Hi Owen,
Just a few nitpicks about your otherwise engaging posting.
First, regarding the data used to illustrate interstate comparison....are these the total number of individual human observations of confirmed MEDU in each state, or the total number of instances of confirmed individual MEDU in each state? I'm assuming the latter, in which case it's probably also true that in Colorado at least the vast majority of initial identifications of a persisting confirmed MEDU are made by very experienced observers. Or, in the few cases where the observer is less experienced, it's quickly confirmed by a very experienced observer or reviewer, and then chased and photographed by less experienced observers in order to gain more experience for the next rare occasion when a potential MEDU is located. How do I know this? Well it's basically my own story. I've had to learn as I go in tiny incremental steps on how to identify MEDU in the field, which also requires knowing how to identify hybrids, but I'll get to that more in a moment.
Second, it's worth noting that although the state box plots are all very comparable-looking in terms of proportions of confirmed MEDU by season, the y-axis is indeed dramatically different, and should not be overlooked. CO MEDU number no more than 10% of the next least populous MEDU state (NM) in these graphs, despite the significantly larger human population (and birding population) of CO compared to NM. I would be careful about extrapolating too much information about what MEDU movements are (or aren't) in CO based on such small number statistics. Especially since CO is the only state in this comparison where the species is out of range.
My last nitpick had to do characterizing MEDU ID as simple given how "dramatically" different they are from MALL. Well, that type of hyperbole understates the difficulty of the problem, and seeing as how ABA articles have had to be written by esteemed Colorado birders (Mlodinow, Leukering, Bushong) on how to identify MEDU, calling the process of separating the MALL complex comparable to separating Northern Shovelers from American Wigeons feels a bit denigrating to those of us who have been struggling for years to do things correctly. Does the process of MALL/MEDU/MALLxMEDU separation start to get easier and more straightforward the more you observe and practice it? Certainly, and perhaps even to a point where an experienced observer does feel as if the visual difference between these birds is to them dramatic and obvious. But learning this is still a process for the community as a whole and only keeps moving when we help each other continually with how to separate these birds. I would argue that given the already very low numbers of MEDU likely present in the state at any given time, and given my previous argument that the MEDU-observers with ID skill are already catching the majority of MEDU likely to be found anyway, it probably serves the community better at this point to just assist them with improving their ID skills instead of criticizing their past efforts. Efforts which, as Mike Thompson aptly pointed out, are already being reviewed by a solid and accountable review team. For my part, I want to continue improving my own recognition of the field-identifiable traits, and to improve communication of that information to the birding community. The more confident birders can feel when looking over a distant MALL flock, the more time they will actually spend doing so. It has less to do with laziness and much more to do with encouragement. I also know this from experience. :)
Eric
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Eric DeFonso
Boulder County, CO
Boulder County, CO
On Mon, Apr 27, 2026 at 3:48 PM 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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