This handsome brown-and-white booby is common in tropical waters throughout the world, occurring in the Atlantic, Pacific, and Indian Oceans and the Caribbean Sea. Its breeding range overlaps considerably with that of the Masked (Sula dactylatra) and Red-footed boobies (S. sula); all 3 may be found nesting and feeding together along with frigatebirds (Fregata spp.) and other tropical seabirds.
The Brown Booby feeds mainly on flying fish that it catches in often spectacular plunge dives from varying heights in the air; it may not feed as far from land as other boobies, but there are few data. Individuals are often seen soaring, banking, and turning without flapping, as they follow air currents. When flapping, wing beats are steady and may alternate with gliding or soaring. Individuals seem to prefer roosting and nesting in windy areas, suggesting that takeoff without wind is difficult. The species generally nests in small colonies, tens to hundreds of pairs, with nests on flat ground, hillsides, or cliff ledges; nests vary from not much more than a scrape in the sand to a fairly substantial, well-formed pile of twigs and grasses. Pairs lay 2 eggs but generally raise only 1 chick, as the first to hatch usually outcompetes its sibling during feedings and frequently pushes it out of the nest.
Significant morphological differences in the size of adults and eggs, even within currently recognized subspecies, suggest that Brown Booby taxonomy needs study. Adults are sexually dimorphic to varying degrees throughout the range—some only in size, soft-part coloration, and voice, others also in plumage coloration.
Nesting colonies are currently restricted mainly to remote islands, owing to extensive human disturbance and introduced predators. Populations worldwide have declined dramatically over the past 200 years and may be only 10% of historic levels (see Table 1 ).
Little research has been published on the Brown Booby and most of the work has been done on Ascension Island. The British Centenary Expedition had ornithologists living on that island from October 1957 through April 1959 (Dorward 1962b). K. E. L. Simmons (Simmons 1967, Simmons 1970b, Simmons 1972) lived there from 1962 through 1963, and for 3 weeks in 1966. These two studies provide the major body of information available on this species. Additionally, E. A. Schreiber studied the species on Johnston Atoll from 1984 to 2001, visiting the Atoll for 30–60 days per year (Schreiber 2001). B. Tershy (Tershy 1998) studied sexual selection in the species on the Tres Marias Islands, Mexico (summer 1990–1994).