| Frigatebird | |
|---|---|
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| Malemagnificent frigatebird (Fregata magnificens) displaying gular pouch,Galápagos Islands. | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | Suliformes |
| Family: | Fregatidae Degland &Gerbe, 1867 |
| Genus: | Fregata Lacépède, 1799 |
| Type species | |
| Pelecanus aquilus (Ascension frigatebird) Linnaeus, 1758 | |
| Species | |
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| Range map | |
Frigatebirds are afamily ofseabirds calledFregatidae which are found across alltropical 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 redgular pouch, which they inflate during the breeding season to attract females. Their wings are long and pointed and can span up to 2.3 metres (7.5 ft), the largest wing area to body weight 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 andsquid, caught when chased to the water surface by large predators such astuna. Frigatebirds are referred to askleptoparasites as they occasionally rob other seabirds for food, and are known to snatch seabird chicks from the nest. Seasonally monogamous, frigatebirds nestcolonially. 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 only able to breed every other year.
The Fregatidae are asister group toSuloidea which consists ofcormorants,darters,gannets, andboobies. Three of the five extant species of frigatebirds are widespread (themagnificent,great andlesser frigatebirds), while two are endangered (theChristmas Island andAscension Island frigatebirds) and restrict their breeding habitat to one small island each. The oldest fossils date to the earlyEocene, around 50 million years ago. Classified in the genusLimnofregata, the three species had shorter, less-hooked bills and longer legs, and lived in a freshwater environment.
The termFrigate Bird itself was used in 1738 by the English naturalist and illustratorEleazar Albin in hisA Natural History of the Birds. The book included an illustration of the male bird showing the redgular pouch.[1] Like the genus name, the English term is derived from the French mariners' name for the birdla frégate—afrigate or fast warship.[2] The etymology was mentioned by French naturalistJean-Baptiste Du Tertre when describing the bird in 1667.[3][a]Alternative names and spellings include "frigate bird", "frigate-bird", "frigate", "frigate-petrel".[4]
Christopher Columbus encountered frigatebirds when passing theCape Verde Islands on his first voyage across the Atlantic in 1492. In his journal entry for 29 September he used the wordrabiforçado, modern Spanishrabihorcado or forktail.[5][6][b] In the Caribbean frigatebirds were called Man-of-War birds by English mariners. This name was used by the English explorerWilliam Dampier in his bookAn Account of a New Voyage Around the World published in 1697:[7]
The Man-of-War (as it is called by the English) 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.[7]
Frigatebirds were grouped withcormorants, andsulids (gannets andboobies) as well as pelicans in thegenusPelecanus byLinnaeus in 1758 in thetenth edition of hisSystema Naturae. He described the distinguishing characteristics as a straight bill hooked at the tip, linear nostrils, a bare face, and fully webbed feet.[8] The genusFregata was introduced by French naturalistBernard Germain de Lacépède in 1799.[9] Thetype species was designated as the Ascension frigatebird by French zoologistFrançois Marie Daudin in 1802.[10][11]Louis Pierre Vieillot described the genus nameTachypetes in 1816 for thegreat frigatebird. The genus nameAtagen had been coined by German naturalistPaul Möhring in 1752, though this has no validity as it predates the official beginning ofLinnaean taxonomy.[12]
In 1874, English zoologistAlfred Henry Garrod published a study where he had examined various groups of birds and recorded which muscles of a selected group of five[c] they possessed or lacked. Noting that the muscle patterns were different among the steganopodes (classicalPelecaniformes), he resolved that there were divergent lineages in the group that should be in separatefamilies, including frigatebirds in their own family Fregatidae.[13] Urless N. Lanham observed in 1947 that frigatebirds bore some skeletal characteristics more in common withProcellariiformes than Pelecaniformes, though concluded they still belonged in the latter group (as suborder Fregatae), albeit as an early offshoot.[14] Martyn Kennedy and colleagues derived acladogram 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, andtropicbirds the most distant lineage.[15] The classification of this group as the traditional Pelecaniformes, united by feet that aretotipalmate (with all four toes linked by webbing) and the presence of a gular pouch, persisted until the early 1990s.[16] TheDNA–DNA hybridization studies ofCharles Sibley andJon Edward Ahlquist placed the frigatebirds in a lineage withpenguins,loons,petrels andalbatrosses.[17] Subsequent genetic studies place the frigatebirds as asister group to the groupSuloidea, which comprises the gannets and boobies, cormorants and darters.[18][19] 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.[20]
Molecular studies have consistently shown thatpelicans, the namesake family of the Pelecaniformes, are actually more closely related toherons,ibises and spoonbills, thehamerkop and theshoebill than to the remaining species. In recognition of this, theorder comprising the frigatebirds and Suloidea was renamedSuliformes in 2010.[21][22]
In 1994, the family name Fregatidae, cited as described in 1867 by French naturalistsCôme-Damien Degland andZéphirin Gerbe, wasconserved under Article 40(b) of theInternational Code of Zoological Nomenclature in preference to the 1840 description Tachypetidae byJohann Friedrich von Brandt. This was because the genus namesAtagen andTachypetes had beensynonymised withFregata before 1961, resulting in the aligning of family and genus names.[23]
TheEocene frigatebird genusLimnofregata 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.[24] Three species have been described from fossil deposits in the western United States, two—L. azygosternon andL. hasegawai—from theGreen River Formation (48–52 million years old) and one—L. hutchisoni—from theWasatch Formation (between 53 and 55 million years of age).[25] Fossil material indistinguishable from living species dating to thePleistocene andHolocene has been recovered fromAscension Island (forF. aquila),[26]Saint Helena Island,[27] both in the southern Atlantic Ocean, and also from various islands in the Pacific Ocean (forF. minor andF. ariel).[28][29] Atarsometatarsus andpedal phalanx from theLower EoceneLondon Clay of theWalton-on-the-Naze resemblesLimnofregata, but being notably larger and distinct in other ways, was tentatively referred toMarinavis longirostris due to similar stratigraphy, geography, size, and presumed frigatebird affinities.[30]
Acladistic study of the skeletal and bone morphology of the classical Pelecaniformes and relatives found that the frigatebirds formed aclade withLimnofregata. Birds of the two genera have 15cervical vertebrae, unlike almost all otherCiconiiformes, Suliformes and Pelecaniformes, which have 17. The age ofLimnofregata indicates that these lineages had separated by the Eocene.[19]
| Frigatebird phylogeny[31] | |||||||||||||||||||||||||||
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Thetype species of the genus is theAscension frigatebird (Fregata aquila).[32] For many years, the consensus was to recognise only two species of frigatebird, with larger birds asF. aquila and smaller asF. ariel. In 1914 the Australian ornithologistGregory Mathews delineated five species, which remain valid.[31][33] Analysis ofribosomal andmitochondrial 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.[31] Two subspecies of the magnificent, three subspecies of the lesser and five subspecies of the great frigatebird are recognised.[34]
| Living species of frigatebirds | |||
| Common and binomial names | Image | Description | Range |
|---|---|---|---|
| Magnificent frigatebird (Fregata magnificens) Mathews, 1914 | .jpg) | With a body length of 89–114 cm (35–45 in), it is the largest species and has the longest bill. The adult male is all-black with a scarletthroat pouch that is inflated like a balloon in the breeding season. Although the feathers are black, the scapular feathers have a purple sheen, in contrast to the malegreat frigatebird's green sheen. The female is brownish-black, but has a white breast and lower neck sides, a brown band on the wings, and a blueish-grey eye-ring.[35] | Widespread in the tropicalAtlantic, it breeds incolonies in trees inFlorida, the Caribbean andCape Verde Islands, as well as along the Pacific coast of the Americas from Mexico to Ecuador, including theGalápagos Islands.[36] |
| Ascension frigatebird (Fregata aquila) (Linnaeus, 1758) |  | Apart from its smaller size, the adult male is very similar to the magnificent frigatebird. The female is brownish black with a rusty brown mantle and chest, and normally lacks any white patches present on the front of female birds of other species. The occasional female observed with a white belly may be breeding before obtaining the full adult plumage.[37] | Found onBoatswain Bird Island just offAscension Island in the tropical Atlantic Ocean, having not bred on the main island since the 1800s.[38] |
| Christmas frigatebird (Fregata andrewsi) Mathews, 1914 |  | The adult male is one of the frigatebird species with white on its belly—an egg shaped patch. It is larger with a longer bill than the related great frigatebird. Its upperparts are black with green metallic gloss on the mantle and scapulars. The female has dark upperparts with brown wing bars, a black head with white belly and white collar (sometimes incomplete) around its neck.[39] | Breeds only onChristmas Island in the eastern Indian Ocean.[40] |
| Great frigatebird (Fregata minor) (Gmelin, 1789) |  | The adult male has black upperparts with green metallic gloss on the mantle and scapulars. It is completely black underneath with subtle brown barring on the axillaries. The upperparts of the female are dark with lighter brown wing bars. Its head is black with a mottled throat and belly. The neck has a white collar.[39] | Found in tropical Indian and Pacific oceans, as well as one colony—Trindade and Martim Vaz—in the south Atlantic, generally where the water is warmer than 22 °C (72 °F), and breeding on islands and atolls with sufficient vegetation to nest in.[41] |
| Lesser frigatebird (Fregata ariel) (G. R.Gray, 1845) |  | With a body length of around 75 cm (30 in), it is the smallest species. The adult male has black upperparts with greenish to purple metallic gloss on the mantle and scapulars, and is black underneath except for bold white axillary spurs. The upperparts of the female are dark with lighter wing bars. The head is black while the belly and the neck collar are white.[39] | Tropical and subtropical waters across the Indian and Pacific Oceans. Atlantic racetrinitatis was limited toTrindade, off Eastern Brazil but may now be extinct.[42][43] |
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 114 cm (45 in) in length, with three of the remaining four almost as large. The lesser frigatebird is substantially smaller, at around 71 cm (28 in) long. Frigatebirds exhibit markedsexual dimorphism; females are larger and up to 25 percent heavier than males,[44] and generally have white markings on their underparts.[45] Frigatebirds have short necks and long, slender hooked bills.[44] Their long narrow wings (male wingspan can reach 2.3 metres (7.5 ft)) taper to points. Their wings have elevenprimary flight feathers, with the tenth the longest and eleventh a vestigial feather only, and 23secondaries. Their tails are deeply forked, though this is not apparent unless the tail is fanned.[46] The tail and wings give them a distinctive 'W' silhouette in flight.[44] 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.[46]
The bones of frigatebirds are markedlypneumatic, making them very light and contributing only 5% to total body weight. Thepectoral 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.[44] The males have inflatable red-coloured throat pouches called gular pouches, which they inflate to attract females during the mating season.[45] The gular sac is, perhaps, the most striking frigatebird feature. These can only deflate slowly, so males that are disturbed will fly off with pouches distended for some time.[44]
Frigatebirds remain in the air and do not settle on the ocean. They produce very little oil from theiruropygial 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.[44]
Frigatebirds are found over tropical oceans, and ride warmupdrafts undercumulus clouds. Their range coincides with availability of food such asflying fish, and with thetrade winds, which provide the windy conditions that facilitate their flying.[44] They occur as rarevagrants totemperate regions and are not found in polar latitudes. Adults are generally sedentary, remaining near the islands where they breed.[44] However, male frigatebirds have been recorded dispersing great distances after departing a breeding colony—one male great frigatebird relocated fromEuropa Island in the Mozambique Channel to theMaldives 4,400 km (2,700 mi) away, and a male magnificent frigatebird flew 1,400 km (870 mi) fromFrench Guiana toTrinidad.[47] In 2015, a magnificent frigatebird was spotted as far north as Michigan.[48] Great frigatebirds marked withwing tags on Tern Island in theFrench Frigate Shoals were found to regularly travel the 873 km (542 mi) toJohnston Atoll, although one was reported inQuezon City in thePhilippines. Genetic testing seems to indicate that the species has fidelity to their site of hatching despite their high mobility.[49] Young birds may disperse far and wide, with distances of up to 6,000 km (3,700 mi) recorded.[44]
Having the largest wing-area-to-body-weight ratio of any bird, frigatebirds are essentially aerial.[44] This allows them to soar continuously and only rarely flap their wings. One great frigatebird, being tracked by satellite in the Indian Ocean, stayed aloft for two months. They can fly higher than 4,000 meters in freezing conditions.[50] Likeswifts they are able to spend the night on the wing, but they will also return to an island to roost on trees or cliffs.[51] Field observations in theMozambique Channel found that great frigatebirds could remain on the wing for up to 12 days while foraging.[52] Highly adept, they use their forked tails for steering during flight and make strong deep wing-beats,[46] though not suited to flying by sustained flapping. Frigatebirds bathe and clean themselves in flight by flying low and splashing at the water surface before preening and scratching afterwards.[44] Conversely, frigatebirds do not swim and with their short legs cannot walk well or take off from the sea easily.[46]
According to a study in the journalNature Communications, scientists attached anaccelerometer and anelectroencephalogram testing device on nine great frigatebirds to measure if they slept during flight. The study found the birds do sleep, but usually only using one hemisphere of the brain at a time and usually sleep while ascending at higher altitudes. The amount of time mid-air sleeping was less than an hour and always at night.[53]
The average life span is unknown but in common with seabirds such as thewandering albatross andLeach's storm petrel, frigatebirds are long-lived. In 2002, 35 ringed great frigatebirds were recovered onTern Island in theHawaiian Islands. Of these ten were older than 37 years and one was at least 44 years of age.[54]
Despite having dark plumage in a tropical climate, frigatebirds have found ways not to overheat—particularly as they are exposed to full sunlight when on the nest. They ruffle feathers to lift them away from the skin and improve air circulation, and can extend and upturn their wings to expose the hot undersurface to the air and lose heat by evaporation and convection. Frigatebirds also place their heads in the shade of their wings, and males frequently flutter their gular pouches.[44]
Unlike most seabirds, frigatebirds arethermal soarers, using thermals to glide.[55][56] This is in contrast to birds like albatrosses, which aredynamic soarers, using winds produced by the waves to stay aloft.
Frigatebirds typically breed on remote oceanic islands, generally in colonies of up to 5000 birds. Within these colonies, they most often nest in groups of 10 to 30 (or rarely 100) individuals.[46] Breeding can occur at any time of year, often prompted by commencement of the dry season or plentiful food.[44]
Frigatebirds have the most elaborate mating displays of all seabirds. The male birds take up residence in the colony in groups of up to thirty individuals.[44] They display to females flying overhead by pointing their bills upwards, inflating their red throat pouches and vibrating their outstretched wings, showing the lighter wing undersurfaces in the process. They produce a drumming sound by vibrating their bills together and sometimes give a whistling call.[46] The female descends to join a male she has chosen and allows him to take her bill in his. The pair also engages in mutual "head-snaking".[44]
After copulation it is generally the male who gathers sticks and the female that constructs the loosely woven nest. The nest is subsequently covered with (and cemented by)guano. Frigatebirds prefer to nest in trees or bushes, though when these are not available they will nest on the ground. A single white egg that weighs up to 6–7% of mother's body mass is laid, and isincubated in turns by both birds for 41 to 55 days. Thealtricial chicks are naked on hatching and develop a white down. They are continuously guarded by the parents for the first 4–6 weeks and are fed on the nest for 5–6 months.[46] Both parents take turns feeding for the first three months, after which the male's attendance trails off leaving the mother to feed the young for another six to nine months on average.[44] The chicks feed by reaching their heads in their parents' throat and eating the part-regurgitated food. It takes so long to rear a chick that frigatebirds generally breed every other year.[46]
The duration of parental care in frigatebirds is among the longest for birds, rivalled only by thesouthern ground hornbill and some largeaccipitrids.[57] Frigatebirds take many years to reach sexual maturity. A study of great frigatebirds in the Galapagos Islands found that they only bred once they have acquired the full adult plumage. This was attained by female birds when they were eight to nine years of age and by male birds when they were ten to eleven years of age.[58]
Frigatebirds' feeding habits arepelagic, and they may forage up to 500 km (310 mi) from land. They do not land on the water but snatch prey from the ocean surface using their long, hookedbills.[46] They mainly catch small fish such asflying fish, particularly the generaExocoetus andCypselurus, that are driven to the surface by predators such astuna anddolphinfish,[51] but they will also eatcephalopods, particularlysquid.[46]Menhaden of the genusBrevoortia can be an important prey item where common, andjellyfish and largerplankton are also eaten. Frigatebirds have learned to follow fishing vessels and take fish from holding areas.[44] Conversely tuna fishermen fish in areas where they catch sight of frigatebirds due to their association with large marine predators.[59] Frigatebirds also at times prey directly on eggs and young of other seabirds, including boobies, petrels,shearwaters and terns, in particular thesooty tern.[44]
Frigatebirds will rob otherseabirds such as boobies, particularly thered-footed booby,tropicbirds, shearwaters, petrels, terns,gulls and evenospreys of their catch, using their speed and manoeuvrability to outrun and harass their victims until they regurgitate their stomach contents. They may either assail their targets after they have caught their food or circle high over seabird colonies waiting for parent birds to return laden with food.[44] Although frigatebirds are renowned for theirkleptoparasitic feeding behaviour, kleptoparasitism is not thought to play a significant part of the diet of any species, and is instead a supplement to food obtained by hunting.[60] A study of great frigatebirds stealing frommasked boobies estimated that the frigatebirds could at most obtain 40% of the food they needed, and on average obtained only 5%.[61]
Unlike most other seabirds, frigatebirds drink freshwater when they come across it, by swooping down and gulping with their bills.[44]
Frigatebirds are unusual among seabirds in that they often carry blood parasites. Blood-borneprotozoa of the genusHaemoproteus have been recovered from four of the five species.[62] Bird lice of theischnoceran genusPectinopygus andamblyceran genusColpocephalum and speciesFregatiella aurifasciata have been recovered from magnificent and great frigatebirds of the Galapagos Islands. Frigatebirds tended to have more parasitic lice than did boobies analysed in the same study.[63]
A heavy chick mortality at a large and important colony of the magnificent frigatebird, located onÎle du Grand Connétable off French Guiana, was recorded in summer 2005. Chicks showed nodular skin lesions, feather loss and corneal changes, with around half the year's progeny perishing across the colony. Analphaherpesvirus was isolated and provisionally namedFregata magnificens herpesvirus, though it was unclear whether it caused the outbreak or affected birds already suffering malnutrition.[64]
Two of the five species are considered at risk.[44] In 2003, a survey of the four colonies of the critically endangered Christmas Island frigatebirds counted 1200 breeding pairs. As frigatebirds normally breed every other year, the total adult population was estimated to lie between 1800 and 3600 pairs. Larger numbers formerly bred on the island, but the clearance of breeding habitat during World War II and dust pollution from phosphate mining have contributed to the decrease.[40][65] The population of the vulnerable Ascension frigatebird has been estimated at around 12,500 individuals.[66] The birds formerly bred on Ascension Island itself, but the colonies were exterminated byferal cats introduced in 1815. The birds continued to breed on a rocky outcrop just off the shore of the island. A program conducted between 2002 and 2004 eradicated the feral cats[67] and a few birds have returned to nest on the island.[68][69]
The other three species are classified by theInternational Union for Conservation of Nature as being ofLeast Concern. The populations of all three are large, with that of the magnificent frigatebird thought to be increasing,[36] while the great and lesser frigatebird decreasing.[41][70] Monitoring populations of all species is difficult due to their movements across the open ocean and low reproductivity. The status of the Atlantic populations of the great and lesser frigatebirds are unknown and possibly extinct.[44]
As frigatebirds rely on large marine predators such as tuna for their prey, overfishing threatens to significantly impact on food availability and jeopardise whole populations.[59] As frigatebirds nest in large dense colonies in small areas, they are vulnerable to local disasters that could wipe out the rare species or significantly impact the widespread ones.[44]
InNauru, catching frigatebirds was an important tradition still practised to some degree. Donald W. Buden writes: "Birds typically are captured by slinging the weighted end of a coil of line in front of an approaching bird attracted to previously captured birds used as decoys. In a successful toss, the line becomes entangled about the bird's wing and bringing [sic] it to ground."[71] Marine birds including frigatebirds were once harvested for food on Christmas Island but this practice ceased in the late 1970s.[65] Eggs and young of magnificent frigatebirds were taken and eaten in the Caribbean.[44] Great frigatebirds were eaten in the Hawaiian Islands and their feathers used for decoration.[72]
The frigate bird appears on the nationalFlag of Kiribati. The design is based on its former colonialGilbert and Ellice Islands coat of arms. The bird also appears on the flag ofBarbuda, and is the national bird ofAntigua and Barbuda.[73]
There are anecdotal reports of tame frigatebirds being kept across Polynesia and Micronesia in the Pacific. A bird that had come from one island and had been taken elsewhere could be reliably trusted to return to its original home, hence would be used as a speedy way to relay a message there.[44] There is evidence of this practice taking place in theGilbert Islands andTuvalu.[74]
The great frigatebird was venerated by theRapa Nui people onEaster Island; carvings of the birdmanTangata manu depict him with the characteristic hooked beak and throat pouch.[75] Its incorporation into local ceremonies suggests that the now-vanished species was extant there between the 1800s and 1860s.[76]
Maritime folklore around the time of European contact with the Americas held that frigatebirds were birds of good omen as their presence meant land was near.[44]
Rostrum edentulum, rectum: apice adunco, unguiculato. Nares lineares. Facies nuda. Pedes digitís omnibus palmatis.
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