| Masked booby | |
|---|---|
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| Adult, subsp.personata,Midway Atoll | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | Suliformes |
| Family: | Sulidae |
| Genus: | Sula |
| Species: | S. dactylatra |
| Binomial name | |
| Sula dactylatra Lesson, 1831 | |
| Subspecies | |
See text | |
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| Range (in green) | |
Themasked booby (Sula dactylatra), also called themasked gannet or theblue-faced booby, is a largeseabird of thebooby andgannet family,Sulidae. First described by the French naturalistRené-Primevère Lesson in 1831, the masked booby is one of six species of booby in the genusSula. 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 75–85 cm (30–33 in) long, it is the largest species of booby. The sexes have similar plumage. This species ranges across tropical oceans, except in the easternAtlantic and eastern Pacific. In the latter, it is replaced by theNazca booby (Sula granti), which was formerly regarded as asubspecies of masked booby.
Nesting takes place incolonies, generally on islands andatolls far from the mainland and close to deep water required for foraging. Territorial when breeding, the masked booby performsagonistic 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 whitedown. The second chick born generally does not survive and is killed by its elder sibling. These birds are spectacular plungedivers, plunging into the ocean at high speed in search of prey—mainlyflying fish. The species faces few threats; although its population is declining, it is considered to be aleast-concern species by theInternational Union for Conservation of Nature (IUCN).
The French naturalistRené Lesson was a member of the crew on theLa Coquille, captained byLouis Isidore Duperrey, on its voyage around the world undertaken between August 1822 and March 1825.[2] In the multi-volume publication by Duperrey about the voyage, Lesson authored the ornithological sections. In his 1829 account of the visit toAscension Island in theSouth Atlantic Ocean, Lesson mentioned encountering masked boobies, and in a footnote proposed thebinomial nameSula dactylatra.[3] Lesson subsequently provided aformal description of the masked booby in 1831.[4] Thespecific epithet combines theAncient Greekδάκτυλος (dáktul), meaning'finger', and the Latinater, meaning'black'. "Black fingers" refers to the splayed wingtips in flight.[5] The Swedish zoologistCarl Jakob Sundevall described the species asDysporus cyanops in 1837[6] from asubadult collected in the Atlantic Ocean on 6 September 1827.[7] The species name was derived from the Ancient Greek wordsκύανος (kúanos), meaning'blue',[8] andὄψ (óps), meaning'face'.[9]
The English ornithologist and bird artistJohn Gould describedSula personata in 1846 from Australia,[10] the species name being the Latin adjectivepersonata, meaning'masked'.[11] Gould adopted the nameSula cyanops in his 1865Handbook to the Birds of Australia.[12] Sundevall's binomial name was followed as Lesson's 1829 record did not sufficiently describe the species; however, in 1911, the Australian amateur ornithologistGregory 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 henceSula dactylactra hadpriority.[13] TheAmerican Ornithological Union followed in the 17th supplement to their checklist in 1920.[14]
"Masked booby" has been designated the officialcommon name by theInternational Ornithologists' Union (IOC).[15] The species has also been called the masked gannet, blue-faced booby, white booby (for its plumage), and whistling booby (for its distinctive call).[5] 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 thered-footed booby (Sula sula).[16]
The masked booby is one of six species of booby in the genusSula.[15] A 2011 genetic study (depicted below) using bothnuclear andmitochondrial DNA showed the masked andNazca boobies (Sula granti) to be each other's closest relatives, their lineage diverging from a line that gave rise to theblue-footed (Sula nebouxii) andPeruvian boobies (Sula variegata). The masked and Nazca boobies were divergent enough to indicate that the latter, formerly regarded as asubspecies 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 tospeciation.[17] Subfossil bones 14,000 years old belonging to the species have been found in deposits onSt. Helena Island.[18]
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There is aclinal 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.[19] Genetic analysis usingmtDNA 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 reducedgene flow between populations that does not correspond with any physical barrier.[20]
Four subspecies are recognized by the International Ornithologists' Union.[15]
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 typicalsulid 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.[30] The sexes have similar plumage with no seasonal variation, but females are on average slightly heavier and larger than males.[31] The bare skin around the face, throat andlores is described either as black or blue-black. It contrasts with the white plumage and gives a mask-like appearance.[32] The bill of thenominate subspecies is pale yellow with a greenish tinge, sometimes greyish at the base.[19] Conical in shape, the bill is longer than the head and tapers to a slightly downcurved tip. Backward-pointingserrations line the mandibles.[33] Theprimaries,secondaries,humerals andrectrices 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 whitecoverts.[34] The legs are yellow-orange or olive.[19] The iris is yellow.[35]
The subspecies differ slightly in size and sometimes also in the colour of the irises, bill, legs and feet. The racemelanops has an orange-yellow bill and olive-grey legs, the racetasmani has dark brown irises and dark grey-green legs and the racepersonata has olive to blueish-grey legs.[19] For the subspeciestasmani and the nominatedactylatra, during the breeding season, the leg colour of male birds contains more yellow-red than those of the females.[28][36]
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,[30] 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.[35]
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.[37]
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.[29] The whitemorph of the red-footed booby is similar but smaller.[19]Abbott's booby (Papasula abbotti) has a more wholly black upperwing, and a longer neck and tail and larger head, while theCape gannet (Morus capensis) and theAustralasian gannet (Morus serrator) have a buff-yellow crown, shorter tail, whitehumerals and a grey rather than yellowish bill. The juvenile masked booby resembles thebrown booby (Sula leucogaster), though adults of that species have clearly demarcated brown and white plumage.[30]
The masked booby is found across tropical oceans between the30th parallel north and30th 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 theDampier Archipelago. In the Pacific, it ranges from Brisbane eastwards. It is found in the Caribbean and Atlantic Ocean south to Ascension Island.[38] In the eastern Pacific off the coast of Colombia and Ecuador, the masked booby is replaced by the Nazca booby.[29] Avagrant was rescued in 2015 inNewport, Oregon.[39]
In the Atlantic, Caribbean birds occasionally wander north to warm southernGulf Stream waters off the eastern seaboard of the United States, with single records fromIsland Beach in New Jersey and New York.[40] There are summer records fromDelaware Bay,[41] andWorcester County, Maryland,[42] as well as waters off the coast of Spain.[43]
During the monsoon season (midyear), the masked booby is an occasional vagrant along the western coast of India, with records fromKerala, Karnataka,[44] andMaharashtra states.[45] It is a vagrant to theCaroline Islands north of New Guinea.[46]
Breeding colonies are located on remote islands, atolls and cays.[30] Lord Howe Island is the southernmost colony.[38] Deep water nearby is important for feeding. As an example, waters aroundRaine Island, at the edge of theGreat Barrier Reef, are anywhere from 180 to 3,700 m (590 to 12,140 ft) deep.[30] 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.[38] 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.[30]
The largest masked booby colony is on Clipperton Island in the eastern Pacific Ocean,[19] 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 offeral 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.[47] Clipperton is on a narrow ridge surrounded by deep water.[48] 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.[49] The masked booby was first recorded breeding onPhilip 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.[50] In 2006, two pairs nested in a brown booby colony on Morros del Potosí (White Friars Rocks) nearZihuatanejo in southern Mexico.[51]
Major nesting areas in the Atlantic includeRocas Atoll off the coast of Brazil,[52] Ascension Island in the south Atlantic,[53] and five islands of theCampeche Bank in theGulf of Mexico.[54][55] The species attempted to nest atDry Tortugas in the Gulf of Mexico over 1984 and 1985;[56] 19 pairs were recorded there in 1998.[57]
.jpg) French Frigate Shoals |
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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. It is often encountered alone, or in a small group when returning to its colony.[30]
Regarding the masked booby's longevity, a bird tagged atNepean Island (off Norfolk Island) in September 1979 was recovered and released after being caught in fishing gear 24 years and 9.9 months later some 713 km (443 mi) away off theIsle of Pines, New Caledonia in July 2004. The longest distance travelled is 3,152 km (1,959 mi); a bird tagged at Raine Island in theGreat Barrier Reef in December 1981 was picked up and released at Phillip Island (off Norfolk Island) in December 1986.[58]
The masked booby begins breeding by around four or five years of age, though can occasionally do so at three years old.[59] Adults formmonogamous relationships with many pairs remaining together over multiple breeding seasons. Highly territorial when nesting, single males and mated pairs engage inagonistic displays to mark their ground against neighbours and interlopers. The male advertises his territory to females byflight circuiting—making a short flight and holding his wings in a 'V' shape and making a call as he lands. The mated pair engages inoutposting as other boobies fly overhead, stretching their necks out and forward. More direct trespassers are confronted with ayes-no headshaking, in which the booby shakes its head from side to side or up and down and ruffles its head feathers to make its head look bigger and facial markings more prominent. It may cock its tail and hold its wings up away from its body.[60] Neighbouring boobies may escalate by jabbing and lunging at each other. In thepelican posture, a bird tucks the tip of its bill into its chest, possibly positioned to avoid injury to others. This posture is used against intruders or as advertising for a mate.[61]
There are several displays related to the establishment and maintenance of pair-bonding. The male initiatessky-pointing when a female approaches or leaves his territory. In this display, he paces slowly with his neck and bill pointed upwards—between vertical and 45 degrees—with wings partly raised and whistling faintly with an open bill. In agazing display, one bird stares at another of the opposite sex; this generally leads to other displays. Pairs engage in a (mostly) gentler form of jabbing display, andallopreening. In anoblique headshake, a bird flings its head vigorously. The male may also parade in front of the female, walking with an exaggerated high-stepping gait and intermittently tucking his head in his breast, after collecting nesting material and before the pair begins laying. The male presents small sticks and debris as nesting material in a gesture of symbolic nest-building, which leads to copulation. Afterwards, the pair engages in more symbolic nest-building.[61] The twigs and debris are cleared away later as none is actually used in adorning the nest while in use.[37]
Breeding takes place at different times of year throughout its range. On theCocos (Keeling) Islands, egg-laying takes place from January to July, peaking in June, with juvenile birds from April to December. On Moulter Cay in theCoral Sea, breeding takes place year-round, with egg-laying peaking from September to early November, while on nearby Raine Island birds begin laying in or after August, likely peaking September to early November.[37] Eggs are laid between May and September on Lord Howe Island,[49] and early July to early January (peaking in September) on Phillip Island.[50]
In the northern hemisphere, egg-laying onKure Atoll can be any time from January to early July, peaking in February and March.[62] On Clipperton Island, egg-laying peaks in November to coordinate with peak fish productivity of the surrounding waters in January (for growing chicks).[48] Masked boobies lay at any time in the Caribbean, peaking between March and September.[59]
The nest is a cleared area 0.75 to 1 m (2 ft 6 in to 3 ft 3 in) in diameter, within which is a clearly demarcated 25 to 30 cm (10 to 12 in) shallow (1–2 cm (0.4–0.8 in) deep) depression. A clutch of two chalky white eggs is laid, with an interval of five to eight days between the laying of each egg. Occasionally nests with three eggs are reported; these are probably due to an egg from another nest rolling downhill into the nest.[37] The eggs have an average size of 64 mm × 45 mm (2.5 in × 1.8 in) and weigh 75 g (2.6 oz).[37] They are incubated by both adults for 45 days.[49] Parents incubate the eggs by resting on theirtarsi and wrapping their webbed feet over the eggs, with the outermost toes resting on the ground. Their feet are more vascular at this time.[63] When first hatched, the chicks are about 10 cm (3.9 in) long and weigh around 40–60 g (1.4–2.1 oz), with a sparse covering of white down over their grey to pinkish-grey skin.Altricial andnidicolous, their eyes are open at birth. Their down thickens as they age, and the chicks are quite fluffy by week 5–6. The primaries and rectrices appear by week 8, andscapulars appear by week 10. They begin losing their down from week 12 onwards, until they are wholly covered by juvenile plumage by week 15 or 16, andfledge at around 120 days (17 weeks) of age.[34] After leaving the nest, young birds are dependent on their parents for 3–4 weeks before dispersing out to sea.[64][65]
Although two eggs are often laid, the younger chick almost always perishes within a few days. This has been observed widely across the species' range. Dorward suspectedsiblicide on Ascension Island.[66] Siblicide has been observed in the Nazca booby on theGalapagos Islands,[67] and is assumed to occur in the masked booby as well.[66][49]
The masked booby is a spectacular diver, plunging vertically or near-vertically from heights of anywhere from 12 to 100 m (40 to 330 ft)—but more commonly 15 to 35 m (50 to 115 ft)—above the water into the ocean at high speed, to depths of up to 3 m (9.8 ft) in search of fish.[68] It generally swallows its catch underwater.[69] Fieldwork at Clipperton Island showed that masked boobies flew on average to 103 km (64 mi) from their colony, with a maximum range of 242 km (150 mi), while feeding their chicks. They did not rest at sea at night, though part of their return trip was at night time for longer expeditions.[48] The masked booby forages with thewhite-bellied storm petrel (Fregetta grallaria) andBulwer's petrel (Bulweria bulwerii) at times.[70]Frigatebirds often harass the species until they disgorge their catch and steal their food.[19]
Fish, particularlyflying fish, up to 28 cm (11 in) long (rarely up to 41 cm (16 in)) form the bulk of its diet, along withcephalopods.[19] Species eaten include various species of flying fish such asblue flyingfish (Exocoetus volitans),mirrorwing flyingfish (Hirundichthys speculiger),sailfin flyingfish (Parexocoetus brachypterus),glider flyingfish (Cheilopogon atrisignis) andAtlantic flyingfish (Cheilopogon melanurus), other fish such asyellowtail amberjack (Seriola lalandi),skipjack tuna (Katsuwonus pelamis),mackerel scad (Decapterus macarellus),pompano dolphinfish (Coryphaena equiselis),mahi-mahi (Coryphaena hippurus),brown chub (Kyphosus bigibbus),redbarred hawkfish (Cirrhitops fasciatus),snake mackerel (Gempylus serpens),frigate tuna (Auxis thazard),Pacific saury (Cololabis saira),ribbon halfbeak (Euleptorhamphus viridis),flat needlefish (Ablennes hians) and mullet of the genusMugil, and thepurpleback flying squid (Sthenoteuthis oualaniensis).[69]
Silver gulls (Chroicocephalus novaehollandiae) andbuff-banded rails (Gallirallus philippensis) prey on eggs and young. On some islands such as Ascension and Saint Helena, feral cats have been a threat to masked boobies.[53][71] The tick speciesOrnithodoros (Alectorobius) muesebecki was described parasitising nesting blue-faced boobies off the Arabian coast.[72] Theargasid tickOrnithodoros capensis and theixodid tickAmblyomma loculosum have also been recorded as parasites, the latter possibly spreadingpiroplasmosis caused byBabesia among boobies.[73] On Raine Island and Pandora Cay, nests have been destroyed bygreen sea turtles (Chelonia mydas) as they pass through booby colonies and dig their own nests in large numbers.[74] Rats prey on eggs and young of many seabirds, though the size of masked boobies probably prevents direct predation. On Clipperton Island, rats prey on the crab that eats vegetation.[47]
TheTaíno ate masked and red-footed boobies that nested onGrand Turk Island around 1000 years ago. The two species subsequently vanished from the Turks and Caicos Islands. A booby yielded around 1–2 kg (2–5 lb) of meat. European sailors in the area also caught and ate tame boobies.[75] Masked booby young and eggs were eaten by the crew ofHMS Supply on Lord Howe Island.[25]
TheInternational Union for Conservation of Nature (IUCN) lists the masked booby as a species ofleast concern, though the population worldwide is decreasing.[1] At Clipperton Island, the colony was benefitted by the presence ofyellowfin tuna (Thunnus albacares), which drove their prey item—flying fish—to the surface, facilitating predation by boobies. It may be that overfishing of tuna adversely impacts the availability of fish there.[48] In 2005, 508 young masked boobies at the colony suffered from "angel wing", a congenital deformity of one or both wings resulting in flightlessness. This coincided with a season of high nestling mortality that was likely related to low numbers of yellowfin tuna due to possible overfishing at a crucial time in the breeding season.[76] The warm phase (El Niño) of theEl Niño–Southern Oscillation in 1982 and 1983 negatively impacted breeding on Christmas Island as the higher water temperatures reduced food supply. Where usually 1500 pairs nested, no young were observed over this period; 50–60 pairs were observed breeding in October 1983.[77] The Australian government has rated both subspecies occurring in Australian waters asvulnerable to climate change. The low-lying colonies of subspeciespersonata are at risk from rising sea levels, and the rising sea temperatures are calculated to reduce food productivity, which may impact on breeding success of both subspecies.[78]
