The Mexican Duck (Anas diazi, and see below) is a species of dabbling duck that breeds in Mexico and the southwestern United States. Most of the population is resident, but some northern birds migrate south to Mexico in winter. The species also occurs widely, but in limited numbers, in Colorado in all seasons and there are photographs of birds referable to this taxon from Utah, Wyoming, Nebraska, and Montana.
It is a bird of most wetlands, including ponds and rivers, and usually feeds by dabbling for plant food or grazing. It nests usually on a river bank, but not always particularly near water.
Mexican ducks are fond of the green shoots of alfafa and feed at night on irrigated fields.
Both sexes of this 51–56 cm length bird resemble a female mallard, but with a slightly darker body. The Mexican duck is mainly brown, with a blue speculum edged with white, obvious in flight or at rest. The male has a brighter yellow bill than the female.
The male has a nasal call, whereas the female has the very familiar “quack” commonly associated with ducks.
Including the Mexican duck in the mallard is a relic from the usual practice of much of the mid-late 20th century, when all North American “mallardines” as well as the Hawaiian and Laysan ducks were included in the mallard proper as subspecies. This was based on the assumption that hybridization, producing fertile offspring, is an indicator of lack of speciation.
Rather, in these birds it indicates a fairly recent allopatric radiation, which has not yet established solid barriers against gene flow on the molecular level; mate choice is conferred by cues of behavior and plumage in the mallardine ducks, and this, under natural conditions, has precluded a strong selective pressure towards establishment of genetic incompatibility.
Those photos I shot years ago but I couldn’t make the ID for the same reason that you mention. Finally I did more research and found the solution to the mystery. And days later Ted Jennings was asking for his mystery ducks and I suggested to check the Mexican Duck. I know I have more pictures of them but I have so many. Thank you, Donna. 🙂
H.J. Ruiz - Avian101 
Interesting and informative HJ, I also like your little quote at the bottom of the page, so true 🙂
Thank you so much, Ashley. 🙂
Interesting, HJ! Wow, they do look like a female Mallard. 🙂
Those photos I shot years ago but I couldn’t make the ID for the same reason that you mention. Finally I did more research and found the solution to the mystery. And days later Ted Jennings was asking for his mystery ducks and I suggested to check the Mexican Duck. I know I have more pictures of them but I have so many. Thank you, Donna. 🙂
Another very informative post, H.J.
There certainly isn’t such a pronounced dimorphism between the sexes in this species than in the mallard.
Thank you so much, D. 🙂