Owen - from a review perspective this is a rather risky approach. Youth and exuberance can often confuse laziness with wisdom and caution.
I have a completely different perspective from Montezuma County where MEDU are more abundant in our waterways than anywhere else in Colorado: the dip in the graph reflects careful birders and a solid, accountable review team. Not the other way around. Wise summertime submissions of MEDU/MALL easily fill the dip in the graph.
And you did not mention the most problematic ID issue with local summertime Anas; young male MALL, not eclipse MALL, are typically quite dark and give a strong vibe for the MEDU-complex. Young male MALL are the primary confusion issue with summertime MEDU reports, not eclipse MALL. And both young and eclipse MALL are confusion issues for hybrids. Yes an experienced birder can separate them, but not usually as carefully as needed in a distant scope view.
A message from a review perspective to all observers is to always positively identity what you can, get the best documentation possible, and use extra caution when reporting MEDU and MEDUxMALL during non-breeding season.
Mike Thompson
mt@reardonsteel.us mobile
On Mon, Apr 27, 2026 at 15:48 Owen Robertson <owen.k.robertson@gmail.com> wrote:
Hi CoBirders!
Perhaps there are few more dreaded phenomena in Colorado birding than late summer Anas duck identification. Mallards molt into their "alternate" plumage (it's still termed "alternate", even though the birds are dull) from roughly June-October, and during those months every pond in the state is teeming with brown-bodied, yellow-billed ducks that may or may not contain a smidgen of diazi DNA. I've been hearing about this problem for a while now (looking at you, Ted), and took the opportunity of a freeform statistics final project to do some analysis on the wonderful eBird data that you all have contributed to! I found that, despite no (known) seasonal movements in Mexican Ducks, they are reported almost half as often in late summer than they are elsewhere in the year. This pattern is extremely strong (p < 0.0001), holds true for all four states where MEDU are regular in the ABA (Texas, Arizona, New Mexico, and Colorful Colorado), and is really quite striking (check out the boxplots and line chart at the bottom of this email for a visualization).
Which brings me to the (slightly) chastising part of this email: male and female Mexican Ducks are eminently identifiable year-round - they look dramatically different from any worn or molting Mallard you could imagine. So why are we not finding them? The answer is that birders are lazy! (And yes, I'm including myself in that statement). We see a flock of five dozen Anas drifting through the heat haze at John Martin Reservoir in August and plop them on the eBird list as Mallard, but in December that one chocolate brown male with a yellow bill really pops, and bam - Bent County pulls another MEDU. (By the way, the disparity in reports of male and female Mexican Ducks must be wild - somebody wanna check that out? And I don't even want to think about the hybrids in summer...) Since Mexican Ducks aren't known to undertake seasonal movements, they should be out there, ready and waiting to be found by the first birder (that's you!) to take a closer look. (And if they do turn out to migrate or disperse in some way, more rigorous eBird data could help confirm that!). In sum - Mexican Ducks being overlooked in summer/fall is a real problem for eBird data, and the solution is wonderfully simple: just look for them!
Good birding!
Owen
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