Masked Booby

Identificazione

The largest species of booby, the masked booby ranges from 75 to 85 cm (30 to 33 in) long, with a 160–170 cm (63–67 in) wingspan and 1.2–2.2 kg (2.6–4.9 lb) weight. It has a typical sulid body shape, with a long pointed bill, long neck, aerodynamic body, long slender wings and pointed tail. The adult is bright white with dark wings and a dark tail. The sexes have similar plumage with no seasonal variation, but females are on average slightly heavier and larger than males. The bare skin around the face, throat and lores is described either as black or blue-black. It contrasts with the white plumage and gives a mask-like appearance. The bill of the nominate subspecies is pale yellow with a greenish tinge, sometimes greyish at the base. Conical in shape, the bill is longer than the head and tapers to a slightly downcurved tip. Backward-pointing serrations line the mandibles. The primaries, secondaries, humerals and rectrices are brown-black. The inner webs of the secondaries are white at the base. The underwing is white except for the brown-black flight-feathers that are not covered by the white coverts. The legs are yellow-orange or olive. The iris is yellow.<p>The subspecies differ slightly in size and sometimes also in the colour of the irises, bill, legs and feet. The race melanops has an orange-yellow bill and olive-grey legs, the race tasmani has dark brown irises and dark grey-green legs and the race personata has olive to blueish-grey legs. For the subspecies tasmani and the nominate dactylatra, during the breeding season, the leg colour of male birds contains more yellow-red than those of the females.<p>The juvenile is a streaked or mottled grey-brown on the head and upperparts, with a whitish neck collar. The wings are dark brown and underparts are white. Its bill is yellowish, face is blue-grey and iris a dark brown. Older immature birds have a broader white collar and rump, and more and more white feathers on the head until the head is wholly white by 14 to 15 months of age. Full adult plumage is acquired three to four months before the bird turns three years old.<p>The masked booby is usually silent at sea, but is noisy at the nesting colonies. The main call of male birds is a descending whistle; that of females is a loud honk.<p>The adult masked booby is distinguished from the related Nazca booby by its yellow rather than orange bill, larger size and less distinctive sexual dimorphism. The latter nests on steep cliffs rather than flat ground. The white morph of the red-footed booby is similar but smaller. Abbott's booby (Papasula abbotti) has a more wholly black upperwing, and a longer neck and tail and larger head, while the Cape gannet (Morus capensis) and the Australasian gannet (Morus serrator) have a buff-yellow crown, shorter tail, white humerals and a grey rather than yellowish bill. The juvenile masked booby resembles the brown booby (Sula leucogaster), though adults of that species have clearly demarcated brown and white plumage.

Tassonomia

The French naturalist René Lesson was a member of the crew on the La Coquille, captained by Louis Isidore Duperrey, on its voyage around the world undertaken between August 1822 and March 1825. In the multi-volume publication by Duperrey about the voyage, Lesson authored the ornithological sections. In his 1829 account of the visit to Ascension Island in the South Atlantic Ocean, Lesson mentioned encountering masked boobies, and in a footnote proposed the binomial name Sula dactylatra. Lesson subsequently provided a formal description of the masked booby in 1831. The specific epithet combines the Ancient Greek δάκτυλος (dáktul), meaning 'finger', and the Latin ater, meaning 'black'. "Black fingers" refers to the splayed wingtips in flight. The Swedish zoologist Carl Jakob Sundevall described the species as Dysporus cyanops in 1837 from a subadult collected in the Atlantic Ocean on 6 September 1827. The species name was derived from the Ancient Greek words κύανος (kúanos), meaning 'blue', and ὄψ (óps), meaning 'face'.<p>The English ornithologist and bird artist John Gould described Sula personata in 1846 from Australia, the species name being the Latin adjective personata, meaning 'masked'. Gould adopted the name Sula cyanops in his 1865 Handbook to the Birds of Australia. Sundevall's binomial name was followed as Lesson's 1829 record did not sufficiently describe the species; however, in 1911, the Australian amateur ornithologist Gregory Mathews pointed out that although Lesson's 1829 account did not describe the bird, his 1831 account did, and thus predated Sundevall by six years, and hence Sula dactylactra had priority. The American Ornithological Union followed in the 17th supplement to their checklist in 1920.<p>"Masked booby" has been designated the official common name by the International Ornithologists' Union (IOC). The species has also been called the masked gannet, blue-faced booby, white booby (for its plumage), and whistling booby (for its distinctive call). The Australian ornithologist Doug Dorward promoted the name "white booby" as he felt the blue coloration of its face was less prominent than that of the red-footed booby (Sula sula).<p>The masked booby is one of six species of booby in the genus Sula. A 2011 genetic study (depicted below) using both nuclear and mitochondrial DNA showed the masked and Nazca boobies (Sula granti) to be each other's closest relatives, their lineage diverging from a line that gave rise to the blue-footed (Sula nebouxii) and Peruvian boobies (Sula variegata). The masked and Nazca boobies were divergent enough to indicate that the latter, formerly regarded as a subspecies of the former, should be classified as a separate species. Molecular evidence suggests they most likely diverged between 0.8 and 1.1 million years ago. Complex water currents in the eastern Pacific may have established an environmental barrier leading to speciation. Subfossil bones 14,000 years old belonging to the species have been found in deposits on St. Helena Island.<p>There is a clinal change in size across the masked booby's range. Birds in the Atlantic are the smallest, with the size increasing westwards though the Pacific to the Indian Ocean, where the largest individuals are found. Genetic analysis using mtDNA control region sequences shows that populations in the Indian and Pacific Oceans greatly expanded around 180,000 years ago, and that these became separated from Atlantic populations around 115,000 years ago. Furthermore, within each ocean, there is evidence of reduced gene flow between populations that does not correspond with any physical barrier.<p>Four subspecies are recognized by the International Ornithologists' Union.

Nome scientifico

Sula dactylatra

Autorità

Lesson, RP, 1831

Ordine

Suliformi

Famiglia

Sulidae

Genere

Sula

Areale e habitat

The masked booby is found across tropical oceans between the 30th parallel north and 30th parallel south. In the Indian Ocean it ranges from the coastlines of the Arabian Peninsula and East Africa across to Sumatra and Western Australia, though it is not found off the coast of the Indian subcontinent. Off the Western Australian coastline it is found as far south as the Dampier Archipelago. In the Pacific, it ranges from Brisbane eastwards. It is found in the Caribbean and Atlantic Ocean south to Ascension Island. In the eastern Pacific off the coast of Colombia and Ecuador, the masked booby is replaced by the Nazca booby. A vagrant was rescued in 2015 in Newport, Oregon.<p>In the Atlantic, Caribbean birds occasionally wander north to warm southern Gulf Stream waters off the eastern seaboard of the United States, with single records from Island Beach in New Jersey and New York. There are summer records from Delaware Bay, and Worcester County, Maryland, as well as waters off the coast of Spain.<p>During the monsoon season (midyear), the masked booby is an occasional vagrant along the western coast of India, with records from Kerala, Karnataka, and Maharashtra states. It is a vagrant to the Caroline Islands north of New Guinea.<p>Breeding colonies are located on remote islands, atolls and cays. Lord Howe Island is the southernmost colony. Deep water nearby is important for feeding. As an example, waters around Raine Island, at the edge of the Great Barrier Reef, are anywhere from 180 to 3,700 m (590 to 12,140 ft) deep. On these landforms, masked boobies select sites of generally flat, bare or exposed open ground that lie above the high-tide level with access to the ocean. During the breeding season, the species remains near the colony. At other times, juveniles and some adults disperse widely, though some remain at the colony year-round. Most (but not all) birds return to breed at the colony of their birth; once they begin breeding at a site, they will return there annually.<p>The largest masked booby colony is on Clipperton Island in the eastern Pacific Ocean, a desert atoll southwest of Mexico. In 2003, 112,000 birds were counted, having recovered from 150 individuals in 1958. The population had suffered from the introduction of feral pigs in the 1890s. These pigs preyed on the crabs that ate the vegetation. After the elimination of pigs in 1964, the crab population rose and vegetation largely disappeared. This was beneficial to the boobies, as they prefer open ground. Clipperton is on a narrow ridge surrounded by deep water. The colony on Lord Howe Island numbered in the thousands at the time of the island's discovery in 1788, but has declined to under 500 pairs—mostly on offshore islets with the remainder on two hard-to-access headlands—by 2005. Hunting by humans is thought to have played a role; although rats were introduced to the island in 1918, there has been no evidence they are able to kill chicks or eggs—possibly due to the size of the adult boobies. The masked booby was first recorded breeding on Philip Island off Norfolk Island in 1908, with devegetation by feral animals creating the open ground preferred by the species. By 2007, an estimated 300 pairs were breeding over the island, though the island flora's regeneration after the removal of feral animals might begin to limit suitable nesting sites. In 2006, two pairs nested in a brown booby colony on Morros del Potosí (White Friars Rocks) near Zihuatanejo in southern Mexico.<p>Major nesting areas in the Atlantic include Rocas Atoll off the coast of Brazil, Ascension Island in the south Atlantic, and five islands of the Campeche Bank in the Gulf of Mexico. The species attempted to nest at Dry Tortugas in the Gulf of Mexico over 1984 and 1985; 19 pairs were recorded there in 1998.

Migrazione

Specie stanziale

Comportamento

Comportamento migratorio: Specie stanziale.

Conservazione

Tendenza della popolazione: in diminuzione.

Tendenza della popolazione

in diminuzione