Fregatidaefrigatebirds

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Fregatidae comprises a single genus (Fregata) and five species.

Frigatebirds have a pantropical marine distribution.

Frigatebirds are pelagic and coastal in tropical and subtropical oceans. Breeding grounds are located on remote oceanic islands or in coastal mainland mangrove regions.

The egg is incubated for 40-55 days. Chicks are altricial and acquire a whitish down. Chicks are brooded and fed by parents. Fledging age is variable and ranges from 4.5-7 months. Post-fledging care may last from four to eighteen months depending on species. Adult plumage may be acquired between four or six years. Age of first breeding varies with species and ranges from six to eleven years. Frigatebirds may live for at least 25-34 years.

Frigatebirds are large birds (89-114 cm; 625-1640 g; 196-244 wingspan). Females tend to be larger and heavier than males. Plumage is mostly iridescent black-brown and some species have white on the breast and/or abdomen. The gular sac is red and becomes greatly enlarged when inflated by males performing mating displays. Wings are long, narrow and pointed, and the tail is long and deeply forked. Head is small and neck is relatively short. The long, cylindrical bill is strongly hooked at the tip and palate is desmognathous. Small feet are totipalmate with a small area of webbing at base of toes. Legs and feet of males are mostly black or brown, whereas females are white or red. Both sexes have brood patches. Coracoid and furcula are fused to the sternum (unique in birds).

Frigatebirds may breed in mixed colonies with other fregatids, red-footed booby (Sula nebouxii), terns and noddies (Sternidae), gulls (Larus), cormorants (Phalacrocorax), shearwaters and petrels (Procellariiformes), pelicans (Pelecanidae).

Frigatebirds feed primarily on flying fish (Cypselurus, Exocoetus), but will also take menhaden (Brevoortia), squid or jellyfish. They also prey upon eggs and chicks of their own species, terns (Sterna), boobies (Sula), and petrels and shearwaters (Procellariiformes).

Predators of frigatebirds include: domestic cats, rats and humans. Frigatebirds may kill chicks and eggs of conspecifics and congeners.

Frigatebirds are considered seasonally monogamous. Males gather in groups to display for females by spreading their wings, inflating their large scarlet gular sacs, and pointing their bills skywards. When a female flies over the group, each male quivers wings and head, and the bill vibrates against the inflated pouch producing a distinctive drumming sound. A female will land next to one male and two or three days of pair-formation ensues with periods of head snaking and the male taking the female's bill into his own. The pair takes two to three weeks to build a nest on the display site. The male collects nesting material (sometimes pilfering from nearby nests) whereas the female defends the site and builds a nest from the materials brought by the male. Copulations occur at the nest site.

Frigatebirds breed in colonies numbering up to several thousand pairs. Breeding is considered biennial, although in some populations females may breed biennially whereas males may breed annually. The beginning of breeding is variable and may coincide with food availability. Nest-sites include trees, bushes, and cliffs. Nesting material consists of twigs, leaves, grass, seaweed and feathers. Females generally lay one whitish egg per clutch.

Both sexes have brood patches and parents take turns incubating for 40-55 days in stints lasting from one to 18 days. Males and females take turns brooding and feeding chicks regurgitated food until chicks fledge at four to seven months. Chicks are guarded constantly during the first month, then left alone frequently while the parents forage. Post-fledging care is prolonged (14-18 months) and the female may do most, if not all, of the post-fledging feeding.

Frigatebirds generally spend the year within range of the breeding colony, yet young birds may disperse widely. Frigatebirds are noted soarers, spending much of the day riding the winds and roosting at night on trees or cliffs. Thermoregulation includes gular fluttering, feather ruffling, and wing extending. Frigatebirds catch most of their prey by flying low over the water and picking prey from near the surface. They will, however, feed opportunistically by taking fish from fishing boats or offal from slaughterhouses. Interestingly, frigatebirds sometimes obtain food by cooperatively stealing prey items from other species. Several frigatebirds soar together and target seabirds (boobies and others) returning with food items. The frigatebirds swoop down from above to pursue the target, pulling at the bird's wings or tail, in an attempt to force the bird to disgorge and drop its prey items. If successful, the dropped food item is plucked from the air by one of the agile frigatebirds.

Frigatebirds form mixed flocks with other Pelecaniformes for breeding and roosting. They generally forage singly or in small groups of two or three individuals, but may flock over an abundant food source.

Frigatebirds are often quiet in flight or when perching. Colonies are often noisy. Males and females have distinctive vocalizations. Adults elicit twittering, drumming, rattling and bill clattering. Young birds chirp or squeal.

Frigatebird eggs and young are collected for human consumption.

Two frigatebird species are included in the IUCN Red List of Threatened Species: Andrew's Frigatebird (Fregata andrewsi) is listed as 'Critically Endangered' and Ascension Frigatebird (F. aquila) is listed as 'Vulnerable'. Major threats include: habitat degradation, introduced predators, and human disturbance at breeding sites.

The evolutionary relationships of frigatids remain unclear. Frigatebirds have been considered related to other totipalmate birds (tropicbirds, anhingas, gannets and boobies, pelicans, cormorants and anhingas), which when taken together, form Pelecaniformes. Frigatebirds have been considered most derived within Pelecaniformes. However, some hierarchies include frigatebirds within Ciconiiformes or Charadriiformes. Morphological, ethological and molecular analyses suggest differing hypotheses of sister group relationships: frigatebirds as sister to a group comprising pelecanids (pelicans), sulids (boobies and gannets), phalacrocoracids (cormorants and anhingas); frigatebirds as sister to procellariids (tubenoses); frigatebirds as sister to a group consisting of spheniscids (penguins), gaviids (loons), procellariids (petrels, shearwaters, albatrosses).

A frigatebird fossil (Limnofregata) from Wyoming has been dated to the Eocene.

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

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.

Johnsgard, P. A. 1993. Cormorants, Darters, and Pelicans of the World. Smithsonian Institution Press Washington.

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