The masked booby (Sula dactylatra) is a major seabird of the Sulidae family of gannets. It is also known as the blue-faced booby or masked gannet. The masked booby is a large, white, tropical seabird-like gannet with black trailing wing edges, a black tail, and a yellow bill. The dark skin of the face around the eyes and the bill resembles a mask. In the genus Sula, it is one of six booby species. With black wings, a black tail, and a dark face mask, the adult is bright white; it is the largest booby species at 75–85 cm (30–33 in) long. The sexes have plumage which is identical. Except in the eastern Atlantic and eastern Pacific, this species occurs through tropical oceans. The plumage of juvenile masked boobies, with a lighter mottled abdomen, the underside of the wings, and collar, is mainly grey-brown above and on the head. The feet and the bill are not as brightly colored as in adults, with the yellow tones getting a blue-green tinge. Compared to juveniles, immature birds exhibit an increased proportion of white in their plumage, particularly a lighter-colored head. Spectacular divers are Masked Boobies, diagonally plunging at high speed into the ocean. They eat small fish mostly, including flying fish. This is a fairly sedentary species but seldom seen far from the breeding colonies, wintering at sea. Caribbean birds, however, sometimes wander north to warm southern Gulf Stream waters off the United States’ eastern seaboard. More strikingly, three Western Palaearctic Masked Booby records, probably dactylatra, were reported, all from Spanish waters, although one of these also entered French territorial areas.
The Masked Booby population that breeds along the Pacific coast of northern South America, including the Galapagos, has recently been recognized as a distinct species, the Nazca Booby. The Nazca Booby has an orange, not yellow, bill, and with a slightly shorter, shallower bill, it is smaller. While the Masked Booby typically nests in low, flat areas, cliffs and steep slopes are used by the Nazca Booby. On offshore islands all over the tropics, masked boobies breed, and have a widespread range at sea reaching into the subtropics. The proximity of breeding grounds to clear, deep waters is significant. In a shallow depression on flat ground away from the vegetation, the female lays two chalky white eggs. Chicks are born featherless but are soon coated in a white bottom. Generally, the second chick born does not survive and is killed by its older sibling. Spectacular plunge divers are these animals, plunging into the ocean at high speed in search of prey, primarily flying fish. When it was found alive in the wild in the Pacific Oceania region, the oldest known Masked Booby was at least 25 years and 3 months old. Fish, squid, and octopi are eaten by masked boobies. Flying fish, needlefish, and various perciforms, as well as flying squid and other cephalopods, are among the fish species taken. There is no data available on the prey of birds breeding on the Kermadec Islands, although there have been records of flying fish. The masked booby is normally quiet at sea, but in the breeding colonies, it is noisy. A falling whistle is the principal call of male birds; a loud honk is that of females.