The male, behind, looks like a true diazi, while the female in front looks somewhat intermediate: The orange bill with dark spots is a character of the female of the nominate race, while the dark body is a character of the diazi race.
493 (1 photos) 2010-07-25 Presa La Colorada, El Llano, Aguascalientes (Agu), Mexico
Male. Yellowgreenish bill and dark markings on belly.
492 (1 photos) 2010-08-11 Presa La Colorada, El Llano, Aguascalientes (Agu), Mexico
Female. No dark markings on the bill.
2288 (2 photos) 2016-05-14 Presa La Vega, west of Gdl, Jalisco (Jal), Mexico
Female with orange bill. Most common duck this day, around 30.
2289 (1 photos) 2016-05-14 Presa La Vega, west of Gdl, Jalisco (Jal), Mexico
Males in front and at the back, greenish bills, female in the middle with paler chest and apparently shorter bill?
2290 (1 photos) 2016-05-14 Presa La Vega, west of Gdl, Jalisco (Jal), Mexico
15 birds. Study of flight silhouette. Notice that size depends on the distance and the angle to the camera.
Male with the greenish bill. With Blue-winged Teal and a Long-billed Dowitcher. Interesting optical effect that makes the Blue-winged Teal look like a dwarf compared to the Mexican Duck. Of course there are size differences between the two, but not as much as is indicated in the first picture. This effect often seems to happen when one bird is standing out of water with the legs and the other is deep in.
3258 (3 photos) 2019-11-24 Bosque de Chapultepec, Ciudad de México (Cdmx), Mexico
Female with orange bill. The dark markings on the bill are variable. Even in the overexposed photo one can appreciate the strong contrast between the color of the body and the neck.
Mallard is rare this far south in Mexico, but an occasional park bird could be causing identification problems, but on the is bird the narrow white trailing edge to the speculum is a good indication of a pure Mexican Duck. And the amount of white above the speculum is a good guarantee that we are not talking about a vagrant Mottled Duck.
3259 (3 photos) 2019-11-24 Bosque de Chapultepec, Ciudad de México (Cdmx), Mexico
Male with the yellow-green bill.
3260 (1 photos) 2019-11-24 Bosque de Chapultepec, Ciudad de México (Cdmx), Mexico
Male showing all the good field marks: Bill color, body-neck contrast and right amount of white in front and behind speculum.