Date: 6/19/25 2:18 pm From: David Sibley <sibleyguides...> 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
