[cobirds] Dinosaur Ridge - Denver Field Ornithologists (30 Apr 2026) 6 Raptors

Dinosaur Ridge - Denver Field Ornithologists
Golden, Colorado, USA

Daily Raptor Counts: Apr 30, 2026
Species	Day's Count	Month Total	Season Total
Black Vulture	0	0	0
Turkey Vulture	0	135	162
Osprey	1	58	64
Bald Eagle	0	14	27
Northern Harrier	0	38	50
Sharp-shinned Hawk	3	174	195
Cooper's Hawk	1	197	249
American Goshawk	0	1	2
Red-shouldered Hawk	0	0	0
Broad-winged Hawk	0	80	80
Red-tailed Hawk	0	214	462
Rough-legged Hawk	0	0	0
Swainson's Hawk	0	52	52
Ferruginous Hawk	0	8	17
Golden Eagle	0	1	8
American Kestrel	0	557	696
Merlin	0	4	7
Peregrine Falcon	1	8	10
Prairie Falcon	0	2	4
Mississippi Kite	0	0	0
Unknown Accipitrine	0	16	22
Unknown Buteo	0	2	5
Unknown Falcon	0	4	5
Unknown Eagle	0	0	0
Unknown Raptor	0	4	5
Total:	6	1569	2122

Observation start time:	09:15:00
Observation end time:	13:30:00
Total observation time:	4.25 hours
Official Counter	Soren Zappia
Observers:

Visitors:
No visitors today. We hope for sunnier skies the next few days! Thank you to Ryan Gannon for volunteering today.

Weather:
Initially, the weather looked like a full cancellation, but as the radar updated to look clear we decided to start the count. Winds from the east and NE appeared to slow the forecasted system arriving from the west. The ridge started clear of precipitation with fog visible to the south. Skies were overcast and temperatures were cool in the low 40s F. The winds were moderate from the northeast but shifted due east after a few hours. The fog intensified, limiting visibility to the south for about an hour, but then cleared again. After a few hours, the promised precipitation was visible to the west, the wind calmed, and the system approached rapidly until visibility was limited and a snow/rain mix was falling. The count was ended at 13:30 MST due to poor visibility and precipitation.

Raptor Observations:
Despite the impending weather, we had a few birds today and an enjoyable flight line. Most birds were in the first two hours at near eye level just off the east side of the ridge. First, an Osprey flew past coming first from below. After a few minutes, an adult Sharp-shinned came past so close that it seemed like it would hit us! Our final bird seen on this path was an adult Peregrine Falcon, which took a second to look at us before continuing past. Besides the close birds, we had a distant Sharp-shinned Hawk to the west, and a Juvenile Cooper's Hawk between the ridge and the west side. In the remaining 2.5 hours, we counted one distant Sharp-shinned Hawk on the west. Today brings our April count to a close! We counted 1569 birds this month, bringing our current season total to 2122. We are excited to see what May will bring.

Non-raptor Observations:
White-throated Swift 1, Broad-tailed Hummingbird 1, Woodhouse's Scrub Jay 1, Black-billed Magpie 3, Common Raven 3, Rock Wren 1, Pine Siskin 1, Spotted Towhee 3, Western Meadowlark 2, Brewer's Blackbird 1

Predictions:
Tomorrow, partly cloudy to mostly sunny skies are forecasted with winds from the NE at 5-10 MPH. Expect cool temperatures in the mid 40s-50s F.

---

Report submitted by Official Counter of the day shown above (dinoridgehw@gmail.com)
Dinosaur Ridge - Denver Field Ornithologists information may be found at: www.dinosaurridgehawkwatch.org
More information at hawkcount.org: [Site Profile] [Day Summary] [Month Summary]
Count data submitted via Trektellen.org - [Project Details]

---

Site Description
Dinosaur Ridge is the only regularly staffed hawk watch in Colorado and is the
best place in the world to see migrating Ferruginous Hawks. Hawk watchers may
see 17 species of migrating raptors; and it is an excellent site to see rare
dark morph buteos including Broad-winged hawk, Swainson's hawk, Ferruginous
hawk, Rough-legged hawk and Red-tailed Hawk. Other raptors we see include Golden
and Bald Eagles, Northern harrier, Osprey, Peregrine Falcons, Prairie Falcons,
Cooper's and Sharp-shinned Hawks, American Kestrels, Merlin, and Turkey
Vultures. American Goshawk is uncommon but also counted each season. Non-raptor
species include Rock Wren, Bushtit, Western Bluebird, Sandhill Crane,
White-throated Swift, and American White Pelican. Birders of any skill level are
always welcome. The hawk watch at Dinosaur Ridge is staffed by Hawk Counter(s)
and volunteers from March through early May.

Directions to site:
From exit 259 on I-70 towards Morrison, drive south under freeway and take left
into first parking lot, the Stegosaurus lot. Follow hawk watch signs from the
southwest end of the parking lot to the hawk watch site. The hike starts heading
east on an old two-track and quickly turns south onto a trail on the west side
of the ridge. When the trail nears the top of the ridge, turn left, and walk to
the flat area at the crest of the ridge. (Distance: 0.56 miles, Elevation gain:
259 feet)

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Re: [cobirds] Mexican Ducks in summer

[cobirds] calloo callay, an oriole today: west of Lagerman Res

I suspect many other early risers are hearing them as well. We've never heard this returnee this early in the season. I need to get his favorite feeder out fast. Linda central Boulder Cnty 2 mi west of Lagerman -- -- You received this message because you are subscribed to the Google Groups "Colorado Birds" group. To post to this group, send email to cobirds@googlegroups.com For more options, visit this group at http://groups.google.com/group/cobirds * All posts should be signed with the poster's full name and city. Include bird species and location in the subject line when appropriate. * Join Colorado Field Ornithologists https://cobirds.org/membership/ --- You received this message because you are subscribed to the Google Groups "Colorado Birds" group. To unsubscribe from this group and stop receiving emails from it, send an email to cobirds+unsubscribe@googlegroups.com. To view this discussion visit https://groups.google.com/d/msgid/cobirds/5BEC57FB-F81E-497D-A159-053AAA2C9E5F%40comcast.net.

[cobirds] CO Field Ornithologists Convention Registration Ends May 7th!

Have you been thinking of attending the Colorado Field Ornithologists convention, based in Loveland CO, but haven't registered yet? Well folks, the registration deadline of May 7th is coming up, so don't delay.

This convention, which takes place May 28th - 31st, offers field trip adventures with fellow birders, a robust science session, a silent auction, and an informative Saturday night banquet and keynote by Marion Clément from Bird Conservancy of the Rockies on the topic of "The Charm and Mystery of Owls and Their Power to Move Us to Action."

But wait, there's more! Our lively Exhibit Hall will have a dozen exhibitors who offer products and services geared toward birders.

Join us for one day or for the whole convention, birders of all levels are welcome. We can't wait to see you!

For more information and to register visit: https://cobirds.org/current-convention/

Linda Lee

Louisville

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Re: [cobirds] Mexican Ducks in summer

Thanks Tony,

One other question. Where do the Mexican ducks in Colorado go to mate.  Do they go back to typical breeding range, or do they just mate with mallards (or something else) and are lost to the MEDU population?

Tom

On Thu, Apr 30, 2026 at 5:45 AM Mike Thompson <mt@reardonsteel.us> wrote:

I’ll add just one more thought to this topic before I crawl back into my digital cave…

The presence of seasonal vegetation providing cover and shade for masses of dabblers is a very significant contributing factor towards lower frequency of reports. 

Mike 

mt@reardonsteel.us mobile

On Apr 29, 2026, at 22:24, Mike Thompson <mt@reardonsteel.us> wrote:



That's all very excellent information and analysis Tony

But I have to point out that most ducks that make it on my lists, regardless of the season, are not in flight. Stationary, flightless MEDU are easier to document than those flying away. This presumably would render their flightlessness less of a factor in the frequency analysis than their seasonal dispersal patterns, high-angle light conditions, etc.

Noted re "eclipse"!

Mike

On Wed, Apr 29, 2026 at 10:10 PM 'Tony Leukering' via Colorado Birds <cobirds@googlegroups.com> wrote:

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): 

Here is the same graph for Mallard:

Here's the same graph for Gadwall and American Wigeon:

Finally, the graph for Northern Pintail and Green-winged Teal:

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:

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.

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...@reardonsteel.us mobile

On Mon, Apr 27, 2026 at 15:48 Owen Robertson <owen.k.r...@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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Re: [cobirds] Mexican Ducks in summer

No one knows. As far as I know, there are no suggestions (other than courtship) of the species breeding in the state.

And I meant to mention this in my original post: Kudos to Owen for doing the initial research and pointing it out in this venue.

Tony

Tony Leukering

Denver, CO

eBird blog

eBird commentary

Photos

On Thursday, April 30, 2026 at 06:21:05 AM MDT, tom none <jtcurt325@gmail.com> wrote:

Thanks Tony,

One other question. Where do the Mexican ducks in Colorado go to mate.  Do they go back to typical breeding range, or do they just mate with mallards (or something else) and are lost to the MEDU population?

Tom

On Thu, Apr 30, 2026 at 5:45 AM Mike Thompson <mt@reardonsteel.us> wrote:

I’ll add just one more thought to this topic before I crawl back into my digital cave…

The presence of seasonal vegetation providing cover and shade for masses of dabblers is a very significant contributing factor towards lower frequency of reports. 

Mike 

mt@reardonsteel.us mobile

On Apr 29, 2026, at 22:24, Mike Thompson <mt@reardonsteel.us> wrote:



That's all very excellent information and analysis Tony

But I have to point out that most ducks that make it on my lists, regardless of the season, are not in flight. Stationary, flightless MEDU are easier to document than those flying away. This presumably would render their flightlessness less of a factor in the frequency analysis than their seasonal dispersal patterns, high-angle light conditions, etc.

Noted re "eclipse"!

Mike

On Wed, Apr 29, 2026 at 10:10 PM 'Tony Leukering' via Colorado Birds <cobirds@googlegroups.com> wrote:

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): 

Here is the same graph for Mallard:

Here's the same graph for Gadwall and American Wigeon:

Finally, the graph for Northern Pintail and Green-winged Teal:

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:

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.

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...@reardonsteel.us mobile

On Mon, Apr 27, 2026 at 15:48 Owen Robertson <owen.k.r...@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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Re: [cobirds] Mexican Ducks in summer

Re: [cobirds] Mexican Ducks in summer

That's all very excellent information and analysis Tony

But I have to point out that most ducks that make it on my lists, regardless of the season, are not in flight. Stationary, flightless MEDU are easier to document than those flying away. This presumably would render their flightlessness less of a factor in the frequency analysis than their seasonal dispersal patterns, high-angle light conditions, etc.

Noted re "eclipse"!

Mike

On Wed, Apr 29, 2026 at 10:10 PM 'Tony Leukering' via Colorado Birds <cobirds@googlegroups.com> wrote:

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): 

Here is the same graph for Mallard:

Here's the same graph for Gadwall and American Wigeon:

Finally, the graph for Northern Pintail and Green-winged Teal:

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:

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.

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...@reardonsteel.us mobile

On Mon, Apr 27, 2026 at 15:48 Owen Robertson <owen.k.r...@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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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): 

Here is the same graph for Mallard:

Here's the same graph for Gadwall and American Wigeon:

Finally, the graph for Northern Pintail and Green-winged Teal:

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:

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.

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...@reardonsteel.us mobile

On Mon, Apr 27, 2026 at 15:48 Owen Robertson <owen.k.r...@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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