Now that Mexican Duck has regained its proper full-species status (in most realms), I am guessing that M ducks (Mexican, Mottled, Mallard and their hybrids -- aka M&M Ducks) will be receiving more attention. I believe that Tony Leukering is working on a Mexican Duck article for Birding Magazine, but I thought a few comments might be worthwhile in the meantime.
1- Status and Distribution in CO
Apparent distribution of Mexican Duck and in CO has a strong observation bias. In other words, they are found where the birders are, or more accurately, where birders who have an interest in finding them are. During most of my tenure in CO (2011 on), those people have been Cathy Sheeter, Ted Floyd, and me. So, records in eBird Mexican Duck records are concentrated in Weld and Boulder (there is a similar concentration of Mexican Duck records in UT around Salt Lake City)
However, my experience leads me to believe that Mexican Duck is probably most numerous in sw. Colorado/ San Luis Valley
I also wonder if there were more Mexican Ducks during my earlier years in CO, when severe drought conditions reigned in sw USA, perhaps driving some birds n. in search of water. It seems that I found far more in my Weld stomping grounds from 2011 to 2014/2015 than thereafter.
2- ID of Females. Separating Mexican Duck females from Mallards is exceptionally difficult. Telling Mexican Ducks from M&M ducks (Mexican x Mallard, Mottled x Mallard and Mexican x Mottled) requires a very good look at a dark-end female; I think that, by-and-large, only those familiar with female Mexican Ducks within their range can make this identification. During my winter visits to AZ, I probably ID 10 male Mexicans for every female. Females are told from Mexican x Mallard, in no small part, by their all brown tails. Note, though, the very young Mallards (mostly July-August) can have all dark tails, or tails with a bit of white on tail edge (which Mexican Duck can have) -- see https://www.flickr.com/photos/36088296@N08/20634269773 and https://www.flickr.com/photos/36088296@N08/9218634367 (though the latter bird might be a Mexican x Mallard)
For photos of apparent "pure" female Mexican Duck from CO see https://www.flickr.com/photos/36088296@N08/8674584022
https://www.flickr.com/photos/36088296@N08/8758111200 looks to be a female Mallard x Mexican Duck
3- ID of Males. I won't/can't get into the details at this time, but do note that one need consider all of the M&M duck combinations. So, not only look for Mallard characters, but also for Mottled Duck characters (black at gape of bill, narrow borders to wing speculum, buffy unstreaked chin...) when attempting to ID a Mexican Duck. Mottled x Mexican Duck and Mottled x Mallard should be rarer than pure Mexican Duck in CO, but more likely than Mottled Duck. For every Mexican Duck that I've identified in CO, I've labelled (perhaps errantly, see below) 2-3 ducks Mexican x Mallard and left another half-dozen as ???.
Some of the males ducks that I've identified as Mexican x Mallard are probably Mallard.
Why?
Work by Andy Engilis and colleagues demonstrates that first-year male Mexican Ducks can show a hint of Mallard traits (the explanation for this is fascinating, but I'll leave it for another time). This explains why so many of the male ducks I thought where Mexican x Mallard were so very similar to Mexican Ducks, rather than truly intermediate. It appears that first year male Mexican Ducks can have a touch of curl to the central tail-feathers, a smidgen of green on head, etc. This issue should be further clarified in the next couple of years.
https://www.flickr.com/photos/36088296@N08/30344793444 is an excellent example of a duck that looks as if it has a bit of Mallard in it but is probably a pure Mexican Duck (first year).
Also, see https://www.flickr.com/photos/36088296@N08/17019286745 (and the 3 or 4 photos that follow) for an apparent Mottled Duck x Mexican Duck from CO
Good Birding
Steve Mlodinow
Longmont CO
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