Frigatebird
Frigatebirds are a family of seabirds called Fregatidae which are found across all tropical and subtropical oceans. The five extant species are classified in a single genus, Fregata. All have predominantly black plumage, long, deeply forked tails and long hooked bills. Females have white underbellies and males have a distinctive red gular pouch which they inflate during the breeding season to attract females. Their wings are long and pointed and can span up to, the largest wing area to body mass ratio of any bird.
Able to soar for weeks on wind currents, frigatebirds spend most of the day in flight hunting for food, and roost on trees or cliffs at night. Their main prey are fish and squid, caught when chased to the water surface by large predators such as tuna. Frigatebirds are referred to as kleptoparasites as they occasionally rob other seabirds for food, and are known to snatch seabird chicks from the nest. Seasonally monogamous, frigatebirds nest colonially. A rough nest is constructed in low trees or on the ground on remote islands. A single egg is laid each breeding season. The duration of parental care is among the longest of any bird species; frigatebirds are able to breed only every other year.
The Fregatidae are a sister group to Suloidea which consists of cormorants, darters, gannets, and boobies. Three of the five extant species of frigatebirds are widespread, while two are endangered and restrict their breeding habitat to one small island each. The oldest fossils date to the early Eocene, around 50 million years ago. Classified in the genus Limnofregata, the three species had shorter, less-hooked bills and longer legs, and lived in a freshwater environment.
Taxonomy
Etymology
The term Frigate Bird itself was used in 1738 by the English naturalist and illustrator Eleazar Albin in his A Natural History of the Birds. The book included an illustration of the male bird showing the red gular pouch. Like the genus name, the English term is derived from the French mariners' name for the bird la frégate—a frigate or fast warship. The etymology was mentioned by French naturalist Jean-Baptiste Du Tertre when describing the bird in 1667.Alternative names and spellings include "frigate bird", "frigate-bird", "frigate", "frigate-petrel".
Christopher Columbus encountered frigatebirds when passing the Cape Verde Islands on his first voyage across the Atlantic in 1492. In his journal entry for 29 September he used the word rabiforçado, modern Spanish rabihorcado or forktail. In the Caribbean frigatebirds were called Man-of-War birds by English mariners. This name was used by the English explorer William Dampier in his book An Account of a New Voyage Around the World published in 1697:
The Man-of-War is about the bigness of a Kite, and in shape like it, but black; and the neck is red. It lives on Fish yet never lights on the water, but soars aloft like a Kite, and when it sees its prey, it flys down head foremost to the Waters edge, very swiftly takes its prey out of the Sea with his Bill, and immediately mounts again as swiftly; never touching the Water with his Bill. His Wings are very long; his feet are like other Land-fowl, and he builds on Trees, where he finds any; but where they are wanting on the ground.
Classification
Frigatebirds were grouped with cormorants, and sulids as well as pelicans in the genus Pelecanus by Linnaeus in 1758 in the tenth edition of his Systema Naturae. He described the distinguishing characteristics as a straight bill hooked at the tip, linear nostrils, a bare face, and fully webbed feet. The genus Fregata was introduced by French naturalist Bernard Germain de Lacépède in 1799. The type species was designated as the Ascension frigatebird by French zoologist François Marie Daudin in 1802. Louis Pierre Vieillot described the genus name Tachypetes in 1816 for the great frigatebird. The genus name Atagen had been coined by German naturalist Paul Möhring in 1752, though this has no validity as it predates the official beginning of Linnaean taxonomy.In 1874, English zoologist Alfred Henry Garrod published a study where he had examined various groups of birds and recorded which muscles of a selected group of five they possessed or lacked. Noting that the muscle patterns were different among the steganopodes, he resolved that there were divergent lineages in the group that should be in separate families, including frigatebirds in their own family Fregatidae. Urless N. Lanham observed in 1947 that frigatebirds bore some skeletal characteristics more in common with Procellariiformes than Pelecaniformes, though concluded they still belonged in the latter group, albeit as an early offshoot. Martyn Kennedy and colleagues derived a cladogram based on behavioural characteristics of the traditional Pelecaniformes, calculating the frigatebirds to be more divergent than pelicans from a core group of gannets, darters and cormorants, and tropicbirds the most distant lineage. The classification of this group as the traditional Pelecaniformes, united by feet that are totipalmate and the presence of a gular pouch, persisted until the early 1990s. The DNA–DNA hybridization studies of Charles Sibley and Jon Edward Ahlquist placed the frigatebirds in a lineage with penguins, loons, petrels and albatrosses. Subsequent genetic studies place the frigatebirds as a sister group to the group Suloidea, which comprises the gannets and boobies, cormorants and darters. Microscopic analysis of eggshell structure by Konstantin Mikhailov in 1995 found that the eggshells of frigatebirds resembled those of other Pelecaniformes in having a covering of thick microglobular material over the crystalline shells.
Molecular studies have consistently shown that pelicans, the namesake family of the Pelecaniformes, are actually more closely related to herons, ibises and spoonbills, the hamerkop and the shoebill than to the remaining species. In recognition of this, the order comprising the frigatebirds and Suloidea was renamed Suliformes in 2010.
In 1994, the family name Fregatidae, cited as described in 1867 by French naturalists Côme-Damien Degland and Zéphirin Gerbe, was conserved under Article 40 of the International Code of Zoological Nomenclature in preference to the 1840 description Tachypetidae by Johann Friedrich von Brandt. This was because the genus names Atagen and Tachypetes had been synonymised with Fregata before 1961, resulting in the aligning of family and genus names.
Fossil record
The Eocene frigatebird genus Limnofregata comprises birds whose fossil remains were recovered from prehistoric freshwater environments, unlike the marine preferences of their modern-day relatives. They had shorter less-hooked bills and longer legs, and longer slit-like nasal openings. Three species have been described from fossil deposits in the western United States, two—L. azygosternon and L. hasegawai—from the Green River Formation and one—L. hutchisoni—from the Wasatch Formation. Fossil material indistinguishable from living species dating to the Pleistocene and Holocene has been recovered from Ascension Island, Saint Helena Island, both in the southern Atlantic Ocean, and also from various islands in the Pacific Ocean. A tarsometatarsus and pedal phalanx from the Lower Eocene London Clay of the Walton-on-the-Naze resembles Limnofregata, but being notably larger and distinct in other ways, was tentatively referred to Marinavis longirostris due to similar stratigraphy, geography, size, and presumed frigatebird affinities.A cladistic study of the skeletal and bone morphology of the classical Pelecaniformes and relatives found that the frigatebirds formed a clade with Limnofregata. Birds of the two genera have 15 cervical vertebrae, unlike almost all other Ciconiiformes, Suliformes and Pelecaniformes, which have 17. The age of Limnofregata indicates that these lineages had separated by the Eocene.
Living species and infrageneric classification
The type species of the genus is the Ascension frigatebird. For many years, the consensus was to recognise only two species of frigatebird, with larger birds as F. aquila and smaller as F. ariel. In 1914 the Australian ornithologist Gregory Mathews delineated five species, which remain valid. Analysis of ribosomal and mitochondrial DNA indicated that the five species had diverged from a common ancestor only recently—as little as 1.5 million years ago. There are two species pairs, the great and Christmas Island frigatebirds, and the magnificent and Ascension frigatebirds, while the fifth species, the lesser frigatebird, is an early offshoot of the common ancestor of the other four species. Two subspecies of the magnificent, three subspecies of the lesser and five subspecies of the great frigatebird are recognised.British-Canadian ornithologist Antony W. Diamond conducted some of the earliest ecological studies of frigatebirds, examining their breeding colonies, feeding ecology, and parental behavior on tropical islands.Diamond’s fieldwork in the Indian Ocean and the Seychelles provided key insights into frigatebird nesting density, interspecific competition, and the influence of food availability on breeding success.Description
Frigatebirds are large, slender, mostly black-plumaged, seabirds, with the five species similar in appearance to each other. The largest species is the magnificent frigatebird, which reaches in length, with three of the remaining four almost as large. The lesser frigatebird is substantially smaller, at around long. Frigatebirds exhibit marked sexual dimorphism; females are larger and up to 25 percent heavier than males, and generally have white markings on their underparts. Frigatebirds have short necks and long, slender, hooked bills. Their long narrow wings taper to points. Their wings have eleven primary flight feathers, with the tenth the longest and eleventh a vestigial feather only, and 23 secondaries. Their tails are deeply forked, though this is not apparent unless the tail is fanned. The tail and wings give them a distinctive 'W' silhouette in flight. The legs and face are fully feathered. The totipalmate feet are short and weak; the webbing is reduced and part of each toe is free.The bones of frigatebirds are markedly pneumatic, making them very light and contributing only 5% to total body weight. The pectoral girdle is strong as its bones are fused. The pectoral muscles are well-developed, and weigh as much as the frigatebird's feathers—around half the body weight is made up equally of these muscles and feathers. The males have inflatable red-coloured throat pouches called gular pouches which they inflate to attract females during the mating season. The gular sac is, perhaps, the most striking frigatebird feature. These can deflate only slowly, so males that are disturbed will fly off with pouches distended for some time.
Frigatebirds remain in the air and do not settle on the ocean. They produce very little oil from their uropygial glands, so their feathers would become sodden if they settled on the surface. In addition, with their long wings relative to body size, they would have great difficulty taking off again.