I'm generally slow to get invested in local identification challenges and only do so if there is an ongoing situation and/or questionable validations in eBird that are brought to my attention. In essence, it is with some hesitation that I bring bad news to Colorado listing, partially echoing what Steve Mlodinow said in a post yesterday. My post is concerning the duck at Cherry Creek recently, as well as the Louisville duck, past Mexican Ducks reported in the Louisville area, and in summation a general caution on IDing female Mexican Ducks.
It looks like reviewers have the current situation at Cherry Creek State Park under control, but I was asked to point out that the purported Mexican Duck there is actually quite pale for a pure bird and more than likely has some Mallard in it. I was told a couple photos that have since been taken down showed flecks of green in the head as well.
On to female ducks. Below are some female ducks confirmed as Mexican Duck from the Louisville area since 2019:
https://ebird.org/checklist/S56765918
https://ebird.org/checklist/S57343772
https://ebird.org/checklist/S61280748
https://ebird.org/checklist/S63807127
https://ebird.org/checklist/S56818169
https://ebird.org/checklist/S56566242
Now, I would like to direct your attention to a quick search of female Mallards, from New York, Virginia, and Washington state. Being recently hatched (retaining juvenile characteristics) will cause dull uncontrasting coloration in the bill, brown tails, and darker than normal throats/heads due to heavier streaking. Older female Mallards also gain dull olive bills as a result of aging and as part of what we refer to as the "intersex" plumage (they take on some male characteristics). Clearly however, these features can also be found in adult females mature enough to raise young, as witnessed below.
https://macaulaylibrary.org/asset/66302751#_ga=2.162978044.1312677016.1613790084-1314321843.1580152271
https://macaulaylibrary.org/asset/113732861#_ga=2.167089278.1312677016.1613790084-1314321843.1580152271
https://macaulaylibrary.org/asset/179080051#_ga=2.167089278.1312677016.1613790084-1314321843.1580152271
https://macaulaylibrary.org/asset/212821981#_ga=2.167632254.1312677016.1613790084-1314321843.1580152271
https://macaulaylibrary.org/asset/254287161#_ga=2.58578410.1312677016.1613790084-1314321843.1580152271
https://macaulaylibrary.org/asset/246278181#_ga=2.58578410.1312677016.1613790084-1314321843.1580152271
https://macaulaylibrary.org/asset/268176821#_ga=2.167663998.1312677016.1613790084-1314321843.1580152271
https://macaulaylibrary.org/asset/59252921#_ga=2.63272303.1312677016.1613790084-1314321843.1580152271
https://macaulaylibrary.org/asset/170046261#_ga=2.192204906.1312677016.1613790084-1314321843.1580152271
https://macaulaylibrary.org/asset/61882151#_ga=2.159176826.1312677016.1613790084-1314321843.1580152271
Finding these examples was easy, and I knew it would be so because I have studied a number of Mallard family groups and fully grown immature birds over the last few summers, and witnessed those kinds of features (brown tail and vent, and dull smudged bill) exist in Mallards well throughout the year. Now, with the very small rate of nicely plumaged male Mexican Ducks that show up in Colorado, I don't think it would make any sense for there to be the rate of weird darkish female ducks that I have seen throughout the year if they were all female Mexican Ducks. I encourage you all to look through female and immature Mallards in eBird's Macaulay library and in the field, to see what I am talking about.
What I'm saying is, none of the birds in the checklists from the Louisville area I linked above show proof of being Mexican Ducks, and likely don't even show evidence of being hybrids. In fact, almost half of those checklists show very average-looking female Mallard rear-ends; pale tails, whitish streaked undertail coverts, and a general lack of dark warm feather edging.
There is much to learn still about the plumage overlap between Mallards and Mexican Ducks, and this topic thus far is very poorly studied, likely as a result of Mexican Duck only being recognized as a full species very recently. But one thing we do know is that there is a lot of hybridization going on, and especially in females, there is a LOT of overlap. The general rule of thumb in ornithology when recording observations of individuals of any species outside their normal range is to definitively rule out a more common, similar looking species by process of finding characteristics outside the variation of that species. With this in mind, for the sake of eBird's data and science, I think we should put IDing female Mexican Ducks in Colorado on hold for a while until we know exactly what to look for in order to eliminate the possibility of a Mallard or hybrid.
Have a wonderful rest of your weekend, and happy birding.
David Andreas Tonnessen
EBIO department, CU Boulder
Explore eBird and iNaturalist, citizen science databases.
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