The Masked Booby, also known as the White or Blue-faced Booby, breeds throughout the world's tropical oceans and is locally abundant near hundreds of oceanic islands. Although its only U.S. breeding records are from the Hawaiian Islands and the Dry Tortugas, Florida, the species is regularly sighted along the Gulf of Mexico and as far north as waters off North Carolina.
Until recently, the Nazca Booby (S. granti) was considered a subspecies of the Masked Booby, and much of the material in the first version of this account applied to that taxon. In 2000 the Nazca was elevated to full species status by the American Ornithologist's Union based on the work of Pitman and Jehl ( Pitman and Jehl 1998 ), which argued that the Nazca Booby (all colonies located on the Nazca tectonic plate, eastern Pacific) was morphologically and ecologically distinct from the Masked Booby and that the two species mate assortatively where sympatric (i.e., Clipperton Island). Molecular evidence of mitochondrial variation at the cytochrome-b gene was subsequently presented by Friesen et al. ( Friesen et al. 2002 ), which confirmed the Nazca Booby as a distinct species.
The breeding ecology and behavior of this pelagic seabird are known from studies of C.B. Kepler on the Hawaiian Islands ( Kepler 1969 ) and of D.F. Dorward on Ascension Island in the south Atlantic ( Dorward 1962b ). J.B. Nelson ( Nelson 1978c ) integrated these two studies with his own research in the Galápagos in his book The Sulidae. Nelson's account, however, must be read with the separation of the Nazca (Galápagos) from the Masked in mind.
In some populations, individuals display little fear of humans, which greatly facilitates both observational and experimental investigations. Feeding tactics of this booby are memorable: dazzling white birds make high velocity plunge-dives in search of fish. Colonial nesting is typical for this species, with inter-nest distances ranging from 2–3 m to over 100 m. Females lay one or two eggs but only one chick is ever raised, the other disappearing soon after hatching. It is likely that obligate siblicide accounts for this pattern as it does in the closely related Nazca booby, but no observations of siblicide exist for the Masked.
Masked Boobies are exceptionally vulnerable to human development and to some introduced predators. In the eastern Pacific, for example, the introduction of pigs onto Clipperton Island caused nearly complete abandonment of that once-immense colony. A visiting ornithologist subsequently destroyed the pigs, and the colony has since made an encouraging recovery. As land development and breeding seem incompatible, prehistoric and modern human colonization of oceanic islands doubtlessly extirpated significant numbers of Masked Booby colonies. Many of the remaining colonies, however, are small and remote, and so are somewhat protected from repeated and large-scale disturbances.