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
[image: Screenshot 2026-04-27 at 5.40.42 PM.png][image: Screenshot
2026-04-27 at 5.22.35 PM.png]
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