Date: 6/20/25 8:24 am From: Peter Pyle <ppyle...> Subject: Re: [BIRDWG01] Colorado Booby ID
Great discussion and I'm pleased to be enlightened about the
distribution of nominate (Atlantic) Brown Boobies in inland North
America. Eric and I have been working a lot on the separation of Cocos
(brewsteri) from Pacific (plotus) BRBO, and at least I am not as
familiar with the separation of nominate from brewsteri. Thus our
leanings to Cocos Booby for the Colorado bird should probably be
straightened up a bit per David's comments.
There is a specimen of plotus from Alaska but otherwise no confirmed
records off our coast north of Mexico (where Eric has identified 1-2
plotus from photographs taken near islands far off Baja). The two best
candidates are an adult female from the Farallones in 1992, prior to the
incursion of brewsteri up our coast, and a more recent record of one on
a boat in the Gulf of Alaska but neither can be confirmed.
To answer Paul's question, I believe most Cocos Boobies dispersing north
of the breeding grounds are young birds, followed by adult females, with
adult males least frequent. This is a common pattern we see in irruptive
species, the adult males apparently displacing the others when food
becomes scarcer. During the 2013-2014 invasion of 100s of Blue-footed
Boobies up the West Coast, 95% of these were first cycle birds. Male
brewsteri begin to get paler heads in the second cycle but don't get the
fully white-headed look until 3-4 years of age or later, and these older
males could be the last to disperse from breeding areas. That said, we
have had some fully white headed males as far north as the Farallones.
One in an aerial photograph was initially identified as a juvenile
Magnificent Frigatebird.
Sea surface temps off the West coast have been very cold over the last
few years. Once we get an ENSO event or the like, I bet the numbers of
Cocos Boobies will increase again.
Best to all, Peter
On 6/20/2025 4:50 AM, Paul Lehman wrote:
> Ah, the long-standing question about why are there so few clear adult male looking Cocos Boobies with obvious pale heads seen along the U.S. West Coast. We've been talking about that for a couple years now. Even here in Southern California, where there are a couple Island colonies of boobies, and where at least up till the last year or so we would see multiple birds per pelagic trip, we would see extremely few clear adult males with pale heads. Here in San Diego County waters, for example, for many years we would typically see between 3 and 8 boobies per trip and we might do almost 10 trips per year, and on average I would see only one or maybe two pale-headed adult males per year. Pretty much everyone else has the same experience In this region. We wondered if perhaps only the oldest adult males get pale heads, and so everything else have dark heads like females, but we really have no idea why this ratio of clear pale-headed male birds is so low...... I would also add that there's been a noticeable decline in numbers in the past 2 years locally.
> Paul Lehman, San Diego
>
> Sent from AOL on Android
>
> On Thu, Jun 19, 2025 at 6:33 PM, Wayne Hoffman<whoffman...> wrote: Hi -
>
> Your mention that both this bird and the previous Colorado bird were females reminded me that when "Brown" Boobies (later split as Cocos Boobies) became regular in small numbers on the Oregon coast a decade or so ago, both adults and subadults were occurring, but no birds identifiable as adult male Cocos Boobies had been documented, at least as of 2018 when I left Oregon. So I wonder if female "Brown" Boobies are more likely to wander than males? Similar sex biases in vagrancy are documented in other birds (e.g. Ancient Murrelets).
>
> Wayne Hoffman
>
>
> ----- Original Message -----
> From: "David Sibley" <sibleyguides...>
> To: "BIRDWG01" <BIRDWG01...>
> Sent: Thursday, June 19, 2025 5:17:22 PM
> Subject: Re: [BIRDWG01] Colorado Booby ID
>
> Following up on this after more photo study. I still think this bird is a better match for Atlantic Brown Booby, but can't rule out Cocos with certainty.
>
> For comparison, this subadult Brown from Maryland is an almost exact match - https://macaulaylibrary.org/asset/624857879 > This Cocos from Costa Rica is also very close - https://macaulaylibrary.org/asset/629034516 >
> Bill color, bill shape, and eye color seem to be no help in ID.
>
> One thing that does seem fairly consistent in photos I've looked at is that Cocos has a wider ring of bare skin behind the eye, and by that feature the Colorado bird matches Brown.
>
> And the bare skin on the chin of Cocos seems to average a little drabber and greener, Brown Booby more yellow, again the CO bird matches Brown.
>
> Those are pretty much the only differences I can see for a subadult female like this, and for now I would not feel confident identifying either species out of range based on those things. But maybe one could argue that Colorado is within the "expected" range for Brown Booby?
>
> Colorado's prior record looks like an adult female Atlantic Brown Booby. Cocos adult female typically has a greener face and darker brownish bill (but there is overlap).
>
> Best,
> David
>
> <sibleyguides...>
> www.sibleyguides.com
>
>
>
>> On Jun 18, 2025, at 11:57 AM, Rachel Hopper <hopkohome8...> wrote:
>>
>> Colorado just had its second record of a booby, found on June 16th & seen early again on the 17th after which it disappeared.
>>
>> It was found in a tree out on the prairie in far SE CO. No water body even close.
>>
>> There are several photos of the bird, which did take flight a few times, always coming back to land in an Elm tree. At one point it was on the ground.
>>
>> We are trying to determine if this is a Brown Booby or Cocos Booby. Possibly not a full adult. Any help would be appreciated.
>>
>> You can view the photos of the bird here on these 3 checklists:
>>
>> https://ebird.org/checklist/S251486607 >>
>> https://ebird.org/checklist/S251577330 >>
>> https://ebird.org/checklist/S251215877 >>
>> -----------------------
>> Rachel Kolokoff Hopper
>> Ft. Collins, CO
>>
>>
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