Date: 6/20/25 4:51 am From: Paul Lehman <00000ae015dd4920-dmarc-request...> Subject: Re: [BIRDWG01] Colorado Booby ID
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
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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.
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
>
>
> Archives: https://listserv.ksu.edu/birdwg01.html
