Monotypic. Until recently treated
as a subspecies of Masked Booby. The split was proposed by Pitman &
Jehl (1998) (Wilson Bulletin 110: p155-170).
Juvenal-plumage: Separated from Brown Booby, by virtue of having: a pale eye, the demarcation between the dark head and white chest being at the base of neck, and the presence of more extensive white in the underwings.
Separation from Masked Booby (particularly the race californica) is more complicated. Roberson (1998) argued that all juvenal Masked Boobies have white cervical collars, except a small percentage (20%) belonging to the race S. d. californica which breed on Clipperton Island and the Revillagigedos off western Mexico. In contrast, c.77% of juvenal Nazca Boobies lack a cervical collar. If present, the cervical collar of Nazca Booby is narrower, less cleanly demarked (fuzzy edged). In most Nazca's, there is a slight wash of brownish collar right at the base of the neck which vaguely connects the head and back. In Masked Booby the collar is normally very broad, cleanly white, and well-demarcated.
In addition, Nazca Booby shows more contrast between the head and back (and may appear more "gray-backed" at a distance). Many Nazca Boobies have a "white frosting" to the tail base (43%), while only the most extreme Masked Boobies (12%, and only in race californica) show this feature. In juvenile Nazca's, the bill is dark gray or bluish-gray with a yellowish tip becoming more orange or even pinkish with age.
Adult Nazca Booby. Galapagos Islands, November 1992. Separated from Masked Booby by the orange base of the bill and orange iris. Photograph copyright of Angus Wilson©, 1999.
For more details and specific examples
(which nicely illustrate the difficulty of the identification), please
see Don Roberson's analysis (with photos) of a presumed juvenile
Nazca Booby that was sighted and photographed in Monterey Bay, California
on 13 Feb 1999.
Where and When
Breeds on the Galapagos Islands but
may range through the eastern tropical Pacific occasionally reaching California.
Photographs on the web
Series of excellent shots of a juvenile
Masked and Nazca Boobies taken by Don Roberson, Richard Ternullo, John
Sorensen and Robert L. Pitman. Posted by Don Roberson on his web site.
Roberson, D. (1998) Sulids Unmasked: which large booby reaches California? Field Notes 52: p276-287.