically vetted as pure or hybrid for 398 Mexican ducks across their ranges in the US and Mexico, as well as mallards from North America. In short, samples are first genetically assigned as pure (i.e., 95% genetic assignment to one group) Mexican duck, pure mallard or hybrid (i.e., 10% genetic assignment to the interspecific group). Next, we perform a discriminant function analysis to determine which phenotypic characters best distinguish between genetically assigned pure Mexican ducks, pure mallards, and hybrids. Among sex-age classes, we report that first-year male Mexican ducks commonly exhibited mallard-like traits (e.g. green feathers on the head, black rump) not observed in adult males. Moreover, the expression of these traits in first-year male Mexican ducks decreased from their northern to the southern range. This finding may explain early reports of decreasing hybrid prevalence with the same clinal fashion. Additionally, we corroborate earlier reports that female Mexican ducks show subtle plumage traits that make them difficult to distinguish from hybrids. Though juvenile males and females may be problematic to separate from hybrids, the accuracy of the key increases with adult individuals. The developed key is currently being deployed in the southwestern US where scored birds are going to be genotyped as to determine whether further key refinement is necessary.