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.
