|Mexican Duck, Anas (platyrhynchos) diazi, at First Dam, Logan, Utah. Copyright Ryan P. O'Donnell.|
First, what is a Mexican Duck? That question is not easy to answer. According to the American Birding Association and the American Ornithologists Union, it is a subspecies of Mallard. But that view is considered by some to be antiquated and inaccurate. Recent genetic work has shown that the Mexican Duck may actually be its own species, and it is at least as unique as several other species such as the Laysan Duck and the Mottled Duck. The picture is complicated, however, because Mallards contain several genetic lineages, and thus to have species boundaries that reflect mitochondrial gene phylogenies, one would either have to split Mallards based on strictly genetics, or else consider "Mallards" to include Laysan Ducks, American Black Ducks, Mottled Ducks, and many others, which obviously does not reflect biology well. This is an area that is in need of additional research, particularly in generating phylogenies and assessing hybridization with nuclear molecular markers.
The definition of species within the Mallard group is confusing, but perhaps even more so in the case of Mexican Ducks, because our understanding of this species is clouded by hybridization with Mallards in Arizona and New Mexico. A classic study of morphology in Mexican Ducks found that there are virtually no pure Mexican Ducks anywhere in the species range, according to a numerical scale of morphology that ranges from pure Mexican Duck to pure Mallard. However, an alternative interpretation of the same dataset is that our definition of what identifies a pure Mexican Duck is too narrow, and that pure Mexican Ducks can show traits that have once been taken to be indicative of Mallards.
With respect to the duck seen as recently as yesterday in Logan, this appears about as close to a pure Mexican Duck as one can expect at the northern part of the range of the (sub)species. There is very little green on the head. The bill is bright yellow. The tail shows no patches of white. The speculum has green iridescence, and is bordered only thinly by white. The rump and undertail coverts match the flanks well in color, showing no obvious indication of the black that a male Mallard has in these areas. The belly is dark, matching the color of the rest of the bird well. The only part that seems to show some obvious Mallard ancestry is that the central retrices (tail feathers) curl up slightly off the plane of the tail, hinting at the curled central tail feathers of an adult male Mallard. But, with how little we know of "pure" Mexican Ducks, perhaps this is not outside the range of variation shown by them? Only an extensive study of morphological variation and nuclear DNA across the range of the Mexican Duck and Mallard can really address this question well.
|The Mexican Duck looks a lot like a female Mallard, but darker and (in a male, such as this one) with a bright yellow bill. Copyright Ryan P. O'Donnell.|
|Unlike a Mallard, the lower belly is not noticeably paler than the flanks or breast. The bright white underwing coverts are a trait shared by male Mexican Ducks and male Mallards. Copyright Ryan P. O'Donnell.|
feeding park ducks, it is a common practice at this park, and if you happen to time your visit when a local is feeding them, this duck might come right out in the open and fight with the other local domestic breeds for bread. Otherwise, you might get lucky and see it swimming around on the water, or it might be sleeping on the far shore of the lake. If it's not out in the open when you get there, try patiently scanning the sleeping ducks along the shoreline.