The masked booby (Sula dactylatra), also called the masked gannet or the blue-faced booby, is a large seabird of the booby and gannet family, Sulidae. First described by the French naturalist René-Primevère Lesson in 1831, the masked booby is one of six species of booby in the genus Sula. It has a typical sulid body shape, with a long pointed yellowish bill, long neck, aerodynamic body, long slender wings and pointed tail. The adult is bright white with black wings, a black tail and a dark face mask; at long, it is the largest species of booby. The sexes have similar plumage. This species ranges across tropical oceans, except in the eastern Atlantic and eastern Pacific. In the latter, it is replaced by the Nazca booby (Sula granti), which was formerly regarded as a subspecies of masked booby.
Nesting takes place in colonies, generally on islands and atolls far from the mainland and close to deep water required for foraging. Territorial when breeding, the masked booby performs agonistic displays to defend its nest. Potential and mated pairs engage in courtship and greeting displays. The female lays two chalky white eggs in a shallow depression on flat ground away from vegetation. The chicks are born featherless, but are soon covered in white down. The second chick born generally does not survive and is killed by its elder sibling. These birds are spectacular plunge divers, plunging into the ocean at high speed in search of prey—mainly flying fish. The species faces few threats; although its population is declining, it is considered to be a least-concern species by the International Union for Conservation of Nature (IUCN). The masked booby primarily feeds on flying fish and squid, which it captures through high-speed plunge diving. Breeding Colonies are typically located on remote islands, where reduced predation allows for successful nesting.
Taxonomy 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. The title page gives the year as 1828 but the livraison containing page 494 was published in 1829 (See Cretella, 2010) Lesson subsequently provided a formal description of the masked booby in 1831. The specific epithet combines the Ancient Greek (), meaning , and the Latin , meaning . "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 6September 1827. The species name was derived from the Ancient Greek words (), meaning , and (), meaning .
The English ornithologist and bird artist John Gould described Sula personata in 1846 from Australia, the species name being the Latin adjective , meaning . 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.
"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).
Evolution 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.
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.
Subspecies Four subspecies are recognized by the International Ornithologists' Union.
S. d. dactylatra Lesson, 1831 Breeds in the Caribbean and some Atlantic islands including Ascension Island. There is significant genetic divergence between birds on Boatswain Bird Island off Ascension and those from Monito Island off Puerto Rico. S. d. melanops Hartlaub, 1859 Breeds in the western Indian Ocean. The German ornithologist Gustav Hartlaub described this taxon in 1859 from Maydh Island off the coast of Somalia near the town of the same name. He noted its black mask and blue-grey feet to be distinct from Sundevall's cyanops with a blue face, and Lesson's dactylatra with yellow feet. The subspecies name is derived from the Ancient Greek words (), meaning , and (), meaning . S. d. tasmani van Tets, Meredith, Fullagar & Davidson, 1988 (includes S. d. fullagari as a junior synonym): Tasman booby The form breeding on Lord Howe and the Kermadec Islands. The New Zealand naturalist Walter Oliver had noted that this bird had dark brown rather than pale irises in 1930, but it was not until 1990 that it was formally investigated by R. M. O'Brien and J. Davies and found to also have longer wings than other populations. They classified it as a new subspecies: S. d. fullagari. Meanwhile, large prehistoric specimens known from the Lord Howe and Norfolk Island had been classified as a separate species, S. tasmani, in 1988, thought to have become extinct due to Polynesian and then European seafarers and settlers. However, the paleoecologist Richard Holdaway and colleagues cast doubt on the distinctness of the fossil taxon in 2001, and a 2010 review by the New Zealand biologist Tammy Steeves and colleagues of the fossil material and DNA found the two overlapped considerably, and hence the extinct and living entities were found to be the same taxon, now known as S. d. tasmani as this name has priority over S. d. fullagari. Fieldwork in the Kermadec Islands indicates the bills of adults are bright yellow, and that adult males had brighter yellow feet than females. S. d. personata Gould, 1846 (includes S. d. californica and S. d. bedouti) Breeds in the central and western Pacific and around Australia, as well as off Mexico and on Clipperton Island in the Pacific Ocean off the coast of Central America. Birds of the latter two locations have been separated as subspecies californica, and the north west Australian population has been named as subspecies bedouti, but neither is usually considered distinct; the American biologist Robert Pitman and colleagues found no consistent differences between these three subspecies. The subspecies differ slightly in size and sometimes also in the colour of the irises, bill, legs and feet. The bill of the nominate subspecies S. d. dactylatra is pale yellow with a greenish tinge, sometimes greyish at the base. The S. d. melanops has an orange-yellow bill and olive-grey legs, the race tasmani has dark brown irises and dark grey-green legs and S. d. personata has olive to blueish-grey legs. During the breeding season, the leg colour of male S. d. dactylatra and S. d. tasmani contains more yellow-red than those of the females.
Description
The largest species of booby, the masked booby ranges from long, with a wingspan and 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. 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.
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.
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.
Similar species 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 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.
Distribution and 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.