A big, chocolate-brown-and-white seabird similar to gannets is the brown booby (Sula leucogaster). It has a pantropical range, overlapping with that of other species of boobies. The gregarious brown booby commutes and forages at low height over inshore waters. It is slightly smaller than a gannet and identical to one in shape and conduct, if not color. Flocks plunge-dive to require small fish, especially when these are driven near the surface by their predators. They only nest on the bottom and roost on solid objects instead of the water surface. With an estimated wingspan of 150 cm and an average weight of 1.3 kg (140 cm and 1.0 kg for males), female brown boobies are slightly bigger than males. The chocolate brown color of the head, collar, back, and upper side of the wings is uniform. Apart from the broad brown edges of the underwing, the brown stops abruptly across the breast, with the belly and undersides of the wings being white. The legs are yellow shading slightly green. The bill is long and conical. Facial color differs between the sexes, with the male having blue skin and blue-grey bill and also the female yellow skin and therefore the bill yellow shading to grey. Immature and juvenile birds differ from adults in having the underparts mottled brown or blue, though still distinctly differentiated from the chocolate brown breast and upperparts. The brown booby was described in his Histoire Naturelle des Oiseaux in 1781 by the French polymath Georges-Louis Leclerc, Comte de Buffon. The current bird genus was introduced by the French zoologist Mathurin Jacques Brisson in 1760. The word Sula is Norwegian for a gannet; the precise leucogaster is from Hellenic leuko for “white” and gastēr for “belly”.
Brown boobies, plus the Caribbean Sea, have a wide range across the tropical seas of the Indian, Pacific, and Atlantic Oceans. The nearest breeding sites to New Zealand are islands and cays of the Australian Great Barrier Reef. Their beaks, which contain many jagged points, are very sharp. They have fairly short wings, but long, tapered tails, resulting in a fast flap pace. Although these birds are usually quiet, bird watchers have recorded occasional sounds similar to grunting or quacking. Brown boobies are not an endangered species, but many breeding colonies are subject to human interference, and rats may take chicks and unguarded eggs. This bird nests in large colonies, laying two chalky blue eggs on the bottom in a very mound of broken shells and vegetation, but usually raises only 1 chick, the other to hatch being unable to compete for food with its older sibling, or maybe ejected from the nest by it. It winters at sea over a wider area. Brown booby pairs may remain together over several seasons. Flight height is typically up to 30 meters with most plunge-diving carried out from 15 meters or less. They usually dive at a shallower angle than gannets. Brown boobies also chase teleost fish (their major food item) while they’re airborne, catching the fish either on the wing or as they re-enter the water. Certain fish and squid are also taken by plunge-diving. Although they are efficient and agile fliers, they are especially clumsy in takeoffs and landings; they use strong winds and high perches to assist their takeoffs.