Date: 4/29/26 9:10 pm From: 'Tony Leukering' via Colorado Birds <cobirds...> Subject: Re: [cobirds] Mexican Ducks in summer
Owen, Mike, CFO:
Unfortunately, Owen's analysis was conducted in something of a vacuum, and
that vacuum is the more retiring nature of flightless ducks compared to
flighted ducks.
Most flying bird species conduct wing molts slowly to limit the resultant
reduction in flight efficiency. For example, most passerine species (which
have either 9 or 10 effective primaries) will drop the innermost primary
(#1), and once the new feather is half-grown (more or less), it will drop
primary #2. Since each primary takes quite a few days to grow to full
length, an individual's flight efficiency can be reduced for a month or
more... and those species are generally small, with much less feather mass
to replace. The wing molt of larger birds takes much more time to regrow
all that feather mass. Very large birds (such as many raptors) do not
replace all wing flight feathers in a given year, such that a complete wing
molt in such species takes three or more years of molts. (Sidebar: That
wing-molt strategy is why there is only one period of a bird's life of
large birds, such as vultures and eagles, in which all flight feathers are
from the same generation: when they are juveniles.)
*The Waterfowl Strategy*:
In the summer, waterfowl drop all of their flight feathers at once (or
virtually so), which, obviously, renders them flightless. It is thought
that this strategy developed to enable them to have the shortest period of
less-effective flight. Yes, during the molt, they are flightless, but
their flight efficiency is reduced for a shorter period because they can
grow all of their new flight feathers at once.
Before conducting their wing molts, waterfowl move to more protected
situations. Dabbling-duck species generally move to aquatic habitats that
are more protected (protected = more emergent vegetation within which to
hide); many diving-duck species have very different strategies, but this
topic involves a dabbling-duck species, so I'll ignore those other
strategies; they are fascinating, though. Males replace their bright finery
with much drabber plumage. Once they're no longer quite as obvious, they
*then* undergo their wing molt. Once the wing molt is complete (or
virtually so), they then conduct their molt into Alternate plumage: the
males re-attain their fancy plumage.
Coming full circle, it is the dabbling-duck molt strategy that is the
primary reason behind reduced eBird detection rates of dabbling ducks in
summer, including Mexican Duck.
eBird frequency in Colorado of Mexican Duck and Mallard/Mexican Duck (as
noted by Mike):
[image: MEDU-MEDU-slash-MALL-CO.jpg]
Here is the same graph for Mallard:
[image: MALL-CO.jpg]
Here's the same graph for Gadwall and American Wigeon:
[image: GADW-AMWI.jpg]
Finally, the graph for Northern Pintail and Green-winged Teal:
[image: NOPI-GWTE.jpg]
It is obvious from the various graphs that the frequency of eBird detection
of all breeding dabbling-duck species declines severely in the state from
May to June (and that doesn't take into account the possible change in how
and where Colorado's birders bird between those two months). Below, I
present the percentage change for each of the six species of dabbling ducks
using the actual frequency percentage numbers from May to June. I averaged
the four eBird-week frequency values for each month, and the numbers below
represent the percentage change in eBird frequency in Colorado from May to
June, with all species' frequency values declining in June relative to May:
[image: May-to-June frequency change.jpg]
As can be seen in the right-most column, Mallard frequency declined the
least, and the Mexican Duck change was the third LOWEST of the six species.
However, given the very low absolute numbers of Mexican Duck in the state
in May (122) and June (38), the error bar on that change estimate would be
quite large, and that doesn't take into account that most Mexican Duck
reports from Colorado do NOT come from the well-birded northern
urban-corridor counties (Larimer, Boulder, Adams, Jefferson, Denver,
Douglas, Arapahoe); most Mexican Ducks occur in Colorado in the relative
boonies.
[image: MEDU-map.jpg]
That strongly suggests that if western Colorado were more heavily birded,
we would turn up an even larger number of the species. Additionally,
Mexican Ducks in those northern Colorado urban counties are chased by
eBirders much more frequently than those farther afield, thus inflating the
frequency values reported from those counties.
Finally, a little bit of a deviation from the subject: A plea to birders to
quit using the phrase "eclipse plumage." Ducks are no different from all
other Colorado species in conducting a substantial molt as adults after
breeding. There is no reason to call the post-breeding molt of Mallards one
thing and that of Yellow-rumped Warblers... and Spotted Sandpipers,
Bobolinks, Swainson's Hawks, California Gulls, and Lazuli Buntings another
thing. The only real difference is that ducks hold that "Basic Plumage" for
a much shorter portion of the calendar year. But it's the same molt.
Thanks.
Tony Leukering
Currently, Cut Bank, MT
On Monday, April 27, 2026 at 4:11:28 PM UTC-6 Mike Thompson 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
>
> <m......> mobile
>
>
> On Mon, Apr 27, 2026 at 15:48 Owen Robertson <owen.k.r......>
> 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
>> [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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