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Boobies and Gannets (Sulidae)

Boobies and gannets

(Sulidae)

Class Aves

Order Pelecaniformes

Suborder Pelecani

Family Sulidae


Thumbnail description
Medium to large-sized seabirds with long, narrow, pointed wings and conical bill, highly adapted to catching fish by plunge-diving, often from great heights

Size
25–39 in (64–100 cm); 1.5–7.9 lb (0.7–3.6 kg)

Number of genera, species
3 genera; 9 species

Habitat
Mainly pelagic waters (open seas), breeding on offshore islands

Conservation status
Critically Endangered: 1 species; Vulnerable: 1 species

Distribution
Widespread in tropical, subtropical, and temperate oceans

Evolution and systematics

Boobies and gannets constitute a distinct family of specialist plunge-divers that are most closely related to the cormorants and anhingas (Phalacrocoracidae). They probably originated in the late Cretaceous, more than 60 million years ago. Boobies seem to have appeared first, while the gannets probably split off at a later stage, about 16 million years ago, and developed in the Northern Hemisphere. Available evidence suggests that they also occupied the North Pacific, where no gannets are currently found.

As of 2001, three genera are recognized (although some authors still keep to the classical grouping of all species under the single genus Sula): the genus Papasula comprises one species, the distinct Abbott's booby, possibly the most ancient of today's sulids. All remaining boobies are grouped under the genus Sula, whereas the gannets have been separated and classified in the genus Morus. This system reflects differences in morphology, biology, and ecology among the species, although their high degree of adaptation to the marine environment results in them sharing many characteristics.

Physical characteristics

Boobies and gannets have long, pointed wings and a characteristic cigar-shaped body. This body is medium to large in size, robust, and ends in a fairly long, wedge-shaped tail. The neck is long and thick, with strong, well-developed muscles. The head is dominated by the stout, conical bill. The bare skin around the neck and bill is often brightly colored, and plays an active role in ritual displays. The eyes are placed at each side of the bill and are orientated towards the front, giving the birds excellent binocular vision, which is essential for active fishing from the air.

Like most fish-eating birds, boobies and gannets are predominantly light colored in the underparts, particularly the belly, but also the neck, head, and underwings. The upper-parts, especially the wings, are most often dark. The white-colored underparts blend in against the brighter sky, thus rendering the predator less visible for the prey fish. The dark pigment or melanin in flight feathers protects them from ultraviolet light and salt. In some species (e.g. gannets) the white color, clearly visible from a great distance, attracts large numbers of birds to a feeding source, often a big school of fish. This may give the gannets an advantage: by attacking simultaneously in large numbers and therefore confusing the prey, the birds are able to feed on a shoal that normally is too large for a single individual to exploit efficiently.

As a further adaptation to their specialized fishing technique, boobies and gannets have subcutaneous fat and well-developed air sacs, which act as cushions and protect the birds from the violent impact of crashing into the water. For the same reason, their external nostrils are closed. Sulids also lack brood patches, which would be disadvantageous in a cold aquatic environment. Instead, they incubate by sitting on their "heels" and wrapping highly vascularized webbing of their feet around the eggs. The short and stout legs are situated far back on their bodies, allowing the birds to swim well and to maintain buoyancy even in rough seas.

Distribution

The family Sulidae occurs across the world's oceans, although the distribution patterns are quite distinct. The gannets are typically found in cold or temperate waters of the northern Atlantic ocean, the South African region, and Australia and New Zealand. In the off-season, they disperse over lower latitudes in the same broad areas. A further group of species, the so-called pantropical boobies (masked, redfooted, and brown), are circumpolar in distribution, occurring over most of the world's oceans between the tropics. The Peruvian and blue-footed boobies are more specialized and occur only in the eastern Pacific, along the Humboldt current area, in the Galápagos Islands, and north to southern North America. The Critically Endangered Abbott's booby is confined to tiny Christmas Island, although it had been much more widespread over the Indian Ocean.

Habitat

Boobies and gannets live primarily at sea, an environment to which they are particularly well adapted so they do not need to set foot on land except during the breeding season. Boobies are found in tropical or subtropical waters, whereas gannets favor more temperate environments, occurring even north of the Arctic Circle. Brown and blue-footed boobies often feed in inshore waters, while red-footed and Abbott's take probably the longest foraging trips and are known to occur several hundred kilometers from the nearest land.

A variety of sites are used for breeding, although this occurs almost invariably on offshore islands and on rocky outcrops. Several species place nests directly on exposed, flat ground, others choose to do so on cliffs. The third most common nesting habitat is on top of tall tropical trees or, alternatively, on the lower scrub of oceanic islands.

Behavior

The social behavior of boobies and gannets is complex and has been the subject of various studies, most notably by J. B. Nelson. All species nest colonially in quite high densities, which has favored the development of ritualized displays, particularly to indicate site-ownership or to obtain a mate. These are most developed in aggressive species, such as the northern gannet, that live in packed-in colonies, often on cliffs or other unstable surfaces. By resorting to ritualized behavior, sulids avoid the risks implicit in physical squabbles.

Feeding ecology and diet

Boobies and gannets are highly specialized fish-eaters. They prey mostly on mobile, schooling fish that frequent open waters, such as mackerel, whiting, pilchard, and anchovy. In tropical waters, flying-fish and squid are also frequent prey. A certain degree of opportunism is common in the family; northern gannets often follow trawlers searching for fish discards.

In order to catch their prey, sulids typically plunge-dive from great heights above the water (generally from 33 to 100 ft [10–30 m], but up to 330 ft [100 m] has been recorded). Once they have located their prey, they close their wings and plunge vertically, head first, into the water. Just before entering the water, they extend their wings backwards alongside their body and thus achieve a torpedo-like shape which probably assists them in reaching a greater depth. Once under water, they may use their wings to penetrate even deeper, perhaps up to 82 ft (25 m). Prey is generally caught on the way up and is usually swallowed underwater, thus avoiding the harassment of more opportunistic feeders such as frigate-birds or gulls.

Reproductive biology

Many species, particularly of gannets, pair for life and reunite annually at the nest-site, having spent the off-season individually at sea. In that context, pair-bonding displays play an important role and can take up a significant part of the

breeding season. They generally consist of exaggerated movements of the head and neck, and also of the whole body, the wings, feet, and tail. Boobies make extensive use of their wings and feet in displays, most notably the blue-footed with its unique aerial greeting, in which the incoming bird salutes with outstretched feet just as it is about to land.

Not all species are strictly seasonal. The pantropical boobies, in particular, time their breeding attempts to local conditions and food availability. Often in the same colony there may be pairs which are at different breeding stages. However, the three gannets and Abbott's and Peruvian boobies time their nesting seasons to make them coincide with the best weather and the most productive conditions at sea, both during the chick-rearing period and immediately after chicks have fledged. This increases the chances of juvenile survival when the environment is highly seasonal.

Breeding density is moderate to very high; all sulids tend to group in colonies which they sometimes share with other

species. Nesting colonies are particularly dense in the case of gannets, especially when located on flat ground. In those cases, nests are spaced regularly, the distance between nests being determined by the maximum length that two neighboring birds can reach with their bills while seated.

Booby and gannet nests are quite rudimentary, especially those of ground-nesting species. Often, they consist only of a slight depression or an accumulation of debris, glued together with the birds' excreta. The tree-nesting red-footed and Abbott's boobies build slightly more elaborate nests, usually a platform of sticks on one of the upper branches.

Most species lay single-egg clutches, as only one chick can be raised successfully in most environments. Only the Peruvian and blue-footed boobies, living in the exceptionally rich waters of the Humboldt current, can expect to raise more than one chick successfully and thus lay three and two eggs respectively. Other species, such as the pantropical masked and brown boobies, often lay two eggs, but subsequently reduce their brood through sibling aggression, the older chick killing its younger sibling and so ensuring that the strongest one receives all of the limited food resources.

Incubation lasts 41–45 days in most species, although Abbott's booby extends its incubation period to an average 57 days. Both sexes contribute in long stints (12–60 hours) and no feeding occurs between the breeding adults. The eggs, which are incubated by wrapping the webbed feet around them, have unusually thick shells.

The chick is born naked and is continuously guarded for the first month, until it can regulate its own body temperature. It is fed on fish remains directly from the parent's mouth and often has to reach into the parent's throat. Chicks grow rapidly, particularly so in the more seasonal gannets which soon attain the adult birds' weight or even go beyond it. In those species, however, no post-fledging care occurs and when young reach fledging age they are left unattended until they jump out to sea on their own. Booby chicks take much longer to fledge (up to five months in Abbott's) and are fed for quite a lengthy period after they fledge.

Conservation status

In its 2000 assessment of the threatened status of the world's birds, BirdLife International classed Abbott's booby as Critically Endangered and the cape gannet as Vulnerable, according to IUCN standards. The booby has its breeding range confined to Christmas Island in the Indian Ocean, and has suffered a considerable reduction of its distribution within historic times. The population on Christmas Island has declined in the past due to habitat destruction through forest clearance. The introduction in the late 1990s of an alien ant species Anoplolepis gracilipes is predicted to cause a rapid decline through predation on nestlings, habitat alteration, and farming of scale insects that damage the trees. The cape gannet is classified as Vulnerable due to having only six breeding colonies. This renders the species at risk from both natural disasters and human-caused hazards. Among the latter are the risk of an oil spill (one incident in 1993 killed 5,000 gannets) and the more worrying collapse of the sardine fishery in Namibia, formerly the stronghold of the species, which has caused the species to decline severely.

The remaining species are not considered to be globally threatened or in danger of extinction but their continuation depends on conservation of their nesting sites and the overall marine environment. Tourism and the overexploitation of fish stocks may adversely affect several, if not all, species of boobies and gannets. Effective protection of offshore islands where sulids place their colonies is an essential part of any conservation program for these species.

Significance to humans

Over the centuries, humans have exploited boobies and gannets, their eggs and chicks, for food. The birds were at one time an important source of protein for certain local communities in the northern Atlantic. The fact that the birds breed in quite large numbers on islands that are generally accessible has probably contributed to their exploitation by humans. This still occurs throughout the tropics, although perhaps to a lesser degree as environmental education has begun to influence human behavior. Many species have seen their numbers artificially being kept to quite low levels for centuries, and have started to recover only in the last few decades. The Peruvian booby, one of the three "guano birds," has suffered severely from direct disturbance and habitat alterations in the past, during times of the intensive exploitation of guano for agricultural purposes, which continued until well into the first part of the twentieth century.

Species accounts

List of Species

Abbott's booby
Northern gannet
Cape gannet
Australasian gannet
Blue-footed booby
Peruvian booby
Masked booby
Red-footed booby
Brown booby

Abbott's booby

Papasula abbotti

taxonomy

Sula abbotti, Ridgway, 1893, Assumption Island. Monotypic.

other common names

French: Fou d'Abbott; German: Abbott-Tölpel; Spanish: Piquero de Abbott.

physical characteristics

31 in (79 cm); 3.2 lb (1.46 kg). Distinctive shape with long, narrow wings. White underparts, neck and head; upperparts dark-brown. Bill slightly hooked and highly serrated, pinkish in female, blue-gray tinged pink in male.

distribution

Breeding currently confined to Christmas Island (Indian Ocean) from where it disperses widely for foraging. Formerly more widespread across Indian Ocean, east to western Pacific.

habitat

Strictly marine and pelagic. Nesting is restricted to tall forest trees in central plateau of Christmas Island. Foraging area not precisely known, but frequently seen in rich upwelling area off Java, often well away from nearest land.

behavior

Nesting site on trees high above ground affects territorial and pair behavior, so Abbott's boobies' displays are the least fervent of all sulids. Territorial disputes are unknown in this species and even chick begging behavior is moderate in comparison with that of its congeners.

feeding ecology and diet

Not precisely known. Thought to prey mostly on flying-fish and squid. Forages well away from nesting island and presumably feeds by plunge-diving like other members of the family.

reproductive biology

Fairly seasonal (laying in May through July.) but only a biennial breeder when successful. Very low reproductive success, mainly due to coincidence of breeding season with monsoons. Nest is platform of twigs and sticks high above ground; only loosely colonial. Lays only one egg, which is incubated for 57 days (longest of all sulids). Chicks fledge at 140–175 days. Post-fledging care period is also very long: 162–280 days. Does not breed until four to six years old.

conservation status

Critically Endangered (BirdLife International, 2000), due to extremely reduced breeding range and small, declining population of only 2,500 active pairs as of 2000. Most highly threatened by ecological alterations on breeding island caused by introduced yellow crazy ant (Anoplolepis gracilipes), which is known to prey on chicks, as well as to kill the red crab (Gecaroidea natalis), and to farm scale insects which damage the trees. A control program for this ant has been initiated. In the past, mining of Christmas Island for phosphate extraction has reduced nesting habitat significantly. Destruction of rainforest on former breeding islands is thought to have caused their extirpation.

significance to humans

None known. Abbott's booby's secretive habits have resulted in few interactions between this species and humans.


Northern gannet

Morus bassanus

taxonomy

Pelecanus Bassanus, Linnaeus, 1758, Bass Rock, Scotland. Monotypic.

other common names

English: (North) Atlantic gannet; French: Fou de Bassan; German: Basstölpel; Spanish: Alcatraz Atlántico.

physical characteristics

34.3–39.4 in (87–100 cm); 5.1–7.9 lb (2.3–3.6 kg); wingspan 65–70.9 in (165–180 cm). Largest of sulids, a strong bird with mainly a strikingly white plumage. Compared with other gannets, bill is slightly stouter and head is paler cream. Juveniles mainly dark brown, gradually gaining white feathers of adult plumage.

distribution

Exclusively in the north Atlantic, where breeds on both sides 46–72° north. More widespread on eastern side, where in winter also enters the Mediterranean Sea and disperses south to subtropical waters. On western side, breeds on islands off Newfoundland and in the Gulf of St. Lawrence (Canada) and disperses south in winter to the Gulf of Mexico.

habitat

Strictly marine, mainly in waters over the continental shelf. Breeds on cliffs on offshore islands or, more rarely, on mainland.

behavior

Breeds in dense colonies where aggressiveness and intense social behavior have given way to complex repertoire of stereo-typed displays. Breeding birds acquire a nest-site, which they then defend against intruders and maintain from year to year. Pair behavior is equally complex and linked to the nest-site. At sea, often occurs in groups particularly congregating around rich feeding sources but with little interaction.

feeding ecology and diet

Feeds on shoaling pelagic fish like herring (Clupea), mackerel (Scomber) and sprat (Sprattus), also sandeels (Ammodytes). Makes spectacular plunge-dives from great heights. Also regularly attends trawlers.

reproductive biology

Highly seasonal, starting March through April. Forms large colonies on cliffs or on flat ground, where builds large nest of seaweed, grass, etc. and a significant amount of excreta. Lays one egg only, incubated by both parents for 44 days. Chick fledges at 90 days; on its own, after it has been deserted by parents. Does not breed until four to five years old.

conservation status

Not threatened. Abundant and widespread throughout its range. Protection of breeding sites and cessation of former direct exploitation of chicks (for food) led to significant recovery over most of twentieth century. Overexploitation of fisheries remains an important threat; also suffers some degree of incidental mortality at sea.

significance to humans

Chicks used to be taken for food in some local communities, a practice that still continues in a few places (e.g., Sula Sgeir, off Scotland). Also present in literature and art. Nowadays colonies may constitute important sources of income locally, as tourist activities are developed around them.


Cape gannet

Morus capensis

taxonomy

Dysporus capensis, Lichtenstein, 1823, Cape of Good Hope. Monotypic.

other common names

English: African gannet; French: Fou du Cap; German: Kaptölpel; Spanish: Alcatraz del Cabo.

physical characteristics

33.5–35.4 in (85–90 cm); 5.7 lb (2.6 kg). Slightly smaller than northern gannet, wings show black wingtips and secondary feathers; tail feathers also black. Black gular stripe much longer than in the other gannets; head darker cream than in northern gannet. Juveniles dark, gradually acquiring adult plumage.

distribution

Breeds coasts of South Africa and Namibia. Disperses north along African coasts, to the Gulf of Guinea in the Atlantic and to Mozambique, exceptionally to Kenya, in the Indian Ocean.

habitat

Strictly marine, mainly in waters of the continental shelf. Nests on flat offshore islands.

behavior

Much as in northern gannet although much less aggressive and site competition less intense, despite nesting in very dense colonies on flat ground. Similarly, sexual behavior and pair-bonding displays are more moderate.

feeding ecology and diet

Feeds mostly on shoaling pelagic fish, particularly pilchard (Sardinops), anchovy (Engraulis), saury (Scomberesox) and mackerel (Scomber). Feeds by plunge-diving from 66 ft (20 m)

above water. Also forms large concentrations attending trawlers.

reproductive biology

Highly seasonal, September through April. Nests in very dense colonies on flat ground, where nests consist of accumulation of debris with central depression. Lays one egg, exceptionally two. Incubation lasts 44 days. Young fledges at 97 days. Does not start breeding until three to four years old.

conservation status

Vulnerable. Only six breeding colonies known. Population has undergone important reductions in the past and, in latter part of twentieth century, has been further reduced through overexploitation of fish stocks, particularly in Namibia. Oil pollution and mortality caused by fishing gear are also known to take a heavy toll.

significance to humans

In the past, heavily exploited for food and for fish-bait. The cape gannet is one of the guano birds, its colonies being used to extract the fertilizer until well into the twentieth century.


Australasian gannet

Morus serrator

taxonomy

Pelecanus serrator, G. R. Gray, 1843, from Sula australis, Gould 1841 (preoccupied), Tasmania. Monotypic.

other common names

French: Fou austral; German: Australtölpel; Spanish: Alcatraz Australiano.

physical characteristics

33.1–35.8 in (84–91 cm); 5.2 lb (2.35 kg); wingspan 63–66.9 in (160–170 cm). Resembles cape gannet but is slightly smaller, has white outer tail feathers and blue orbital ring is more intensely colored. Juveniles dark, gradually acquiring adult plumage.

distribution

Breeds coasts of New Zealand, Tasmania, and Australia. Disperses over those waters and along both coasts of Australia, reaching as far as Tropic of Capricorn.

habitat

Strictly marine, occurs mostly over continental shelf. Breeds on offshore islets.

behavior

Much as in cape gannet, which it most closely resembles. Compared to northern gannet, less aggressive and not so competitive over nest-sites. Also complex but not so intense sexual behavior and pair-bonding displays.

feeding ecology and diet

Feeds mostly on shoaling pelagic fish, especially pilchard (Sardinops), anchovy (Engraulis), and jack mackerel (Trachurus). Feeds by plunge-diving. Also attends trawlers, where large numbers may concentrate.

reproductive biology

Highly seasonal, October through May. Nests in rather small but dense colonies. Builds rough nest of accumulated seaweed

and grass, cemented together with excreta. Lays one egg, exceptionally two. Incubation lasts 44 days. Young fledges at 102 days. Does not start breeding until five to six years old.

conservation status

Not threatened. During twentieth century, population gradually recovered from earlier heavy persecution although some colonies (e.g. Tasmania) continued to decline markedly during second part of the century. Total world population is smallest of all gannets and species still suffers some degree of direct exploitation (eggs and chicks). Sometimes caught accidentally during fishing activities.

significance to humans

Breeding colonies have been traditionally raided for eggs and chicks. Species present in indigenous folklore in New Zealand. Currently some tourist activities are being developed around nesting colonies.


Blue-footed booby

Sula nebouxii

taxonomy

Sula nebouxii, Milne-Edwards, 1882, Chile. Two subspecies recognized, S. n. nebouxii Milne-Edwards, 1882 and S. n. excisa Todd, 1948.

other common names

French: Fou à pieds bleus; German: Blaufusstölpel; Spanish: Piquero Camanay.

physical characteristics

29.9–33.1 in (76–84 cm); wingspan 59.8 in (152 cm). Upper-parts generally dark brown; underparts white. Distinctive blue feet. Iris pale. Female averages larger. Race excisa appears larger and paler.

distribution

Continental coasts of east Pacific Ocean, from northwest Mexico in north to Peru in south (S. n. nebouxii) and Galápagos Islands (S. n. excisa).

habitat

Strictly marine. Frequents cool, rich waters in areas of upwelling, often close inshore. Breeds along rocky coasts, on cliffs and islets with little or no vegetation.

behavior

Spectacular and elaborate pair-bonding displays, both aerial and on ground, where blue feet play important role.

feeding ecology and diet

Active fisher, preying mostly on sardine (Sardinops), anchovy (Engraulis), and mackerel (Scomber); also flying-fish (Exocoetus). Highly gregarious, often feeds in quite large groups, plunge-diving in unison. May fish with other species. Often feeds in shallow water.

reproductive biology

Not markedly seasonal. Usually nests on ground, sometimes also on vegetation. Forms large colonies where nest is mere circle of accumulated excreta around a slight depression. Lays two eggs on average (one to three), which are incubated for 41 days. Chicks fledge at 102 days and afterwards are cared for 56 days on average. First breeds at two to three years of age.

conservation status

Not threatened. World population quite small but quite abundant locally. Suffers predation from alien predators, at least in Galápagos. Most breeding sites currently protected.

significance to humans

Subject to exploitation for food in the past.


Peruvian booby

Sula variegata

taxonomy

Dysporus variegatus, Tschudi, 1843, islands off Peru. Monotypic.

other common names

English: Variegated booby; French: Fou varié; German: Guanotölpel; Spanish: Piquero Peruano.

physical characteristics

28–29.9 in (71–76 cm). Smaller version of blue-footed booby, which it most closely resembles. Lacks blue feet and averages paler, with white on head and neck. Upperwing and back are mottled white.

distribution

Exclusive to Humboldt current area of Pacific, breeding from northern Peru to central Chile. Occurs in Ecuador.

habitat

Strictly marine. Found quite close inshore, where it feeds in cool, rich waters of upwelling zones. Breeds on rocky islets and on cliff ledges.

behavior

Behaviorally, the Peruvian booby most closely resembles the blue-footed although its displays are somewhat more moderate. It breeds in densely packed colonies, yet site-tenancy is less intense than in other species.

feeding ecology and diet

Almost an exclusive feeder on anchoveta (Engraulis ringens) when this was abundant. After stocks were depleted in 1970s

and 1980s, resorted to sardine (Sardinops), mackerel (Scomber), and other fish. Feeds by plunge-diving in groups of 30–40, often more, individuals.

reproductive biology

Only moderately seasonal, September through February in Peru, much later in Chile. Breeds in immense colonies where nests consist of loose pile of seaweed and debris, stuck together with excreta. Lays three eggs on average (one to four), which are incubated for 42 days. Chicks fledge at 78–105 days and are cared for a further 62 days on average. First breeding occurs at two to three years of age.

conservation status

Not threatened. Has undergone significant reductions in the past (through direct exploitation, disturbance at breeding colonies, and depletion of fish stocks) but is somewhat stable at present. However, always subject to risk by regular El Niño phenomena, which can cause severe mortality of both adults and young.

significance to humans

One of the main guano birds, together with Guanay cormorant (Phalacrocorax bouganvillii) and Peruvian pelican (Pelecanus thagus). Subject to intense disturbance at breeding colonies and to direct exploitation of eggs and chicks up to middle twentieth century; practice is perhaps still maintained at a smaller scale although nesting sites are legally protected.


Masked booby

Sula dactylatra

taxonomy

Sula dactylatra, Lesson, 1831, Ascension Island. Five subspecies generally recognized: S. d. dactylatra, Lesson, 1831; S. d. melanops, Heuglin, 1859; S. d. personata, Gould, 1846; S. d. fullagari, O'Brien and Davies, 1990; S. d. granti, Rotschild, 1902.

other common names

English: Blue-faced booby, white booby; French: Fou masqué; German: Maskentölpel; Spanish: Piquero Enmascarado.

physical characteristics

31.9–36.2 in (81–92 cm); wingspan 59.8 in (152 cm). Largest of all boobies, body feathers mostly white; flight and tail feathers black. Bare parts mostly dark, bill usually yellow in males, duller in females. Females average slightly larger in size.

distribution

Pantropical, race dactylatra occurs in Caribbean and Atlantic; melanops in west Indian Ocean; personata in east Indian Ocean and central Pacific; fullagari in north Tasman Sea; granti in east Pacific.

habitat

Strictly marine and fairly pelagic, prefers more offshore waters than other booby species. Nests on bare ground and cliffs on rocky offshore islands.

behavior

Breeds in less dense colonies than other boobies. Accordingly, defends nest site less tenaciously and whole behavior is less aggressive. Much territorial behavior is based on ritualized displays. Pair-bonding behavior is also less intense than in other species.

feeding ecology and diet

Feeds mostly on shoaling fish, especially flying-fish, which it catches by plunge-diving from great heights. Feeds farther offshore than other species, also taking larger prey.

reproductive biology

Only loosely colonial, very simple nest of accumulated excreta on cliff, slope or flat ground. Usually lays two eggs; brood size subsequently reduced to one chick through sibling aggression. Incubates eggs for 44 days. Chick fledges at 120 days and is further cared for another 156 days. Does not breed until two to three years old.

conservation status

Not threatened. Much widespread and locally abundant, total population may number several hundred thousand individuals. Known to have undergone some declines locally, particularly as a consequence of predation by introduced animals. Eggs and chicks also taken for food locally. Booming tourist industry may pose further threat.

significance to humans

Subject to a moderate degree of exploitation for food, perhaps also for fish-bait. Some breeding colonies may be of interest for local tourist industry.


Red-footed booby

Sula sula

taxonomy

Pelecanus Sula, Linnaeus, 1766, Barbados, West Indies. Three subspecies generally recognized: S. s. sula, Linnaeus, 1766; S. s. rubripes, Gould, 1838; S. s. websteri, Rotschild, 1898.

other common names

French: Fou à pieds rouges; German: Rotfusstölpel; Spanish: Piquero Patirrojo.

physical characteristics

26–30.3 in (66–77 cm); 1.9–2.2 lb (0.9–1.0 kg); wingspan 35.8–39.8 in (91–101 cm). Smallish, polymorphic sulid. Some individuals mostly white, with only flight feathers black (tail remains white in most plumages); others are wholly brown, with flight feathers always looking darker. Feet and cere around bill reddish in most plumages. Females average slightly larger.

distribution

Pantropical, race sula occurs in Caribbean and southwest Atlantic Ocean, rubripes in tropical west and central Pacific and also Indian Ocean, websteri in east Pacific.

habitat

Strictly marine and largely pelagic, feeding largely offshore. Nests on offshore islands with abundant vegetation.

behavior

Repertoire of ritualized displays more moderate than in other species, adapted to breeding habitat on trees. Role of bright-red feet largely unknown.

feeding ecology and diet

Feeds mostly offshore, preying on flying-fish and squid. Catches prey by plunge-diving from considerable height, but also takes flying-fish in flight. Partially nocturnal habits.

reproductive biology

Not seasonal, may start breeding in any month. Highly colonial, builds nest of sticks on top of tree or bush. Lays one egg, incubated for 45 days. Chick fledges at 100–139 days, later cared for 190 days. First breeds at two to three years old.

conservation status

Not threatened. Widely scattered, reasonably large population. Subject to direct exploitation and disturbance, most important threat comes from destruction of nesting habitat.

significance to humans

Traditionally exploited for food over much of its range.


Brown booby

Sula leucogaster

taxonomy

Pelecanus Leucogaster, Boddaert, 1783, Cayenne. Four subspecies recognized: S. l. leucogaster, Boddaert, 1783; S. l. plotus; J. R. Forster, 1844; S. l. brewsteri, Goss, 1888; S. l. etesiaca, Thayer and Bangs, 1905.

other common names

English: White-bellied booby; French: Fou brun; German: Weissbauchtölpel; Spanish: Piquero Pardo.

physical characteristics

25.2–29.1 in (64–74 cm); 1.6–3.4 lb (0.7–1.6 kg); wingspan 52–59.1 in (132–150 cm). Wholly dark, except for white belly. Color of head and bare parts varies with race. Females average slightly larger.

distribution

Pantropical, race leucogaster occurs in Caribbean and tropical Atlantic, plotus in Red Sea and west Indian Ocean east to central Pacific, brewsteri in northeast tropical Pacific, etesiaca in central east Pacific.

habitat

Strictly marine, feeding mostly in inshore waters. Nests on cliffs, slopes, or bare ground on offshore islands or coral atolls.

behavior

Ample repertoire of ritualized displays, including some aerial elements. Rather aggressive on breeding grounds.

feeding ecology and diet

Feeds close to the shore mostly on flying-fish and squid caught by plunge-diving from lower heights, often at an oblique angle. Uses feet and wings for underwater propulsion. Also commonly feeds on the wing, catching flying-fish or harassing other birds.

reproductive biology

Only locally seasonal. Forms colonies on flat ground or among vegetation. Nest is only a small depression, sometimes lined with grass. Lays two eggs but brood size subsequently reduced through sibling aggression. Incubation lasts 43 days. Chick fledges at 85–105 days, then cared for a further 118–259 days. Does not breed until two to three years old.

conservation status

Not threatened. Numerous and widespread, though numbers significantly reduced in historic times through direct exploitation. Locally threatened with alien predators, tourist development, and lack of protection at nest-sites.

significance to humans

In the past, widely taken for food and fish-bait. Such practices still persist in some areas. Due to presence of widely scattered breeding colonies, may give rise to incipient tourist activities in places.


Resources

Books

Barnes, K. N., ed. The Eskom Red Data Book of Birds of South Africa, Lesotho and Swaziland. Johannesburg: BirdLife South Africa, 2000.

BirdLife International. Threatened Birds of the World. Barcelona: Lynx Edicions; and Cambridge, United Kingdom: Bird Lifse International, 2001.

del Hoyo, J., A. Elliott, and J. Sargatal, eds. Handbook of the Birds of the World. Vol. 1, Ostrich to Ducks. Barcelona: Lynx Edicions, 1992.

Garnett, S. T., and G. M. Crowley. Revised Action Plan for Australian Birds. Canberra: Environment Australia and Birds Australia, 2001.

Harrison, J. A., D. G. Allan, L. G. Underhill, M. Herremans, A. J. Tree, V. Parker, and C. J. Brown, eds. The Atlas of Southern African Birds. Vol. 1, Non-passerines. Johannesburg: BirdLife South Africa, 1997.

Nelson, J. B. The Sulidae: Gannets and Boobies. Oxford: Oxford University Press, 1978.

Nelson, J. B. The Gannet. London: T. & A. D. Poyser, 1978.

Organizations

BirdLife International. Wellbrook Court, Girton Road, Cambridge, Cambridgeshire CB3 0NA United Kingdom. Phone: +44 1 223 277 318. Fax: +44-1-223-277-200. E-mail: [email protected] Web site: <http://www.birdlife.net>

Carles Carboneras

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