Mexican Duck is the southernmost member in the New World of a suite of species in the Mallard (Anas platyrhynchos) complex. Mallard is sexually dimorphic, but in Mexican Duck and related species (such as American Black Duck Anas rubripes and Mottled Duck Anas fulvigula) the sexes are similar in plumage (although differing in more subtle ways, such as in bill color). Mexican Duck occurs from the southwestern United States south to central Mexico. Its breeding distribution probably was allopatric or parapatric to Mallard until recently, but the two have come into contact in Arizona and New Mexico, where they hybridize. On this basis, some authorities have classified Mexican Duck as only a subspecies of Mallard, although genetic evidence suggests that Mexican Duck may be more closely related to Mottled Duck than it is to Mallard. It was feared, in the late 20th century, that Mexican Duck was endangered in the United States, but in recent years its populations have rebounded, and in fact it may be expanding its range in Mexico, by taking advantage of irrigated farmlands.