The red-tailed tropicbird (Phaethon rubricauda) is a tropicbird, one of three closely related species of seabird of tropical oceans. Superficially resembling a tern in appearance, it has amost all-white plumage with a black mask and a red bill. Most adults have red that are about two times their body length, which gives rise to its common name. Pieter Boddaert described the species in 1783. There are four subspecies recognized, though there is evidence there is a clinal change with smaller birds in the north and larger in the south (and hence no grounds for any subspecies).
Phaethon phoenicuros Gmelin 1789
Nesting takes place in loose colonies on oceanic islands, the nest itself a scrape found on a cliff face. This bird is considered to be a least-concern species according to the International Union for Conservation of Nature (IUCN).
The red-tailed tropicbird was described by Dutch naturalist Pieter Boddaert in 1783, from Mauritius. The genus name is derived from Ancient Greek phaethon, "sun" while the species epithet comes from the Latin words ruber "red" and cauda "tail". English ornithologist John Latham wrote about the red-tailed tropicbird in 1785 in his General Synopsis of Birds, recording it as common in Mauritius and the South Pacific. He also reported a black-billed tropicbird collected from Palmerston Island that ended up in the collection of British naturalist Sir Joseph Banks. Latham did not give them binomial names, however. It was left to German naturalist Johann Friedrich Gmelin to describe the species, which he did as Phaeton phoenicuros and P. melanorhynchos respectively in the 13th edition of Systema Naturae in 1788. Latham later described this black-billed specimen as the New Holland tropicbird, also described as Phaethon novae-hollandiae.
Walter Rothschild reviewed the described names and specimens in 1900 and concluded that the original use of P. erubescens lacked a description and was hence a nomen nudum. He concluded populations of Lord Howe, Norfolk and Kermadec Islands belonged to a distinct form that he named P. rubicauda erubescens and was larger overall with a more robust bill and prominent reddish tinge to its plumage. He also classified P. melanorhynchus and P. novae-hollandiae as juveniles. Gregory Mathews then applied the name P. rubicauda roseotinctus to Rothschild's P. rubicauda erubescens.
"Red-tailed tropicbird" has been designated the official name by the International Ornithologists' Union (IOC). Other common names include red-tailed bos'nbird or silver bos'nbird, the name derived from the semblance of the tail feathers to a boatswain's marlin spikes, and strawtail. The Maori called it amokura, tawake, ko’ae or ’ula, while the Hawaiians call it koa'e’ula.
Four subspecies are recognized by the IOC:
- P. r. rubricauda Boddaert, 1783 the nominate subspecies, from the western Indian Ocean. Subsequent specimens from the Cocos-Keeling Islands were allocated to this taxon.
- P. r. westralis Mathews, 1912 from the eastern Indian Ocean. Mathews described it as separate on account of its larger wings. More extensive analysis in 1989 showed that the wing and beak size overlap between this and the nominate subspecies, leaving intensity of colour as the only possible distinguishing feature.
- P. r. roseotinctus (Mathews, 1926) from the southwestern Pacific Ocean, including populations on Kermadec, Lord Howe, Norfolk and Raine Islands.
- P. r. melanorhynchos Gmelin, 1789 from the western, central and southern Pacific Ocean, including populations on The Cook Islands, Tonga, Samoa, the Marquesas and Society Islands.
Ornithologist Mike Tarburton reviewed the known subspecies and concluded that none were valid, noting that there was a clinal change in size in the species, with those from Kure Atoll in the North Pacific being the smallest ranging to those from the Kermadec Islands in the South Pacific being the largest. He also noted that the pink coloration was more intense in new plumage and faded after a few years in museum specimens.
The red-tailed tropicbird measures 95 to 104 cm (37 to 41 in) on average, which includes the 35 cm (14 in)-long , and weighs around 800 g (28.2 oz). It has a wingspan of 111 to 119 cm (44 to 47 in). It has a streamlined but solid build with almost all-white plumage, often with a pink tinge. A dark brown comma-shaped stripe extends back from the lores, through and over the eyes and reaching the ear coverts. The iris is dark brown. The bill is bright red, slightly paler at the base and black around the nostrils. The legs and base of the toes are pale blue-mauve, while the webbing and rest of the toes are black. The white feathers of the head and rump have concealed dark brown bases, while those of the mantle, back, tail retrices and tail coverts have dark brown shaft bases. The two long tail feathers are orange or red with white bases for around a tenth of their length, and can be hard to see when the bird is flying. The white wings are marked by dark chevron-shaped patches on the tertials, and the dark shafts of the primary feathers are visible. The pink tinge is often more pronounced in the remiges of the uipper wing.
Newborn chicks are covered in thin long grey-white down, which is paler on the head. The lores are bare. The down is greyer in older chicks. The primaries, retrices and scapulars are evident in the third week, and chicks are mostlyfeathered with residual down on underparts and under the wings after six weeks, and fully feathered by 11 weeks. Juvenile birds have a glossy white forehead, chin, throat and underparts, and prominent black barring and scaling on their crown, nape, mantle, back, rump and upper wing coverts. Their bills are blackish grey with a light blue-grey base, and grey legs and feet.
In Australian waters it could be confused with the silver gull or various tern species, though is larger and heavier-set, with a wedge-shaped tail. Its red bill and more wholly white wings distinguish it from the adult white-tailed tropicbird. Immature red-tailed tropicbirds likewise can be distinguished from immature white-tailed tropicbirds by their partly red rather than yellow bills.
Distribution and habitatEdit
The red-tailed tropicbird ranges across the southern Indian, and western and central Pacific Oceans, from the African coast, breeding the Seychelles, Mauritius, Madagascar, Indonesia, the waters around the southern reaches of Japan, Fiji, New Caledonia, French Polynesia, the Cook Islands, Pitcairn Island to Chile, and the Hawaiian Islands, where they are more common on the northwestern islands, In Madagascar they nest on the tiny island of Nosy Ve. In Australia, they nest on Queensland's coral islands (including Raine Island and Lady Elliot Island), and Ashmore Reef and Rottnest Island off Western Australia, as well Sugarloaf Rock at Cape Naturaliste and Busselton on the Western Australian coastline itself, and the offshore territories of the Cocos (Keeling) Islands, Christmas, Norfolk and Lord Howe islands. In New Zealand territory they breed on the Kermadec Islands.
They frequent areas of ocean with water temperatures from 24 to 30 C and salinity under 35% in the southern hemisphere and 33.5% in the northern hemisphere. In the Pacific Ocean, the southern boundary of their range runs along the 22 C summer surface isotherm. The warm waters of the Leeuwin Current facilitate the species nesting at Cape Leeuwin in southwestern Australia, yet is only a rare visitor to New South Wales at corresponding latitudes on the Australian east coast.
It is an occasional visitor to Palau, with breeding recorded from the Southwest Islands, and were first recorded from Guam in 1992. It is an uncommon vagrant to New Zealand proper, where it has been recorded from the northern reaches of North Island, especially Three Kings Islands.It is a very rare vagrant to North America, with records from California and Vancouver.
They disperse widely after breeding, birds ringed in Hawaii have been recovered as far away as Japan and the Philippines. Strong winds can blow them inland on occasions, which explains some sighting records away from the coast and their preferred habitats.
The red-tailed tropicbird is thought to be monogamous, with pairs remaining bonded over successive breeding seasons, although information such as age at first breeding and pair-formation is not known. It nests in loose colonies, on offshore islands and stacks, rocky cliffs, coral atolls and cays. It rarely nests on large bodies of land, though has done so in southern Western Australia. The nest itself is a shallow scrape, in either shaded sand or a rocky crevice. Because this birds does not walk well, and enters its nest by stalling into the wind, falling, and walking to its nest, the nest is often located within one metre (3.3 ft) of the edge of the shrub (or other shaded area) for easy access. Additionally, the tropicbird often chooses shrubs with less stems, also for accessibility.
The species is territorial to a degree, aggressively defending the nest site and pecking radius around it, commencing around three months before breeding. Birds are more aggressive at crowded colonies, where numbers are large or suitable nest sites less common. They adopt a defence posture, which consists of raising the humeri up and bringing the wrists together, drawing the neck into the body and shaking the head sideways, fluffing up the head feathers and squawking. Bill-jabbing and fights can break out, the two combatants locking bills and wrestling for up to 90 minutes.
In the leadup to breeding, males initiate an aerial courtship display of flying in large circles, alternating between gliding, short periods of rapid wing-beating, and low flight within a few metres of the water, while making sharp cackling calls. Initially flying in small groups, birds then pair off to repeat the display in pairs before bonding. Once pairs have established a nest, they do not perform the display
The timing of breeding depends on location; in some places, birds breed in a defined breeding season, whereas in others, there is none. South of the equator, the latter is likely to be true. On islands near the equator, laying usually occurs from June to November, with the majority of chicks fledging around January to February. Some birds may remain at the breeding site year-round.
This bird lays a clutch of one egg, which both parents incubate for 42 to 46 days. Ranging from 5.4 to 7.7 centimetres (2.1 to 3.0 in) long (averaging between 6.3 and 6.8 centimetres (2.5 and 2.7 in), depending on location) and 4.5 to 4.8 centimetres (1.8 to 1.9 in) wide, the oval eggs are pale tan with brown and red-black markings that are more prominent on the larger end. Born helpless and unable to move around (nidicolous and semi-altricial), the chicks are initially blind, opening their eyes after 2–3 days. Until they are a week old, they only open their beak upon touch, so the parents have to stroke the base of the bill to initiate feeding. Feeding takes place once or twice a day, generally around midday. They are constantly brooded by the parents until they are a week old, after which time they are sheltered under the parent's wings. They also rise up and gape at any nearby bird for food. Both parents feed the young. Initially covered with grey or white down, they grow their first feathers—scapulars—at 16–20 days. Their feet and beaks grow rapidly, outpacing the rest of their bodies. Chicks remain in the nest for 67 to 91 days until they fledge.
Vagrant red-billed tropicbirds have been implicated in egg loss of nests in Hawaii.
The red-tailed tropicbird is mostly a , diving anywhere from 6 to 50 metres (20 to 164 ft) above the water. Flying fish, although, is sometimes caught in the air. Birds remain briefly submerged—one study on Christmas Island coming up with an average time of 26.6 seconds—generally swallowing their prey before surfacing.
During incubation, foraging trips are relatively long, with an average excursion taking about 153 hours. These trips are to very productive areas. After the chicks hatch, on the other hand, the parents adopt a strategy where one takes long trips (these averaging about 57 hours) for self feeding, and the other takes short trips (about three hours long) to feed the chicks. The bimodality of the length of foraging trips is likely to be because of the fact that it is the optimal balance of self feeding and provisioning for chicks. On Christmas Island, birds generally forage far out to sea in the early morning and closer to shore in the afternoon.
Squid and flying fish make up a large portion of this bird's diet, in addition to some crustaceans, depending on location. Fieldwork in the Mozambique Channel revealed the diet of birds there to be mostly fish by mass but equal numbers of fish and squid caught. Fish recorded include the mirrorwing flyingfish (Hirundichthys speculiger) and spotfin flyingfish (Cheilopogon furcatus) and several other unidentified species of the flying fish family Exocoetidae, the pompano dolphinfish (Coryphaena equiselis) and common dolphinfish (C. hippurus), needleflish including the houndfish (Tylosurus crocodilus), and unidentified members of Hemiramphidae, Scombridae, and Carangidae. The purpleback flying squid (Sthenoteuthis oualaniensis) was by far the most common cephalopod eaten, followed by the common blanket octopus (Tremoctopus violaceus). A field study in Hawaii found flyingfish dominated the prey species, with the tropical two-wing flyingfish (Exocoetus volitans) and members of the genus Cypselurus) prominent, followed by squid of the family ommastrephidae including the purpleback flying squid and the glass squid (Hyaloteuthis pelagica), and carangid fish including the shortfin scad (Decapterus macrosoma). The red-tailed tropicbird has also been recorded eating porcupinefish (Diodontidae), although adults have been troubled when the victim fish inflates resulting in it being urgently regurgitated.
A strong flyer with large mouth and bill, the red-tailed tropicbird can carry relatively large prey for its size, with parent birds commonly bearing dolphin fish that weighed 120 g—16% of their own weight—to their chicks.
When incubating during the day in a shaded nest, this bird has an average temperature of 39 °C (102 °F), compared to its average temperature when incubating at night of 37.1 °C (98.8 °F). The difference is likely due to activity levels, as the air temperature during these times does not differ significantly with a bird in the nest. After flying, the average body temperature is 40.9 °C (105.6 °F). The temperature of the feet, although, is always lower than that of the body temperature during flight, but always high than the air temperature. Thus, the feet are likely used to dissipate heat during flight.
The large range and apparently stable population indicate that the red-tailed tropicbird is classified as a least-concern species according to the IUCN, with a range of up to 20 thousand square kilometres (7,700 sq mi), with an estimated 32,000 mature individuals. Human presence generally affects the species adversely, by destruction of habitat or introduction of pests. Within Australia, it is classified as near threatened, due to unexpected declines in some populations, the impact of humans, and the yellow crazy ant overruning Christmas Island. It is listed as vulnerable in New South Wales.
The red-tailed tropicbird nests on Johnston Atoll in the north Pacific Ocean, where the Johnston Atoll Chemical Agent Disposal System (JACADS) was burning stockpiled chemical weapons until 2000. It was studied over eight years to see if there were effects from potential contaminants. There appeared to be no impact on survival during the study period, although young birds from downwind of the plant were less likely to return there than those upwind of the plant—possibly due to the more intact vegetation at the latter site.
Yellow crazy ants were discovered on Johnston Atoll in 2010, hordes of which overrun nesting areas and can blind victims with their spray.
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