Date: 6/18/25 2:16 pm From: Joseph Morlan <jmorlan...> Subject: Re: [BIRDWG01] Booby ID
I corresponded with Eric about this problem and he suggested that Cocos has a more curved bill, more tapered than Brown which has a straight culmen. This apparently works with immatures as well as adults. Another possible clue is eye color which is pale on Brown and dark on Cocos. This works with adults, but Eric said that it may be helpful on immatures as well, but more data is needed.
Bill looks curved and tapered to me. Eye is mostly dark but somewhat ambiguous.
I'll forward this to Eric to see if he has an opinion.
On Wed, 18 Jun 2025 15:59:23 -0400, David Sibley <sibleyguides...> wrote:
>I did a lot of work on this question a year ago, starting from the excellent work by Eric VanderWerf and looking at lots of photos. This bird is a subadult, and I could not find any reliable way of separating Brown and Cocos Booby at that age. The yellow facial skin and pinkish bill seem more typical of Atlantic (but can be matched by some Cocos), and the impression that the neck is darker than the back favors Atlantic. Other features like forehead color and underwing pattern can be helpful on adults but not on subadults. > >As Paul says the location strongly favors Brown. All of the identifiable photos (adults) that I saw from eastern North America are Brown, but there is one Cocos record from the Caribbean off Panama. > >Best, >David > ><sibleyguides...> >www.sibleyguides.com > > > >> On Jun 18, 2025, at 2:25?PM, Paul Lehman <00000ae015dd4920-dmarc-request...> wrote: >> >> Slide presentation from wfo meeting in San Diego last October. Bill color potential differences would suggest Colorado bird may be a Cocos, given all the brownish in the bill, which seems surprising in Southeast Colorado where one would think it's a bird up from the Gulf of Mexico and therefore a Brown. >> Paul Lehman >> Sent from AOL on Android >> >> Archives: https://listserv.ksu.edu/birdwg01.html > > >Archives: https://listserv.ksu.edu/birdwg01.html -- Joseph Morlan, Pacifica, CA
