|Austropacific masked booby (S. d. personata) with chick (background)|
The masked booby (Sula dactylatra) is a large seabird of the booby family, Sulidae. First described by French naturalist René-Primevère Lesson in 1831, the masked booby is one of six species of booby in the genus Sula. This species breeds on islands in tropical oceans, except in the eastern Atlantic; in the eastern Pacific it is replaced by the Nazca booby, Sula granti, which was formerly regarded as a subspecies of masked booby. It is also called the masked gannet or the blue-faced booby.
The masked booby nests in small colonies, laying two chalky white eggs on sandy beaches in shallow depressions. The first chick usually kills the second one.
French naturalist René-Primevère Lesson described the masked booby in 1829 in Louis Isidore Duperrey's work Voyage autour du Monde, Exécuté par Ordre du Roi, Sur la Corvette de Sa Majesté, La Coquille, pendant les années 1822, 1823, 1824 et 1825, after encountering it in Ascension Island in the South Atlantic. The species name is from the Ancient Greek word dactyl "finger" and Latin ater "black". "Black fingers" refers to the splayed wingtips in flight. Swedish zoologist Carl Jakob Sundevall described it as Sula cyanops in 1837, the species name derived from the Ancient Greek words cyanos "blue", and ops "face".
John Gould described Sula personata in 1846 from Australia, the species name being the Latin adjective personata "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 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 their 17th supplement to their checklist in 1920.
"Masked booby" has been designated the official name by the International Ornithologists' Union (IOC). It has also been called masked gannet, blue-faced booby, white booby (for its plumage), and whistling booby (for its distinctive call).
The masked booby is one of six species of booby in the genus Sula. A genetic study using both nuclear and mitochondrial DNA showed the Masked and Nazca boobies to be each other's closest relatives, their lineage diverging from a line that gave rise to the blue-footed and Peruvian boobies. The masked and Nazca boobies were divergent enough to indicate 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.
There is a clinal change in size across its range, where birds in the Atlantic are the smallest, with the size increasing westwards to the eastern Pacific, where the largest individuals are found. Four subspecies are recognized by the IOC.
- S. d. personata Gould, 1846: Austropacific masked booby
- Breeds in the central and western Pacific and around Australia, as well as off Mexico and on Clipperton Island. 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 valid.
- S. d. dactylatra Lesson, 1831: Atlantic masked booby
- Breeds in the Caribbean and some Atlantic islands including Ascension Island. It has recently started breeding off Tobago, formerly being known in this area only from a single sight record from an oil rig off Trinidad.
- S. d. melanops Hartlaub, 1859: Western Indian Ocean masked booby
- Breeds in the western Indian Ocean. Hartlaub described this taxon in 1859 from Maydh Island off the coast of Somalia near Maydh. 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 mela(no)s "black", and ops "face".
- S. d. tasmani (including S. d. fullagari) van Tets, Meredith, Fullagar & Davidson, 1988: Tasman booby or Lord Howe masked booby
- The form breeding on Lord Howe and the Kermadec Islands. New Zealand naturalist Walter Oliver had noted that this bird had dark brown rather than pale irises since 1930, but it was not until 1990 that it was formally investigated by O'Brien and Davies and found to have longer wings than other populations as well. They classified it as a new subspecies—S. d. fullagari—in 1990. Meanwhile, large prehistoric specimens known from the former and Norfolk Island were classified as a separate species—S. tasmani—in 1988, thought to have become extinct from Polynesian and then European seafarers and settlers. However, Holdaway and colleagues cast doubt on the distinctness of the fossil taxon in 2001, and a 2010 review by 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 it 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.
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 almost wholly bright white with a dark face mask. The sexes have similar plumage, and there is no seasonal variation. The bill is yellow while the bare skin around the face, thoat and lores is black. The wings and tail are brown-black, the individual feathers are dark with white bases and shafts. The underwing is white.
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. Full adult plumage is acquired three to four months before the bird turns three years old.
The masked booby is silent at sea, but has a reedy whistling greeting call at the nesting colonies. While on the breeding grounds, these birds display a wide range of hissing and quacking notes.
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 Dampier Arch. In the Pacific, it ranges from Brisbane eastwards. It is found in the Caribbean and Atlantic Ocean south to Ascension Island.
The masked booby is a fairly sedentary bird, wintering at sea, but rarely seen far away from the breeding colonies. However, Caribbean birds occasionally wander north to warm southern Gulf Stream waters off the eastern seaboard of the United States. More remarkably, there have been three western Palaearctic records of masked booby, presumably dactylatra, all from Spanish waters, although one of these also entered French territorial areas.
The masked booby generally flies at least 7 m (23 ft) in height, and at speeds of up to 70 km/h (43 mph). It alternates between gliding and active flying with strong periodic wingbeats.
The masked booby nests in small colonies, laying two chalky white eggs on sandy beaches in shallow depressions, with an interval of five to eight days between the laying of each egg. Occasionally nests with three eggs are reported; these are due to an egg from another nest rolling downhill into the nest.
The eggs are incubated by both adults for 45 days.
In most cases, the first chick will kill its smaller, weaker sibling after it hatches. Siblicide has been well studied in this species; researchers such as David Anderson have demonstrated that while the boobies can manage to feed two chicks if siblicide is prevented, they do so at a steep penalty to health and future reproductive success.
Compared to other species of boobies such as the blue-footed booby, siblicide is obligatory in the masked booby. One reason is because the masked boobies build very shallow flat nests, so older chicks can expel their younger siblings with relative ease. Blue-footed booby parents, meanwhile, build nests with steeper sides, thus preventing some older chicks from engaging in siblicidal behaviour.
The masked booby is a spectacular diver, plunging vertically or near-vertically from heights of anywhere from 12 to 100 m (but more commonly 15 to 35 m) above the water into the ocean at high speed, to depths of up to 3 m (120 in).
Fish, particularly flying fish, up to 28 cm (11 in) long (rarely up to 41 cm (16 in)) form the bulk of its diet, along with cephalopods. Species eaten include yellowtail amberjack (Seriola lalandi), skipjack tuna (Katsuwonus pelamis), various species of flying fish, mullet of the genus Mugil, and others.
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