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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Mike,Thanks for your response. I want to clarify that I’m not in favor of reckless/undocumented Mexican Duck reports - such a program would not improve this gap in data I’m describing. I’m simply saying that in addition to overreporting of MEDU, underreporting (as a result of a lack of observer effort) is perhaps an even greater problem. There will always be Anas ducks that trip us up (the Waneka bird last year is a great example) no matter how well-photographed they are, or how many experienced observers chase them, but such birds will be problematic regardless of season. Conversely, the many well-documented and unimpeachable Mexican Duck records that Colorado possesses should be acceptable as such in June just as much as in January. And similarly, a dubious Mexican Duck in January should not be accepted simply because it is January - Mallard molt timing is variable, and odd plumages can occur at any time of year. I firmly believe that the kind of data gap we see in late summer is fixable - perhaps not to the levels of certainty that we have about MEDU in other times of year, but it is not unimprovable. This improvement, in my opinion, can most strongly begin with observers taking the time to thoroughly check Anas flocks in summer, documenting oddballs, and reporting them - either as MEDU, a hybrid, or as MEDU/MALL (or, perhaps, Anas sp.). If errors are made in identification, they can be corrected, but a lack of effort is not so easy to remedy. If experienced observers can separate MEDU reliably (albeit with care) from odd Mallards, then our goal should be to get birders out there checking duck flocks and learning about the ID, rather than giving up the ghost from the start. Perhaps we’re in total agreement about that, but treating an ID like something that cannot be done, or is reserved for those with decades of experience, is (in my opinion) deeply harmful, both to the Colorado birding community and to the eBird data we’re both striving to improve.
Best,
Owen
On Mon, Apr 27, 2026 at 6:11 PM Mike Thompson <mt@reardonsteel.us> wrote:
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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Mike,
Thanks for your response. I want to clarify that I’m not in favor of reckless/undocumented Mexican Duck reports - such a program would not improve this gap in data I’m describing. I’m simply saying that in addition to overreporting of MEDU, underreporting (as a result of a lack of observer effort) is perhaps an even greater problem. There will always be Anas ducks that trip us up (the Waneka bird last year is a great example) no matter how well-photographed they are, or how many experienced observers chase them, but such birds will be problematic regardless of season. Conversely, the many well-documented and unimpeachable Mexican Duck records that Colorado possesses should be acceptable as such in June just as much as in January. And similarly, a dubious Mexican Duck in January should not be accepted simply because it is January - Mallard molt timing is variable, and odd plumages can occur at any time of year. I firmly believe that the kind of data gap we see in late summer is fixable - perhaps not to the levels of certainty that we have about MEDU in other times of year, but it is not unimprovable. This improvement, in my opinion, can most strongly begin with observers taking the time to thoroughly check Anas flocks in summer, documenting oddballs, and reporting them - either as MEDU, a hybrid, or as MEDU/MALL (or, perhaps, Anas sp.). If errors are made in identification, they can be corrected, but a lack of effort is not so easy to remedy. If experienced observers can separate MEDU reliably (albeit with care) from odd Mallards, then our goal should be to get birders out there checking duck flocks and learning about the ID, rather than giving up the ghost from the start. Perhaps we’re in total agreement about that, but treating an ID like something that cannot be done, or is reserved for those with decades of experience, is (in my opinion) deeply harmful, both to the Colorado birding community and to the eBird data we’re both striving to improve.
Best,
Owen
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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| Route # | Route Name | County Location |
| 17-43 | Gordon Creek | Washington |
| 17-44 | Bethune | Kit Carson |
| 17-55 | Alamosa | Conejos, Alamosa |
| 17-153 | Rio Grand Res | Mineral, Hinsdale |
| 17-214 | Kirk | Kit Carson |
| 17-314 | Burlington | Kit Carson, Yuma |
| 17-321 | Sheridan Lake | Kiowa, Prowers |
| 17-324 | South Fork | Rio Grande |
