The Brown booby's head and upper body (back) are covered in dark brown to black plumage, with the remainder (belly) being a contrasting white. The bare part colors vary geographically, but not seasonally. The species also displays sexual dimorphism of the bare part colors, the males having a blue orbital ring, as opposed to the yellow orbital ring of the female. In addition the male of subspecies S. l. brewsteri is distinctly plumaged in having the forehead, forecrown and chin white, merging to a greyish brown neck and breast. Unlike other species of sulid the juvenile plumage already resembles that of the adult. They are gray-brown with darkening on the head, upper surfaces of the wings, and tail, while the lower breast and underpart plumages are heavily flecked brown on white. Juveniles of subspecies S. l. brewsteri are once again distinct in having the underpart plumage more evenly mouse brown. The beaks of Brown boobies are quite sharp and contain many jagged edges. They have fairly short wings resulting in a fast flap rate, but long, tapered tails.
Brown boobies breed on islands and coasts in the pantropical areas of the Atlantic and Pacific oceans. They frequent the breeding grounds of the islands in the Gulf of Mexico and the Caribbean Sea. After the nesting season Brown boobies winter at sea over a wider area.
Brown boobies are diurnal gregarious birds. They feed during the day alone or gather in flocks, or in mixed-species flocks with other seabirds or other species of booby. They forage at low heights over inshore waters. Flocks plunge-dive to take small fish, especially when these are driven near the surface by their predators. Along with plunge-diving, some fledglings and some adults practice kleptoparasitism, where they steal prey from other seabirds. Although Brown boobies are powerful and agile fliers, they are particularly clumsy in takeoffs and landings; they use strong winds and high perches to assist their takeoffs. These birds are typically silent but may occasionally produce sounds similar to grunting or quacking.
Brown boobies have a carnivorous (piscivorous, molluscivorous) diet. They mainly eat small fish (such as flying fish, mullet, halfbeaks, anchovies, goatfish, crowned squirrelfish, and Indian mackerel), squid or shrimp which gather near the surface; they may also catch leaping fish while skimming the surface.
Brown boobies are monogamous and form pairs that may remain together over several seasons. They perform elaborate greeting rituals, and are also spectacular divers, plunging into the ocean at high speed. Brown boobies nest in large colonies and each pair defends a small nesting territory. Females lay 2 chalky blue eggs on the ground in a mound of broken shells and vegetation; pairs usually raise just one chick because the second one to hatch is unable to compete for food with its older sibling, or even ejected from the nest by it. The incubation period lasts about 42-45 days. The chick fledges 85-103 days after hatching and may remain dependent on its parents for up to 1 year.
The main threats to Brown boobies include hunting, human disturbance, and pollution. With the rise in pollution in the world, Brown boobies have been using marine debris to make their nests. 90.1 percent of these nests consisted of plastic, while nests near shipwrecks have a high percentage of wreckage debris.
According to IUCN Red List, the total population size of the Brown booby is above 200,000 individuals. The national population consists of around 10,000-100,000 breeding pairs and around 1,000-10,000 individuals on migration in Taiwan and approximately 10,000-100,000 breeding pairs, around 1,000-10,000 individuals on migration and around 1,000-10,000 wintering individuals in Japan. Currently, this species is classified as Least Concern (LC) on the IUCN Red List but its numbers today are decreasing.