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:
{| width="98%" class="wikitable"
! align="center" style="background:#D3D3A4" colspan="4" | Subspecies
|-
! width="20%" | Trinomial name
! width="18%" | Photograph
! align="center" | Discovery
! width="18%" | Range
|-
| P. a. aethereus <small>Linnaeus, 1758</small>
| 500x150px|alt=A high contrast image of the nominate, taken from behind the bird while it is on the ground, can be seen.
| The nominate subspecies, described in 1758 by Linnaeus. The latter subspecies is not recognized as distinct by the IOC.
| East Pacific, Caribbean, and East Atlantic.
| Persian Gulf, Gulf of Aden, Red Sea.
|}
Description
The red-billed tropicbird measures on average, It has a wingspan of . 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. 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.
thumb|P. a. mesonauta chick, [[Little Tobago | alt=A chick of the subspecies mesonauta can be seen with a yellow beak turning its head to look at the camera.]]
When the chicks hatch, they are covered with gray down. Juvenile red-billed tropicbirds have more heavily barred upper parts than juveniles of other species. This is completed before courtship and lasts between 19 and 29 weeks, with most being completed in 24 weeks. The subspecies P. a. mesonauta is found in the east Atlantic, the east Pacific, and in the Caribbean. 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. 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, 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.
Behavior
The red-billed tropicbird can reach speeds of when flying out at sea, 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. during which this bird performs aerial courtship displays to potential mates. fighting with each other and ousting species such as shearwaters, petrels, and white-tailed tropicbirds. They have also been recorded taking over nests of white-tailed tropicbirds and raising their young if they failed to destroy their eggs.
Born helpless and unable to move around (nidicolous and semi-altricial), the chicks are constantly brooded by the parents until they are 3 to 5 days old, when they can thermoregulate their body temperature. 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. It usually dives into waters away from the coastline, diving from the air, at heights up to .
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.
Status
Accurate assessment of red-billed tropicbird numbers is difficult due to the remote locations of nesting sites and vast areas of the sea where they might be found. In Mexico 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. On Saba, the problem has only arisen 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 Galápagos 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.
Under normal conditions about 75% of the eggs hatch. Hatching success can drop to about 35% in unusually hot conditions. About 78% of chicks fledge in normal years, with that percentage only dropping slightly, to 77%, in abnormally hot years. The lifespan of this bird is anywhere from 16 to 30 years.