Date: 8/18/25 8:05 pm From: Robert O'Brien <baro...> Subject: Re: [BIRDWG01] Neotropic vs. Double-crested Cormorant
And I promise not to harass the Listserver any more; after this last
observations from the eBird map I included above. Most hybrids are
reported far away from the locations where they are most expected. That
is, they are being overlooked among all the individual, abundant species.
And, further, it is likely that some (many?) eBird reviewers are not making
such reports public (due to uncertainty in the ID) so that the actual
number of far flung hybrids is even higher than reported. So, unless
a. All these reports are erroneous or
b. There is likely a much larger proportion of hybrids in the hybrid zones
than is currently documented. Or
c. Such hybrids have a strange propensity for dispersal to distant
places. This seems unlikely.
Bob OBrien Portland
On Mon, Aug 18, 2025 at 7:28 PM Robert O'Brien <baro...> wrote:
> These eBurd reports notwithstanding, there are definitely some out there.
> Here is what my friend ChatGPT came up with. Bob OBrien Portland
>
> -
>
> *Wakodahatchee Wetlands, Delray Beach, Palm Beach Co., Florida* —
> repeated, well-documented mixed pairings inside the Double-crested colony:
> -
>
> A female *Neotropic Cormorant* *“paired with a Double-crested
> Cormorant”* and was *seen copulating* on *2 Jan 2016*. Florida
> Ornithological Society Records Committee (FOSRC) “Twenty-sixth Report.” Internet
> Archive
> <https://archive.org/stream/floridafieldnat46flor/floridafieldnat46flor_djvu.txt?<utm_source...> > -
>
> Earlier, at the same site, the *Neotropic* *“formed a pair bond
> with a male Double-crested Cormorant, copulated with him, and successfully
> reared young.”* FOSRC “Twenty-fourth Report.”
> -
>
> Additional FOSRC language for this site calls out a *mixed-pair
> nest with ongoing incubation* in 2015. FOSRC “Twenty-fifth Report.” Digital
> Commons USF
> <https://digitalcommons.usf.edu/cgi/viewcontent.cgi?article=2332&context=ffn&<utm_source...> > -
>
> A summary piece by Kenn Kaufman also notes that since *2012* one or
> more *Neotropics joined the Delray Beach DCCO colony and interbred*,
> creating hybrids there. DigitalCommons
> <https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1322&context=usfwspubs&<utm_source...> >
> If you want the exact passages (page context) from the FFN/FOSRC reports,
> I can pull those out for you—but the three committee reports above
> explicitly document the *pair bond*, *copulation*, and *mixed-pair
> nesting* events at Wakodahatchee.
>
> Yes I do.
> *Summary Table*
> Site / Source Observation Details
> *FOSRC 23rd Report (2015)* Adult Neotropic paired with Double-crested →
> hatched 4 young, fledged 2
> *FOSRC 26th Report (2018)* Female Neotropic “paired with” Double-crested
> Cormorant—explicit pairing confirmed
> *Birding Blog (Nemesisbird)* Adult Neotropic confirmed breeding with
> either Double-crested or hybrid mate
> *Tropical Audubon / Birds of S. Florida* Adult Neotropics on nests and
> apparently hybridizing since 2012
> ------------------------------
> Broad Takeaway
>
> The *only well-documented, explicit examples* of *mixed pairs* (adult
> Neotropic × adult Double-crested Cormorants) being *seen together and
> reproducing* come from *Wakodahatchee Wetlands* in Palm Beach County:
>
>
> -
>
> *2015 (FOSRC 23):* Verified mixed pair hatched and fledged offspring.
> -
>
> *2018 (FOSRC 26):* Explicit observation of pairing behavior.
> -
>
> *Additional corroborative birding sources* reinforce these occurrences
> since 2012.
>
>
> On Mon, Aug 18, 2025 at 6:59 PM Robert O'Brien <baro...> wrote:
>
>> And then there are the hybrids. One reported even so far away as here in
>> Portland Oregon. Note I said 'reported'.
>> Bob OBrien Portland.
>> [image: image.png]
>>
>> On Sat, Aug 16, 2025 at 6:10 AM Marcel Gahbauer <
>> <marcel...> wrote:
>>
>>> Hi everyone,
>>>
>>> I'm curious for feedback on an unusual cormorant observation I had
>>> earlier this week.
>>>
>>> I was attending an event on private property adjacent to Lake Ontario,
>>> when my 8-year-old son came to tell me there was a cormorant 'trapped' in
>>> the rocks along the shore. It wasn't trapped as such - but it was just
>>> floating there in a small space between rocks, and occasionally being
>>> pushed into them by the waves. It looked like one eye was damaged or
>>> diseased, and locals told me some cormorants had washed up recently and
>>> were presumed to have died from botulism, so my initial thoughts were
>>> focused on the health of the bird more than its ID (given that the only
>>> regularly occurring species in Ontario is Double-crested).
>>>
>>> However, upon sharing a few photos with friends, the suggestion was
>>> raised that it could be a juvenile Neotropic Cormorant, given the
>>> proportionately long tail, relatively darker upper breast, and generally
>>> sleek shape. In retrospect, it was relatively small - I didn't notice that
>>> at the time given the circumstances but I was as close as 5 m and so it
>>> seemed like a large bird based on proximity more than reality perhaps. On
>>> the other hand, the bill still looks to me quite sturdy and more like that
>>> of a Double-crested Cormorant.
>>>
>>> Although I've seen Neotropic Cormorant once previously in Ontario (more
>>> conveniently among a group of Double-crested) and a few times in its
>>> regular range, I don't have enough experience to be confident in resolving
>>> these seemingly conflicting aspects of ID, and would welcome any insights.
>>> Photos are posted in my eBird report at
>>> https://ebird.org/checklist/S267572991 >>>
>>> Thanks!
>>>
>>> Marcel Gahbauer
>>> Ottawa, Ontario
>>> <marcel...><mailto:<marcel...> >>>
>>> Archives: https://listserv.ksu.edu/birdwg01.html >>>
>>
Thanks Tony, Andy, and Bob for your insights and useful links to further materials for reference.
It seems that although somewhat atypical, many features (plus location) lean at least slightly toward Double-crested – but certainly it’s possible that it’s a hybrid.
Probably best to leave this one as Cormorant sp., but it has been an interesting exercise in learning more about an ID challenge that I had underestimated.
Marcel
From: Robert O'Brien <baro...>
Sent: August 18, 2025 10:29 PM
To: Marcel Gahbauer <marcel...>
Cc: <BIRDWG01...>
Subject: Re: [BIRDWG01] Neotropic vs. Double-crested Cormorant
These eBurd reports notwithstanding, there are definitely some out there. Here is what my friend ChatGPT came up with. Bob OBrien Portland
* Wakodahatchee Wetlands, Delray Beach, Palm Beach Co., Florida — repeated, well-documented mixed pairings inside the Double-crested colony:
If you want the exact passages (page context) from the FFN/FOSRC reports, I can pull those out for you—but the three committee reports above explicitly document the pair bond, copulation, and mixed-pair nesting events at Wakodahatchee.
Yes I do.
Summary Table
Site / Source
Observation Details
FOSRC 23rd Report (2015)
Adult Neotropic paired with Double-crested → hatched 4 young, fledged 2
FOSRC 26th Report (2018)
Female Neotropic “paired with” Double-crested Cormorant—explicit pairing confirmed
Birding Blog (Nemesisbird)
Adult Neotropic confirmed breeding with either Double-crested or hybrid mate
Tropical Audubon / Birds of S. Florida
Adult Neotropics on nests and apparently hybridizing since 2012
________________________________
Broad Takeaway
The only well-documented, explicit examples of mixed pairs (adult Neotropic × adult Double-crested Cormorants) being seen together and reproducing come from Wakodahatchee Wetlands in Palm Beach County:
* 2015 (FOSRC 23): Verified mixed pair hatched and fledged offspring.
* 2018 (FOSRC 26): Explicit observation of pairing behavior.
* Additional corroborative birding sources reinforce these occurrences since 2012.
On Mon, Aug 18, 2025 at 6:59 PM Robert O'Brien <baro...><mailto:<baro...>> wrote:
And then there are the hybrids. One reported even so far away as here in Portland Oregon. Note I said 'reported'.
Bob OBrien Portland.
[cid:<image001.png...>]
On Sat, Aug 16, 2025 at 6:10 AM Marcel Gahbauer <marcel...><mailto:<marcel...>> wrote:
Hi everyone,
I'm curious for feedback on an unusual cormorant observation I had earlier this week.
I was attending an event on private property adjacent to Lake Ontario, when my 8-year-old son came to tell me there was a cormorant 'trapped' in the rocks along the shore. It wasn't trapped as such - but it was just floating there in a small space between rocks, and occasionally being pushed into them by the waves. It looked like one eye was damaged or diseased, and locals told me some cormorants had washed up recently and were presumed to have died from botulism, so my initial thoughts were focused on the health of the bird more than its ID (given that the only regularly occurring species in Ontario is Double-crested).
However, upon sharing a few photos with friends, the suggestion was raised that it could be a juvenile Neotropic Cormorant, given the proportionately long tail, relatively darker upper breast, and generally sleek shape. In retrospect, it was relatively small - I didn't notice that at the time given the circumstances but I was as close as 5 m and so it seemed like a large bird based on proximity more than reality perhaps. On the other hand, the bill still looks to me quite sturdy and more like that of a Double-crested Cormorant.
Although I've seen Neotropic Cormorant once previously in Ontario (more conveniently among a group of Double-crested) and a few times in its regular range, I don't have enough experience to be confident in resolving these seemingly conflicting aspects of ID, and would welcome any insights. Photos are posted in my eBird report at https://ebird.org/checklist/S267572991
Earlier, at the same site, the *Neotropic* *“formed a pair bond with
a male Double-crested Cormorant, copulated with him, and successfully
reared young.”* FOSRC “Twenty-fourth Report.”
-
If you want the exact passages (page context) from the FFN/FOSRC reports, I
can pull those out for you—but the three committee reports above explicitly
document the *pair bond*, *copulation*, and *mixed-pair nesting* events at
Wakodahatchee.
Yes I do.
*Summary Table*
Site / Source Observation Details
*FOSRC 23rd Report (2015)* Adult Neotropic paired with Double-crested →
hatched 4 young, fledged 2
*FOSRC 26th Report (2018)* Female Neotropic “paired with” Double-crested
Cormorant—explicit pairing confirmed
*Birding Blog (Nemesisbird)* Adult Neotropic confirmed breeding with either
Double-crested or hybrid mate
*Tropical Audubon / Birds of S. Florida* Adult Neotropics on nests and
apparently hybridizing since 2012
------------------------------
Broad Takeaway
The *only well-documented, explicit examples* of *mixed pairs* (adult
Neotropic × adult Double-crested Cormorants) being *seen together and
reproducing* come from *Wakodahatchee Wetlands* in Palm Beach County:
*2018 (FOSRC 26):* Explicit observation of pairing behavior.
-
*Additional corroborative birding sources* reinforce these occurrences
since 2012.
On Mon, Aug 18, 2025 at 6:59 PM Robert O'Brien <baro...> wrote:
> And then there are the hybrids. One reported even so far away as here in
> Portland Oregon. Note I said 'reported'.
> Bob OBrien Portland.
> [image: image.png]
>
> On Sat, Aug 16, 2025 at 6:10 AM Marcel Gahbauer <
> <marcel...> wrote:
>
>> Hi everyone,
>>
>> I'm curious for feedback on an unusual cormorant observation I had
>> earlier this week.
>>
>> I was attending an event on private property adjacent to Lake Ontario,
>> when my 8-year-old son came to tell me there was a cormorant 'trapped' in
>> the rocks along the shore. It wasn't trapped as such - but it was just
>> floating there in a small space between rocks, and occasionally being
>> pushed into them by the waves. It looked like one eye was damaged or
>> diseased, and locals told me some cormorants had washed up recently and
>> were presumed to have died from botulism, so my initial thoughts were
>> focused on the health of the bird more than its ID (given that the only
>> regularly occurring species in Ontario is Double-crested).
>>
>> However, upon sharing a few photos with friends, the suggestion was
>> raised that it could be a juvenile Neotropic Cormorant, given the
>> proportionately long tail, relatively darker upper breast, and generally
>> sleek shape. In retrospect, it was relatively small - I didn't notice that
>> at the time given the circumstances but I was as close as 5 m and so it
>> seemed like a large bird based on proximity more than reality perhaps. On
>> the other hand, the bill still looks to me quite sturdy and more like that
>> of a Double-crested Cormorant.
>>
>> Although I've seen Neotropic Cormorant once previously in Ontario (more
>> conveniently among a group of Double-crested) and a few times in its
>> regular range, I don't have enough experience to be confident in resolving
>> these seemingly conflicting aspects of ID, and would welcome any insights.
>> Photos are posted in my eBird report at
>> https://ebird.org/checklist/S267572991 >>
>> Thanks!
>>
>> Marcel Gahbauer
>> Ottawa, Ontario
>> <marcel...><mailto:<marcel...> >>
>> Archives: https://listserv.ksu.edu/birdwg01.html >>
>
Date: 8/18/25 7:00 pm From: Robert O'Brien <baro...> Subject: Re: [BIRDWG01] Neotropic vs. Double-crested Cormorant
And then there are the hybrids. One reported even so far away as here in
Portland Oregon. Note I said 'reported'.
Bob OBrien Portland.
[image: image.png]
On Sat, Aug 16, 2025 at 6:10 AM Marcel Gahbauer <
<marcel...> wrote:
> Hi everyone,
>
> I'm curious for feedback on an unusual cormorant observation I had earlier
> this week.
>
> I was attending an event on private property adjacent to Lake Ontario,
> when my 8-year-old son came to tell me there was a cormorant 'trapped' in
> the rocks along the shore. It wasn't trapped as such - but it was just
> floating there in a small space between rocks, and occasionally being
> pushed into them by the waves. It looked like one eye was damaged or
> diseased, and locals told me some cormorants had washed up recently and
> were presumed to have died from botulism, so my initial thoughts were
> focused on the health of the bird more than its ID (given that the only
> regularly occurring species in Ontario is Double-crested).
>
> However, upon sharing a few photos with friends, the suggestion was raised
> that it could be a juvenile Neotropic Cormorant, given the proportionately
> long tail, relatively darker upper breast, and generally sleek shape. In
> retrospect, it was relatively small - I didn't notice that at the time
> given the circumstances but I was as close as 5 m and so it seemed like a
> large bird based on proximity more than reality perhaps. On the other
> hand, the bill still looks to me quite sturdy and more like that of a
> Double-crested Cormorant.
>
> Although I've seen Neotropic Cormorant once previously in Ontario (more
> conveniently among a group of Double-crested) and a few times in its
> regular range, I don't have enough experience to be confident in resolving
> these seemingly conflicting aspects of ID, and would welcome any insights.
> Photos are posted in my eBird report at
> https://ebird.org/checklist/S267572991 >
> Thanks!
>
> Marcel Gahbauer
> Ottawa, Ontario
> <marcel...><mailto:<marcel...> >
> Archives: https://listserv.ksu.edu/birdwg01.html >
Date: 8/16/25 10:09 pm From: Andrew Birch <0000171b9d979956-dmarc-request...> Subject: Re: [BIRDWG01] Neotropic vs. Double-crested Cormorant
Hi Marcel, interesting bird and I agree appears quite long-tailed. Of course, young apparently dark-lored Double-cresteds are a not too uncommon problem. However, this view of your bird (when zoomed in) does actually show some yellowish lores:https://macaulaylibrary.org/asset/640357003
I find bill structure and even gular shapebelow the eye a little variable and can be hard to assess on lone birds, so not sure there's anything obviously wrong with the bill on your bird eg there's not much of a difference between this Double-crested and this Neotropic in terms of bill structure. The difference in the shape of the gular below the eye can be pretty subtle too even in these close range shots:
https://macaulaylibrary.org/asset/300498091https://macaulaylibrary.org/asset/422118571
On Saturday, August 16, 2025 at 06:09:26 AM PDT, Marcel Gahbauer <marcel...> wrote:
Hi everyone,
I'm curious for feedback on an unusual cormorant observation I had earlier this week.
I was attending an event on private property adjacent to Lake Ontario, when my 8-year-old son came to tell me there was a cormorant 'trapped' in the rocks along the shore. It wasn't trapped as such - but it was just floating there in a small space between rocks, and occasionally being pushed into them by the waves. It looked like one eye was damaged or diseased, and locals told me some cormorants had washed up recently and were presumed to have died from botulism, so my initial thoughts were focused on the health of the bird more than its ID (given that the only regularly occurring species in Ontario is Double-crested).
However, upon sharing a few photos with friends, the suggestion was raised that it could be a juvenile Neotropic Cormorant, given the proportionately long tail, relatively darker upper breast, and generally sleek shape. In retrospect, it was relatively small - I didn't notice that at the time given the circumstances but I was as close as 5 m and so it seemed like a large bird based on proximity more than reality perhaps. On the other hand, the bill still looks to me quite sturdy and more like that of a Double-crested Cormorant.
Although I've seen Neotropic Cormorant once previously in Ontario (more conveniently among a group of Double-crested) and a few times in its regular range, I don't have enough experience to be confident in resolving these seemingly conflicting aspects of ID, and would welcome any insights. Photos are posted in my eBird report at https://ebird.org/checklist/S267572991
Date: 8/16/25 11:37 am From: Wayne Hoffman <whoffman...> Subject: Re: [BIRDWG01] Neotropic vs. Double-crested Cormorant
I agree. In addition to the eye surroundings, the bare facial skin on DCCO extends back and down from the gape to form a fairly extensive bare-skinned gular pouch. In Neotropic, the boundry between bare skin and feathers angles down and forward from a sharp angle at the gape, leaving much more of the chin/throat area feathered. This bird shows the skin/feathers boundary of a NeotropicaL Cormorant.
Wayne Hoffman
----- Original Message -----
From: "<greatgrayowl...>" <0000012933c40dff-dmarc-request...>
To: "BIRDWG01" <BIRDWG01...>
Sent: Saturday, August 16, 2025 2:26:27 PM
Subject: Re: [BIRDWG01] Neotropic vs. Double-crested Cormorant
Marcel:
As depicted by Pyle (2008), the eyes of Neotrop are outside the bare facial skin, whereas in DC and Great, the eyes are within that bare-skin area, which suggests that your bird at least has NECO genes.
Photos of juv/imm NECO:
ML543513911 - Neotropic Cormorant - Macaulay Library
Pyle, P. 2008. Identification Guide to North American Birds, part II. Slate Creek Press, Bolinas, CA. [slatecreekpress.com]
Tony
Tony Leukeringcurrently Denver, COeBird blog
Photo quizPhotos
On Saturday, August 16, 2025 at 07:09:24 AM MDT, Marcel Gahbauer <marcel...> wrote:
Hi everyone,
I'm curious for feedback on an unusual cormorant observation I had earlier this week.
I was attending an event on private property adjacent to Lake Ontario, when my 8-year-old son came to tell me there was a cormorant 'trapped' in the rocks along the shore. It wasn't trapped as such - but it was just floating there in a small space between rocks, and occasionally being pushed into them by the waves. It looked like one eye was damaged or diseased, and locals told me some cormorants had washed up recently and were presumed to have died from botulism, so my initial thoughts were focused on the health of the bird more than its ID (given that the only regularly occurring species in Ontario is Double-crested).
However, upon sharing a few photos with friends, the suggestion was raised that it could be a juvenile Neotropic Cormorant, given the proportionately long tail, relatively darker upper breast, and generally sleek shape. In retrospect, it was relatively small - I didn't notice that at the time given the circumstances but I was as close as 5 m and so it seemed like a large bird based on proximity more than reality perhaps. On the other hand, the bill still looks to me quite sturdy and more like that of a Double-crested Cormorant.
Although I've seen Neotropic Cormorant once previously in Ontario (more conveniently among a group of Double-crested) and a few times in its regular range, I don't have enough experience to be confident in resolving these seemingly conflicting aspects of ID, and would welcome any insights. Photos are posted in my eBird report at https://ebird.org/checklist/S267572991
Date: 8/16/25 11:27 am From: <greatgrayowl...> <0000012933c40dff-dmarc-request...> Subject: Re: [BIRDWG01] Neotropic vs. Double-crested Cormorant
Marcel:
As depicted by Pyle (2008), the eyes of Neotrop are outside the bare facial skin, whereas in DC and Great, the eyes are within that bare-skin area, which suggests that your bird at least has NECO genes.
Photos of juv/imm NECO:
ML543513911 - Neotropic Cormorant - Macaulay Library
Pyle, P. 2008. Identification Guide to North American Birds, part II. Slate Creek Press, Bolinas, CA. [slatecreekpress.com]
Tony
Tony Leukeringcurrently Denver, COeBird blog
Photo quizPhotos
On Saturday, August 16, 2025 at 07:09:24 AM MDT, Marcel Gahbauer <marcel...> wrote:
Hi everyone,
I'm curious for feedback on an unusual cormorant observation I had earlier this week.
I was attending an event on private property adjacent to Lake Ontario, when my 8-year-old son came to tell me there was a cormorant 'trapped' in the rocks along the shore. It wasn't trapped as such - but it was just floating there in a small space between rocks, and occasionally being pushed into them by the waves. It looked like one eye was damaged or diseased, and locals told me some cormorants had washed up recently and were presumed to have died from botulism, so my initial thoughts were focused on the health of the bird more than its ID (given that the only regularly occurring species in Ontario is Double-crested).
However, upon sharing a few photos with friends, the suggestion was raised that it could be a juvenile Neotropic Cormorant, given the proportionately long tail, relatively darker upper breast, and generally sleek shape. In retrospect, it was relatively small - I didn't notice that at the time given the circumstances but I was as close as 5 m and so it seemed like a large bird based on proximity more than reality perhaps. On the other hand, the bill still looks to me quite sturdy and more like that of a Double-crested Cormorant.
Although I've seen Neotropic Cormorant once previously in Ontario (more conveniently among a group of Double-crested) and a few times in its regular range, I don't have enough experience to be confident in resolving these seemingly conflicting aspects of ID, and would welcome any insights. Photos are posted in my eBird report at https://ebird.org/checklist/S267572991
Date: 8/16/25 6:09 am From: Marcel Gahbauer <marcel...> Subject: [BIRDWG01] Neotropic vs. Double-crested Cormorant
Hi everyone,
I'm curious for feedback on an unusual cormorant observation I had earlier this week.
I was attending an event on private property adjacent to Lake Ontario, when my 8-year-old son came to tell me there was a cormorant 'trapped' in the rocks along the shore. It wasn't trapped as such - but it was just floating there in a small space between rocks, and occasionally being pushed into them by the waves. It looked like one eye was damaged or diseased, and locals told me some cormorants had washed up recently and were presumed to have died from botulism, so my initial thoughts were focused on the health of the bird more than its ID (given that the only regularly occurring species in Ontario is Double-crested).
However, upon sharing a few photos with friends, the suggestion was raised that it could be a juvenile Neotropic Cormorant, given the proportionately long tail, relatively darker upper breast, and generally sleek shape. In retrospect, it was relatively small - I didn't notice that at the time given the circumstances but I was as close as 5 m and so it seemed like a large bird based on proximity more than reality perhaps. On the other hand, the bill still looks to me quite sturdy and more like that of a Double-crested Cormorant.
Although I've seen Neotropic Cormorant once previously in Ontario (more conveniently among a group of Double-crested) and a few times in its regular range, I don't have enough experience to be confident in resolving these seemingly conflicting aspects of ID, and would welcome any insights. Photos are posted in my eBird report at https://ebird.org/checklist/S267572991
This past week I have had a confusing Oriole in my yard. I live in Santa Barbara CA and Hooded Orioles nest nearby. My original sighting was brief and I managed just three shots.
Date: 8/11/25 12:07 pm From: Eric VanderWerf <eric...> Subject: Re: [BIRDWG01] Brown/Cocos Booby ID
Hi everyone, here are my thoughts on the identity of this bird. Some of you may have seen some of this already.
• It is a female, based on the dark loral spot.
• It is a subadult, and I agree it is probably a second cycle bird.
• Bill color. The adult bill color and facial skin color are starting to appear, and this is useful for species identification. The facial skin is yellow and the bill is pink. These characters are shared by female leucogaster (Atlantic) Brown Booby and female Cocos Booby. In female Atlantic Brown Booby the bill is bright pink, in female Cocos Booby the bill is darker pink-brown. The bill of the Colorado bird is not bright pink, more of a darker pink, which fits better with Cocos. However, whether a bird this age would already show the full bright pink adult bill color of Atlantic Booby is unknown. This would be difficult to determine precisely because the timing at which the adult coloration appears may vary from bird to bird depending on its nutritional state and development rate, and with age on a month by month basis. Still, given the current state of knowledge it fits better with Cocos.
• Iris color. Adult Cocos Boobies usually have a dark iris, but there is some variation in female Cocos and in female Atlantic Brown Booby. The eye color of this bird is intermediate in my estimation (score of 3 for those that have read my recent paper in Western Birds). However, iris color of immature birds is more variable in both species from what I could see during my analyses, and may not be useful for identification at this age.
• Bill curvature. I measured the curvature of the culmen and mandible using photos from one of the observers. The mandible (lower bill) curvature was 13 degrees, which is right in the range of Cocos but almost entirely outside the range of Brown. The culmen curvature was 20 degrees, which fits with all of the taxa except plotus Brown Booby, and thus is not helpful in this case. This is the only other character (in addition to bill color) that I think is useful for identification of this bird, and it suggests Cocos.
• I still believe the underwing coverts are not useful for identifying birds of this age. The underwing pattern is variable in immature birds, and generally more like that of Cocos in almost all individuals I looked at, with a brown bar in the white polygon. I would be curious to see any data showing consistent differences between the species.
• Location. The location seems equally unlikely for both species to me. Maybe it is a little closer to the Gulf of Mexico, but weather patterns seem more likely to carry a bird from the southwest.
Date: 8/6/25 8:09 pm From: Paul Hurtado <paul.j.hurtado...> Subject: [BIRDWG01] Lake Tahoe Booby ID
An immature Cocos/Brown Booby has been delighting birders at beaches in South Lake Tahoe over the past few days (it has yet to be confirmed in NV waters). It was first reported by Denise Belisle on 1 August.
It's being reported as Cocos, due to the location. Thoughts on this?
I'm not sure the CBRC will review it, given that they show up on the coast, unless it flies over to NV in which case we'd definitely review it (it would be the 4th state record of Cocos Booby, and the first Brown Booby for Nevada).
I wanted to pass along eBird checklist with various photos of the bird, just in case it's older than I think, and in case it might actually identifiable to species (as I understand it, it is not).
