Mexican Duck added to AOU Checklist

Tom Edell

Sixty-first Supplement to the American Ornithological Society’s Check-list

of North American Birds

The latest supplement to the AOU now lists Mexican Duck as a full species. This adds the species to the county list (and Morro Bay CBC list) based on an adult male photographed by Curtis Marantz in downtown SLO along San Luis Obispo Creek on 15 Dec 2018 and is the reason for mentioning the county in the account below.  If it happened once, it can happen again, but it should be considered a very rare vagrant to our area and any claims of one should be documented with good photos. There are lots of hybrid Mallards in the county!




Tom Edell

Cayucos, CA



Sixty-first Supplement to the American Ornithological Society’s Check-list of North American Birds

Volume 137, 2020, pp. 1–24


Anas diazi Ridgway. Mexican Duck.


Anas diazi Ridgway, 1886, Auk 3: 332. (San Ysidro, Puebla, Mexico.)


     Habitat.—Freshwater Marshes (0–2500 m).

     Distribution.—Breeds from southeastern Arizona, southern New Mexico, and west-central Texas south in the highlands of Mexico to Jalisco, Michoacán, México, Distrito Federal, Tlaxcala, and Puebla.

     Winters in the breeding range and east to southern Coahuila, San Luis Potosí, and eastern Tamaulipas.

     Nonbreeding birds occur casually throughout the year north through much of Colorado and in Utah north to Great Salt Lake, west to the Lower Colorado River Valley, and east to the Lower Rio Grande Valley. Accidental west to San Luis Obispo County, California, north to Albany County, Wyoming, and east to southwestern Nebraska. Difficulties distinguishing this species from A.  fulvigula may be decreasing detection east of its usual range.

     Notes.—Formerly (e.g., AOU 1983,  1998)  considered conspecific with A.  platyrhynchos, although prior to this (until  AOU 1973) the 2 were treated as separate species. Newly separated based on assortative mating in the narrow contact zone between these species (Bellrose 1976, Hubbard 1977, Brown 1985) and genomic data that indicate restricted gene flow between them (Lavretsky et al. 2015, 2019a).


Anas diazi is treated as a species separate



Tom Edell
Cayucos, CA