The red-billed tropicbird (Phaethon aethereus) is a tropicbird, one of three closely related species of seabird of tropical oceans. Superficially 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 that are about two times their body length, with those in males being generally longer than those in females. The red-billed tropicbird itself has three subspecies recognized, including the nominate. 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 orange bill. This species ranges across the tropical Atlantic, eastern Pacific, and Indian Oceans. The nominate is found in the southern Atlantic Ocean, the subspecies indicus in the waters off of the Middle East and in the Indian Ocean, and the subspecies mesonauta in the eastern portions of both the Atlantic and the Pacific Oceans and in the Caribbean. It was one of the many species described by Carl Linnaeus in his 1758 10th edition of Systema Naturae.
Nesting takes place in loose colonies, as they nest a scrape found on a cliff face that is easy to take off from. A single egg is laid and is incubated by both sexes for about six weeks. Whether the egg hatches or not can be influenced by pollution and weather, although the latter has a minimal effect on whether a chick fledges or not. After a chick fledges, the parents will usually stop visiting the nest and the chick will leave. Birds of all ages feed on fish and squid, catching them by diving from the air into the water. However, the red-billed tropicbird sometimes follows surface-feeding predators. The predators will drive the prey to the surface, which are then seized by the tropicbird.
In some areas, introduced black and brown rats raid nests for eggs and young. Cats also threaten the red-billed tropicbird. 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 English naturalist Francis Willughby wrote about the red-billed tropicbird in the 17th century, having seen a specimen held by the Royal Society. It 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 name comes from Latin aetherius, "heavenly". 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". In a 1945 paper, American ornithologist Waldo Lee McAtee proposed it be called the barred-backed tropicbird after its most distinguishing feature.
The red-billed tropicbird is basal (the earliest offshoot) in the genus Phaethon, the sole extant genus in the family Phaethontidae, 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.
There are three subspecies, including the nominate, of this tropicbird:
Subspecies Trinomial name Photograph Discovery Range P. a. aethereus Linnaeus, 1758 The nominate subspecies, described in 1758 by Linnaeus. Central Atlantic (breeding on islands south of the equator) P. a. mesonauta J. L. Peters, 1930 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 Galápagos Islands. He reported this taxon resembled P. a. mesonauta but had a yellower bill. Peters insisted this color was not an artefact of preservation. The latter subspecies is not recognized as distinct by the IOC. East Pacific, Caribbean, and East Atlantic. P. a. indicus A. O. Hume, 1876 Allan Octavian Hume wrote of this taxon in 1876 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. Persian Gulf, Gulf of Aden, Red Sea.
Description The red-billed tropicbird measures on average, which includes the -long . Without them the tropicbird measures about . It has a wingspan of . 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 gray mottling usually seen near the nape and hindneck. The tail has black shaft streaks, as do tail streamers. The 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, base of the central toe, and parts of the outer toes are orange-yellow while the rest of the feet are black. Although the sexes are similar, the males are generally larger than females, with the tail streamers being around 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 upper wing, and the more solid look of the black on the outer wing. The subspecies P. a. indicus can be distinguished by its smaller size, its smaller mask on the face, which often does not extend far behind the eye, and its more orange bill with a black cutting edge.
When the chicks hatch, they are covered with gray down. This down is eventually cleared in about 40 to 50 days. The young chicks lack 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 under tail coverts.
The red-billed tropicbird can be differentiated from the other tropicbirds 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 upper wing coverts, and its yellow-orange bill. Juvenile red-billed tropicbirds have more heavily barred upper parts than juveniles of other species. In flight, 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.
This tropicbird moults once every year as an adult, following a complex basic strategy. This 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 screams. If disturbed at the nest, the chicks will vocalize a loud and piercing shriek, either rasping or reeling.
Distribution and habitat The red-billed tropicbird has the smallest range of the three tropicbird species, yet it still ranges across the Neotropics, as well as the tropical Atlantic, eastern Pacific, and Indian oceans. The nominate subspecies Phaethon aethereus aethereus breeds on islands in the Atlantic south of the equator, including Ascension, and Saint Helena on the Mid-Atlantic Ridge, and Fernando de Noronha and Abrolhos Archipelago in Brazilian waters. It is a vagrant to the coastline of Namibia and South Africa. The subspecies P. a. mesonauta is found in the east Atlantic, the east Pacific, and in the Caribbean. This subspecies was restricted to the Cape Verde Islands in the eastern Atlantic but it has colonised the Canary Islands in the 21st century, especially Fuerteventura but also on other islands in that archipelago. The Indian Ocean subspecies, P. a. indicus is found in waters off Pakistan, western India, southwestern Sri Lanka, the Horn of Africa and Arabian Peninsula. The subspecies is also a rare but regular vagrant to Seychelles.
Within the West Indies, this species is most common in the Lesser Antilles, Virgin Islands and small islands east of Puerto Rico. Breeding in the Western Palearctic occurs on the Cape Verde Islands and the Îles des Madeleines off Senegal. In 2000, the total number of pairs there was probably less than 150. In the Pacific Ocean, it breeds from the Gulf of California and Revillagigedo Islands, Mexico in the north, to the Galápagos Islands, Isla Plata, Ecuador and San Lorenzo Island, Peru. Researchers Larry Spear and David Ainley estimated the minimum population of the Pacific at around 15,750 birds in 1995 after 15 years of field observations. Red-billed tropicbirds disperse widely when not breeding, the juveniles more so than the adults, with birds 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 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, likely the same bird that later visited Seal Island and Matinicus Rock, Maine every summer from 2005 to 2021.
Behavior The red-billed tropicbird can reach speeds of when flying out at sea, cruising a minimum of above the sea. It cannot stand and is not proficient at walking, and requires an unobstructed takeoff to fly from land. Conversely it can lift off the sea without much effort. Its plumage is waterproof and it floats on water.
Breeding
The red-billed tropicbird usually nests on isolated cliff faces, in loose colonies. It uses a simple scrape nest, located in a place it can easily take off from. 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. For example, for islands in the California Current, breeding starts in November or December, while it occurs year round in the Galápagos. 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 usually returns to its partner and nest location from the previous breeding cycle.