Brown booby (Sula leucogaster)

French: Fou brun
GenusSula (1)

Classified as Least Concern (LC) on the IUCN Red List (1).

Meaning “white stomach”, the Latin name of the brown booby is a reference to its striking white lower breast and belly, which are sharply demarcated from the brown upper breast, body and head (3) (4). The brown booby is a relatively large seabird, with long, narrow, pointed wings and a bullet shaped body. It possesses a bare patch of skin around the face, typically blue-grey in the male and bright yellow in the female, and a long, powerful bill which can vary in colour from bright yellow to light pink and greyish (5). This species produces a variety of calls including harsh quacks and honking by the female and high-pitched whistles by the male (4) (5). There are four recognised subspecies that vary in appearance and location. Sula leucogaster plotus is the largest subspecies, and has more uniformly dark upperparts than Sula leucogaster leucogaster. The male Sula leucogaster etesiaca has a pale greyish fore-crown, while the head of the male Sula leucogaster brewsteri is mostly greyish-white (2).

The brown booby is found worldwide in tropical oceans, but is sparsely distributed or absent in the Indian Ocean east of the Seychelles (4). Subspecies Sula leucogaster leucogaster occupies the Caribbean and tropical Atlantic; Sula leucogaster brewsteri is found in the north-east tropical Pacific; Sula leucogaster plotus occurs in the central Pacific, Red Sea and west Indian Ocean; and.Sula leucogaster etesiaca inhabits the central-eastern Pacific (2).

The brown booby mostly feeds around inshore waters and nests on cliffs or slopes on bare rocky islands, or on flat coral atolls. Unlike most boobies, it can also be found nesting amongst vegetation, and will occasionally roost in trees (2). Outside the breeding season adult brown boobies are commonly found around the breeding colony, although some individuals, particularly juveniles and non-breeders, may be found soaring hundreds of miles from land (2) (3).

A gregarious species, the brown booby forms nesting colonies on oceanic islands, which are usually small in size, but may number thousands of pairs when food is abundant. The brown booby feeds on squid and various fish species that live near the sea-surface, such as flying fish and anchovies. Foraging is frequently carried out alone or in small groups, by flying over inshore waters while searching for shoals of small fish driven to the surface by marine predators. At other times, however, large groups of brown boobies may gather on rocky cliffs and shorelines waiting for gulls to locate such shoals, before joining the large aggregation of feeding seabirds. Techniques used to catch prey include diving at a shallow angle and snatching prey from the surface, as well as steep plunge dives from up to 20 metres above the water surface, reaching prey over two metres below (3). Some individuals also snatch unwary fish while floating on the surface, or steal food from other seabirds (2) (3).

While the brown booby is a seasonal breeder in some areas, it generally breeds throughout year, with peaks in nesting activity occurring seasonally according to the colony location (2) (3). After securing a territory, often by fighting, the male brown booby advertises for a mate by holding its head and neck erect, bill pointing upwards, in a display known as sky-pointing. Interested females are then courted through behaviours such as mutual preening and bill touching. Once formed, breeding pairs may remain together for many years, continually reusing the same nesting site. Nests comprise a shallow depression in the ground, lined with marine flotsam, feathers and vegetation. A clutch of two eggs is laid asynchronously, and incubated by both parent birds for around 43 days, with the birds using the webbing of the feet to transfer heat to the developing eggs. In most cases, the second chick to hatch is eventually pushed out of the nest by the older, stronger chick, where it perishes; hence only one offspring is usually raised to fledging. The complete juvenile plumage is obtained at around 100 days, allowing the young bird to make its first flight, but parent birds continue to provide food for a further 30 to 260 days. The brown booby usually breeds at an age of four to five years, and may live for over 25 years (3).

Historically the brown booby has been severely impacted by hunting for food, egg collection, and the introduction of non-native species such as rats, cats, goats and pigs (2). Fortunately, this species’ habit of nesting on small, isolated islands generally unfit for permanent habitation means that it has remained widespread and apparently relatively abundant (3). There are, however, some concerns that global populations numbers may be greatly overestimated and that this species could be experiencing a worrying decline (4).

While some brown booby colonies currently receive official protection, many more, particularly in the Caribbean, Central America, and Indonesia, are completely unprotected and in urgent need of legislative action to prevent further disturbance and habitat destruction (5).

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  1. IUCN Red List (July, 2014)