Phaethontidaetropicbirds

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Phaethontidae comprises a single genus (Phaethon) with three species (P. aetherus, P. rubricauda, P. lepturus).

Tropicbirds are pan tropical, ranging throughout the warm tropical and subtropical waters of the Pacific, Atlantic and Indian oceans.

Tropicbirds are largely pelagic and breed on remote oceanic islands.

Eggs are incubated for 42-46 days and young are semi-altricial. Newly hatched chicks weigh about 20 g and have dense white or gray down. Chicks are brooded and fed by adults until they fledge at about 70-91 days and begin a stage of post-fledging dispersal. Adult plumage is attained at about two to three years of age. Tropicbirds may live for 16 to 30 years.

Tropicbirds are medium sized birds (220-835 g; 70-105 cm (30-56 cm including tail streamers); 90-119 cm wingspan). Tropicbirds appear quite elegant with satiny plumage and slender, elongated central tail feather retrices (streamers). Their plumage is primarily white or tinted slightly pink or yellow, with black markings on the flight feathers and black eyestripes. Sexes are similar in appearance, although tail streamers may be longer in males.

The red or yellow bills are stout and slightly decurved with serrated edges. The gular pouch is completely feathered. The legs are short (short tarsus) and the feet are small and totipalmate (all four toes joined by webs). The nares are pervious (exposed external nares), external nostrils are slit-like, the palate is schizognathous, and the skull lacks a supraorbital groove for a nasal gland.

Tropicbirds feed primarily on flying fish (Exocoetidae), other small fish, squid (Ommastrephidae) and crustaceans.

Predators of tropicbirds include rats (Rattus norvegicus, R. rattus. R. exulans), cats and humans.

Tropicbirds are considered monogamous. Pairs may breed together for several years and return to the same nest site repeatedly. Courtship displays are aerial and noisy. Pair establishment begins with groups of six to twelve birds circling the breeding grounds vocalizing loudly. A pair or trio will fly away from the group and begin to perform synchronized aerial displays. These displays may include zigzag flying or downward gliding, oftentimes with the tail streamers undulated. Both adults build or locate the nest-site. Copulation occurs at the nest sites. Pair formation sometimes includes fights for nest-sites or partners. Adults may grip an adversary with the bill and feet in order to drive the adversary from a nest-site. These fights can result in scars and gashes around the head and neck area.

Tropicbirds breed in colonies on remote islands. Breeding may be continuous (year round) or seasonal. Seasonal breeders are also migratory. Nest sites include crags or ledges on cliffs, scrapes on the ground, or hollows of trees. Little or no nesting material is present. If nesting on soft substrate, the adults will use their bodies and feet to create the shallow depression. The female lays one egg, which is red-brown to gray-purple, either uniform or speckled in color.

Both sexes take turns incubating for 40-46 days. Duration of the incubation stints vary, and may be as long as 16 days. Parents feed chicks by regurgitating semi-digested food. Both parents brood the chicks, continuously at first, then more sporadically as the chicks develop. There appears to be no post-fledging parental care.

Tropicbirds spend most of their time alone or in pairs flying over the ocean, searching for prey items. Tropicbirds are noted for their plunge-diving foraging technique. While hovering 6-50 m above the sea, the birds will dive into the water after prey items. Most prey items are caught by plunge diving but some flying fish are caught in flight.

Tropicbirds may be found in pairs while foraging at sea or in colonies for breeding.

While flying over the sea, tropicbirds may elicit sharp piercing whistles. Shrill trilling vocalizations are thought to be similar to the sound of a boatswain's pipe. During courtship flight, vocalizations include repetitive strident cries. When disturbed at the nest adults may hiss or screech. Adults may click or chuck softly when bringing food to the nest.

Some oceanic islanders use tropicbird tail feathers as ornaments. The birds are generally captured but not killed for this purpose. However, tropicbird chicks and adults are sometimes collected and killed for human consumption.

None of the tropicbird species are included in the IUCN Red List of Threatened Species. However, threats to present populations include human exploitation, habitat destruction, and predator introductions into breeding areas.

The evolutionary relationships of the tropicbirds remain uncertain. Traditionally, tropicbirds along with other totipalmate birds (pelicans, boobies, gannets, cormorants, anhingas, frigatebirds) together form Pelecaniformes. Tropicbirds are frequently considered most primitive within Pelecaniformes. However, some hierarchical systems place phaethontids within Ciconiiformes, Charadriiformes, or Procellariiformes. Morphological, ethological and molecular analyses have generated differing hypotheses of tropicbird sister group relationships including: phaethontids as sister to a group comprising pelecanids (pelicans), sulids (boobies and gannets), phalacrocoracids (cormorants and anhingas); phaethontids as sister to tubenoses forming a group within lariids (gulls, terns, jaegers, skimmers).

A tropicbird fossil from England, (Prophaethon), has been described from the early Eocene.

Campbell, B., and E. Lack, editors. 1985. A Dictionary of Birds. Buteo Books, Vermillion, SD.

Cracraft, J. 1985. Monophyly and phylogenetic relationships of the Pelecaniformes: a numerical cladistic analysis. Auk 102: 834-853.

del Hoyo, J., Elliott, A. & Sargatal, J. (eds.) 1992. Handbood of the Birds of the World. Vol. 1. Lynx Edicions, Barcelona.

Feduccia, A. 1999. The Origin and Evolution of Birds, 2nd edition. Yale University Press New Haven.

Sibley, C. G. & J. E. Ahlquist. 1990. Phylogeny and Classification of Birds, A Study in Molecular Evolution. Yale Univ. Press.

Contributors

Laura Howard (author), Animal Diversity Web.

Glossary

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

visual

uses sight to communicate