|P. a. mesonauta, Little Tobago|
The red-billed tropicbird (Phaethon aethereus) is a tropicbird, one of three closely related seabirds of tropical oceans. A graceful bird resembling a tern in appearance, it has mostly white plumage with some black markings on the wings and back, a black mask and, as its common name suggests, a red bill. Most adults have tail streamers that are about two times their body length, with those in males being generally longer than those in females. It ranges across the tropical Atlantic, eastern Pacific and Indian Oceans, feeding on fish and squid. It was one of the many species described by Carl Linnaeus in his 1758 10th edition of Systema Naturae. Three subspecies are recognized. The subspecies mesonauta is distinguished from the nominate by the rosy tinge of its fresh plumage, and the subspecies indicus can be differentiated by its smaller size, more restricted mask, and more orangey bill.
Nesting takes place in loose colonies, the nest itself a scrape. A single egg is laid. In some areas, introduced black and brown rats raid nests for eggs and young. This bird is considered to be a least-concern species according to the International Union for Conservation of Nature (IUCN), though populations are thought to be declining. In some places, such as Brazil and Mexico, this bird is considered to be threatened.
Taxonomy and etymology
The red-billed tropicbird was one of the many bird species originally described by Linnaeus in the landmark 1758 10th edition of his Systema Naturae, and still bears its original scientific name, Phaethon aethereus. The genus name is derived from Ancient Greek phaethon, "sun" while the species epithet comes from Latin aetherius, "heavenly".
There are three subspecies, including the nominate, of this tropicbird:
- P. a. aethereus Linnaeus, 1758 – South Atlantic.
- P. a. mesonauta J. L. Peters, 1930 – East Pacific, Caribbean, and East Atlantic. In 1930, James L. Peters described this subspecies from an adult female specimen from Almirante Bay, Panama. He noted the greater primary coverts were more wholly black than those of the nominate taxon, which were edged with white. In the same paper, he described P. a. limatus from a specimen collected on the Galapagos Islands. He reported this taxon resembled P. a. mesonauta but had a yellower bill. Peters insisted this colour was not an artefact of preservation. The latter subspecies is not recognised as distinct.
- P. a. indicus A. O. Hume, 1876 – Persian Gulf, Gulf of Aden, Red Sea. Allan Octavian Hume wrote of this taxon after shooting a bird near Cherbaniani Reef off the Indian west coast. He noted that the Indian Ocean birds were smaller overall than the nominate subspecies, and tentatively classified it as a separate species.
The red-billed tropicbird is basal (the earliest offshoot) in the genus Phaethon, the tropicbirds. The split between this tropicbird and the other tropicbirds, the red-tailed and white-tailed tropicbird, is thought to have occurred about six million years ago.
This bird is called the red-billed tropicbird due to its red bill and its location in the tropicbird genus. An alternative common name was "bosun bird", also spelt "boatswain bird", from the similarity of its shrill call to a boatswain's whistle. An alternative derivation of the name is from the semblance of the tail feathers to marlin spikes. Local names used in the West Indies include "truphit", "trophic", "white bird", "paille-en-queue", "paille-en-cul", "flèche-en-cul", and "fétu".
The red-billed tropicbird measures 90 to 105 centimetres (35 to 41 in) on average, and without the tail streamers that measure 46 to 56 centimetres (18 to 22 in) long, this tropicbird measures about 48 centimetres (19 in). It has a wingspan of 99 to 106 centimetres (39 to 42 in). In overall appearance it is tern-like in shape. Its plumage is white, with black wing tips, and a back that is finely barred in black. It has a black mask that extends up from just above the lores to the sides of its nape, with grey mottling usually seen near the nape and hindneck. The tail has black shaft streaks, as do tail streamers. The underparts are white, with some black on the outermost primaries and tertials and occasionally with black markings on the flanks. The iris is blackish-brown, and the bill is red. The legs and feet are two colours, orangey-yellow and black; the orangey-yellow colour is found on the legs, base of the central toe, and parts of the outer toes, with the black colour being found everywhere else. Although the sexes are similar, the males are generally larger than females, with the tail streamers being longer on the male than on the female.
The subspecies of this bird can usually be distinguished by their difference in size and plumage. The subspecies Phaethon aethereus mesonauta can be differentiated by its slightly rosy tinge when its plumage is fresh, the bolder look of the black barring on the upperwing, and the more solid look of the black on the outer wing. The subspecies P. a. indicus can be distinguished by its smaller size, the fact that the mask on the face does not usually extend far behind the eye, and its more orange bill compared to the nominate.
When the chicks hatch, they are covered with grey down. This down is eventually cleared in about 40 to 50 days. The young chicks are usually missing their tail streamers. The juvenile looks similar to the adult with a mostly white crown. In the juvenile, the stripes above the eye usually are connected at the nape. The tail feathers usually have black tips or subterminal dots and without the tail streamers that are distinctive on the adult. Occasionally, a juvenile will have black markings on its flanks and undertail coverts.
The red-billed tropicbird can be identified by its red bill in combination with its white tail streamers. The slightly smaller red-tailed tropicbird has red rather than white tail streamers, and the white-tailed tropicbird can be differentiated by its smaller size, black stripe along its upperwing coverts, and its yellow-orange bill. Juvenile red-billed tropicbirds have ore heavily barred plumage on their upperparts than juveniles of other species. The royal tern can be confused with the adult red-billed tropicbird but can be distinguished by the former's less direct flight pattern and its lack of tail streamers.
P. a. mesonauta, chick Little Tobago
P. a. mesonauta, nesting, Little Tobago
P. a. mesonauta, in flight, Little Tobago
P. a. mesonauta, typical clumsy landing, Little Tobago
North Seymour Island, Galapagos Islands
P. a. indicus; illustration by Joseph Smit, 1898
This tropicbird moults once every year as an adult, following a complex basic strategy. This prebasic moult is completed before courtship and lasts between 19 and 29 weeks, with most being completed in 24 weeks. Birds gain their adult plumage at two to three years of age.
The red-billed tropicbird usually only calls near breeding colonies, where it joins in with groups of other adults, numbering from 2 to 20, in circling above the sea and making loud, harsh kreeeee-kreeeee-kri-kri-kri-kr screams. If disturbed at the nest, the chicks will vocalize a loud and piercing shriek, either rasping or reeling.
Distribution and habitat
The nominate subspecies P. aethereus aethereus breeds on islands in the Atlantic south of the equator, including Ascension, and St. Helena on the Atlantic Ridge, and Fernando Noronha and Abrolhos in Brazilian waters. It is a vagrant to the coastline of Namibia and South Africa.
P. aethereus aethereus is found across the neotropics. Within the West Indies, it is most common in the Lesser Antilles, Virgin Islands and small islands east of Puerto Rico. Breeding in the Western Palaearctic occurs on the Cape Verde Islands and attempted breeding on the Îles des Madeleines off Senegal. Total numbers in 2000 were probably less than 150 pairs. In the Pacific Ocean, it breeds from the Gulf of California and Revillagigedo Islands, Mexico in the north, to the Galapagos Islands, Isla Plata, Ecuador and San Lorenzo Island, Peru. Spear and Ainley estimated the minimum population of the Pacific at around 15,750 birds in 1995 after 15 years of observations.
The Indian Ocean subspecies, P. aethereus indicus is found in waters off Pakistan, western India and southwestern Sri Lanka.
Red-billed tropicbirds disperse widely when not breeding, in the Pacific reaching the 45th parallel north off Washington State and 32nd parallel south off Chile, with 19 records as of 2007 from Hawaii—some 4,300 kilometres (2,700 mi) from Mexico. It sometimes wanders further, including five records from Great Britain, and two from Australia: October-December 2010 on Lord Howe Island and September 2014 on Ashmore Reef. In July 2005, one was found in eastern New Brunswick, Canada, and another sighting at Matinicus Rock, Maine. Subspecies indicus is a rare but regular vagrant to the Seychelles.
The red-billed tropicbird can reach speeds of 44 kilometres (27 mi) per hour when flying out at sea. It is unable to walk easily on land, and requires a cliff, like those that it nests on, to take off without complications. Conversely it can lift off the water without much effort. Its plumage is waterproof and it floats on water.
Although it is a poor swimmer, the red-billed tropicbird feeds on fish and squid. The fish are usually small, between about 10 and 20 centimetres (3.9 and 7.9 in), although some caught are up to 30 centimetres (12 in). The aquatic prey is mostly caught by diving into the water from the air, although flying fish, the preferred fish of this species, are sometimes caught while in the air. Fish species eaten include Pacific thread herring (Opisthonema libertate), sharpchin flyingfish (Fodiator acutus), flyingfish of the genus Hirundichthys, sailfin flyingfish (Parexocoetus brachypterus), ornamented flying fish (Cypselurus callopterus), bigwing halfbeak (Oxyporhamphus micropterus), longfin halfbeak (Hemiramphus saltator), mackerel scad (Decapterus macarellus), shortjaw leatherjacket (Oligoplites refulgens), and mackerels (Scomber spp.).
The chicks are, like the adult, fed fish and squid. They are fed increasingly larger quantities of food by their parents. Most fish that the chicks are fed are below 6 centimetres (2.4 in) in length, although some fish fed to larger chicks can be up to 30 centimetres (12 in) in length.
This species of tropicbird usually forages alone. It usually dives into the water from the air, at heights up to 40 metres (130 ft). It will usually hover over the water before diving. Sometimes, this bird will work with predators that feed near the surface, such as dolphins or tuna. The red-billed tropicbird will feed on the fish driven either to or above the surface by the aforementioned predators. It usually forages in warmer waters, though does hunt in areas of cooler currents such as the Gulf of California. The species has also been recorded foraging in salt wedge estuaries.
The red-billed tropicbird usually nests on isolated cliff faces, in loose colonies. The nests are usually able to be flown out of directly. It uses a simple scrape nest. The age of first breeding is usually five years, although this age is variable; a three year-old tropicbird was once seen breeding. In some locations, breeding happens year-round, while in others, breeding occurs seasonally. Breeding is influenced by the availability of food, with an increase in food generally causing an increase in breeding. Individually, this bird only breeds every nine to twelve months. A breeding bird returns to its partner and nest location from the previous breeding cycle.
Courtship and pairing usually lasts three to five weeks, during which this bird performs aerial courtship displays to potential mates. The courtship displays include flying in the air which takes the form of gliding interspersed with short periods of rapid wing-beating. At nest sites, battles sometimes occur between two or more pairs before the original owners declare themselves as the owner of the nest. This tropicbird is aggressive at nest sites.
This tropicbird usually lays a clutch of one white buff to pale purple egg with reddish-brown spots. The egg usually measures 45 by 60 millimetres (1.8 by 2.4 in) and weighs around 67 grams (2.4 oz)—10% of the adult female's weight. It is incubated by both sexes for 42 to 46 days. If the egg cannot survive the first few days in the nest, the female will usually lay a replacement egg. The chicks that hatch eventually fledge in about 10 to 15 weeks after hatching, although most fledge after about 80 to 90 days. Normally, the maximum weight of the chicks is about 725 grams (1.6 lb), but on years that are hotter than average, this can drop to about 600 grams (1.3 lb). Highly aggressive, red-tailed tropicbirds fight with each other and oust species such as shearwaters, petrels, and white-tailed tropicbirds from nest sites. They have also been recorded taking over nests of white-tailed tropicbirds and raising their young if they failed to destroy their eggs. Vagrant red-billed tropicbirds have been implicated in egg loss in red-tailed tropicbird nests in Hawaii.
Chicks are constantly brooded by the parents until they are 3 to 5 days old, when they can thermoregulate their body temperature. They are attended by the parents more between the 30th and 60th days; a behaviour possibly related to the greater food requirements of the chicks during those days. The parents can be seen at normal rates with the chicks up to about the 70th day, after which the attendance by the parents falls rapidly. Chicks receive no care after fledgling, with only about one out of seven chicks receiving food after the 80th day, and almost no chicks are visited after about 90 days. After fledging, the chicks will leave the nest, with few remaining after about 100 days.
Accurate assessment of red-billed tropicbird numbers is difficult due to the remote locations of nesting sites and vast areas of sea they might be found. This bird is considered to be a least-concern species according to the IUCN. This is due to the fact that the range, declination, and numbers of this species, although small, do not meet the criteria required to be considered a vulnerable species. The range of this species is believed to be 86.3 million square kilometres (33.3 million sq mi), with an estimated 3,300 to 13,000 mature individuals. In the western Atlantic, a more precise number was given for the population there in 2000; about 4,000 to 5,000 pairs. The population is declining, mainly due to use by humans and predation by invasive species, such as rats. These predators have the potential to drive populations of the red-billed tropicbird into serious decline. In Mexico and Brazil, this bird is considered to be threatened.
Threats and survival
The eggs and chicks of red-billed tropicbirds is prey for both brown and black rats in places like Abrolhos Marine National Park, where these rats are invasive species. Feral cats are also predators of breeding tropicbirds, with the birds providing about 3% of the diet of the cats in locations such as the Caribbean island Saba. The threat presented by these cats has only appeared in recent years; tropicbirds on Saba have only been threatened since about 2000. On Ascension Island, the effect of the eradication of feral cats was the increase of the red-billed tropicbird population there by about 1.6% in a year. On the Galapagos Islands, the short-eared owl (Asio flammeus) occasionally eats young birds.> Toxoplasma gondii, an intracellular parasite, can be found in this bird. About 28% of red-billed tropicbirds produce antibodies for T. gondii.
The egg that is laid hatches about 75% of the time under normal conditions. When the region that the red-billed tropicbird is inhabiting is abnormally hot when it is nesting, although, the hatching success can drop to about 35%. Egg shell thinning, a potential cause of egg mortality, can be caused by pollutants. About 78% of chicks fledge in normal years, with that percentage only dropping slightly, to 77%, in abnormally hot years. Most egg and chick mortality during periods of normal climate is caused by nest fights between the parents and other birds.
Breeding adults usually survive the year, with only about 18% dying every year. The young have a lower survival rate, with about 29% dying each year. The lifespan for this bird is anywhere from 16 to 30 years.
Relationship with humans
The red-billed tropicbird, a bird not indigenous to Bermuda, was displayed in error on the $50 Bermudian dollar and was replaced in 2012 by the white-tailed tropicbird, the tropicbird that can be found in Bermuda.
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