How do you even tell Colombian from Brown Booby?
I'm not great with separating them, they seem to be fairly variable, and the descriptions in Birds of the World are confusing to me. But I beleive it's primarily the amount and extent of white on the head and the coloration of the bare parts. Brewster's has more white and a blueish base to the bill. Here's, I believe, a male Brewster's that I photographed off the coast of Costa Rica in January:
The BOW description is:
S. l. brewsteri Goss, 1888. Breeds in Gulf of California, on Tres Marias Is., I. Isabela, San Pedro Már-tir I., islands off w. Mexico, including Clipperton and Revillagigedo Is. Ranges at sea near breeding grounds, casually north through Gulf of California to the Southwest (sw. Arizona and s. California; e.g., lower Colorado River valley and Salton Sea) and north along coast to central California. Highly sexually dimorphic; adult male has extensive whitish color on head and pale neck (see
Figure 2A), although variable in extent; Clipperton I., Mexico (10°N, 101°W), white extends down neck in most; San Pedro Mártir I., Mexico (28°N, 112°W), degree of white most variable. Bill gray to horn, skin at base of bill blue with some greenish especially at base of mandible, iris near-black with narrow dull yellowish outer ring. Female much as nominate in bare-part colors, but head and neck concolorous with rest of upperparts. Includes as synonym “
S. l. nesiotes ” (type locality Clipperton I., given by Heller and Snodgrass in 1901) and “
S. l. albiceps ” (type locality I. Isabela, given by van Rossem in 1938).
S. l. etesiaca Thayer and Bangs, 1905. Breeds on islands along Pacific coast from Honduras and Costa Rica to Panama and Colombia. Ranges at sea throughout breeding range; although likely in waters as far south as Ecuador; records there uncertain (
Ridgely and Greenfield 2001aa).
Smallestvideo subspecies; similar to
S. l. brewsteri in having sexually dichromatic plumages, but head and neck said to be not as extensively pale as that race. Extensive variation in degree of white, both throughout range and within colony (EAS, USNM specimens), but not well described except on Cocos I., Costa Rica (5°N, 87°W), white of male mainly restricted to upper head (
Tershy 1998). Female much as previous races, iris dark with narrow pale yellowish outer ring, and
male has bare parts like
plotusphoto , perhaps darker gray on bill, but clearly differs from
leucogaster and plotus in having neck slightly paler than rest of upperparts and conspicuously paler on head where becomes whitish on crown, face and throat, making the sexual dimorphism obvious.
