Fregata maggiore

Identificazione

The great frigatebird measures 85 to 105 cm (33 to 41 in) in length and has a wingspan of 205–230 cm (81–91 in). Male great frigatebirds are smaller than females, but the extent of the variation varies geographically. The male birds weigh 1,000–1,450 g (2.20–3.20 lb) while the heavier female birds weigh 1,215–1,590 g (2.679–3.505 lb).<p>Frigatebirds have long narrow pointed wings and a long narrow deeply forked tail. They have the highest ratio of wing area to body mass and the lowest wing loading of any bird. This has been hypothesized to enable the birds to use marine thermals created by small differences between tropical air and water temperatures. The plumage of males is black with scapular feathers that have a green iridescence when they refract sunlight. Females are black with a white throat and breast and have a red eye ring. Juveniles are black with a rust-tinged white face, head, and throat.

Tassonomia

The great frigatebird was formally described in 1789 by the German naturalist Johann Friedrich Gmelin in his revised and expanded edition of Carl Linnaeus's Systema Naturae. He placed it in the genus Pelecanus and coined the binomial name Pelecanus minor. Gmelin based his description on the "lesser frigate pelican" that had been described in 1785 by the English ornithologist John Latham in his book A General Synopsis of Birds. Latham, in turn, largely based his account on the "man of war bird" that had been described and illustrated by the English naturalist George Edwards in 1760. The great frigatebird is now one of five closely related species of frigatebird that make up their own genus (Fregata) and family (Fregatidae). Its closest relative, within the group, is the Christmas Island frigatebird (Fregata andrewsi). Due to the rules of taxonomy, the species name of minor has been retained. This has led to the discrepancy of minor (Latin for "smaller") in contrast with its common name.<p>A Late Pleistocene fossilised wing phalanx and proximal end of humerus (indistinguishable from the extant great frigatebird) were recovered from Ulupau Head on Oahu.<p>Five subspecies are usually recognised:

Nome scientifico

Fregata minor

Autorità

(Gmelin, JF, 1789)

Ordine

Suliformi

Famiglia

Fregatidi

Genere

Fregata

Areale e habitat

The great frigatebird has a wide distribution throughout the world's tropical seas. Hawaii is the northernmost extent of their range in the Pacific Ocean, with around 10,000 pairs nesting mostly in the Northwestern Hawaiian Islands. In the Central and South Pacific, colonies are found on most island groups from Wake Island to the Galápagos to New Caledonia with a few pairs nesting on Australian possessions in the Coral Sea. Colonies are also found on numerous Indian Ocean islands, the most well-known include the Seychelles islands of Aldabra and Aride, Christmas Island, the Maldives and Mauritius. The Atlantic population is very small, restricted to Ascension Island, St. Helena, and the Brazilian islands of Trindade and Martin Vaz.<p>Great frigatebirds undertake regular migrations across their range, both regular trips and more infrequent widespread dispersals. Birds marked with wing tags on Tern Island in the French Frigate Shoals were found to regularly travel to Johnston Atoll (873 km), one was reported in Quezon City in the Philippines. One male great frigatebird relocated from Europa Island in the Mozambique Channel to the Maldives 4400 km away for four months, where it fed on rich fishing grounds. Despite their extended range, birds also exhibit philopatry, breeding in their natal colony even if they travel to other colonies.

Areale riproduttivo

TrO : widespread, islands of tropical Indian and Pacific oceans, local e of e Brazil (tropical Atlantic Ocean)

Areale non riproduttivo

pelagic, ranging throughout tropical Indian and tropical and subtropical Pacific oceans

Migrazione

Resident species

Comportamento

The great frigatebird forages in pelagic waters within 80 km (50 mi) of the breeding colony or roosting areas. Flying fish from the family Exocoetidae are the most common item in the diet of the great frigatebird; other fish species and squid may be eaten as well. Prey is snatched while in flight, either from just below the surface or from the air in the case of flying fish flushed from the water. Great frigatebirds will make use of schools of predatory tuna or pods of dolphins that push schooling fish to the surface. Like all frigatebirds they will not alight on the water surface and are usually incapable of taking off should they accidentally do so. They are often seen foraging in large, mixed-species flocks, especially flocking with sooty terns and wedge-tailed shearwaters.<p>Great frigatebirds will also hunt seabird chicks at their breeding colonies, taking mostly the chicks of sooty terns, spectacled terns, brown noddies, black noddies and even from other great frigatebirds. Studies show that only females (adults and juveniles) hunt in this fashion, and only a few individuals account for most of the kills. Great frigatebirds will also feed opportunistically in coastal areas on turtle hatchlings and fish scraps from commercial fishing operations.<p>Great frigatebirds will attempt kleptoparasitism, chasing other nesting seabirds (boobies, tropicbirds and gadfly petrels in particular) in order to make them regurgitate their food. This behaviour is not thought to play a significant part of the diet of the species, and is instead a supplement to food obtained by hunting. A study of great frigatebirds stealing from masked boobies estimated that the frigatebirds could at most obtain 40% of the food they needed, and on average obtained only 5%.<p>Great frigatebirds are seasonally monogamous, with a breeding season that can take two years from mating to the end of parental care. The species is colonial, nesting in bushes and trees (and on the ground in the absence of vegetation) in colonies of up to several thousand pairs. Nesting bushes are often shared with other species, especially red-footed boobies and other species of frigatebirds.<p>Both sexes have a patch of skin at the throat that is the gular sac; in male great frigatebirds this skin is red and can be inflated to attract a mate. Groups of males sit in bushes and trees and force air into their sac, causing it to inflate over a period of 20 minutes into a startling red balloon. As females fly overhead the males waggle their heads from side to side, shake their wings and call. Females will observe many groups of males before forming a pair bond. Having formed a bond the pair will sometimes select the display site, or may seek another site, to form a nesting site; once a nesting site has been established both sexes will defend their territory (the area surrounding the nest that can be reached from the nest) from other frigatebirds.<p>Pair bond formation and nest-building can be completed in a couple of days by some pairs and can take a couple of weeks (up to four) for other pairs. Males collect loose nesting material (twigs, vines, flotsam) from around the colony and off the ocean surface and return to the nesting site where the female builds the nest. Nesting material may be stolen from other seabird species (in the case of black noddies the entire nest may be stolen) either snatched off the nesting site or stolen from other birds themselves foraging for nesting material. Great frigatebird nests are large platforms of loosely woven twigs that quickly become encrusted with guano. There is little attempt to maintain the nests during the breeding season and nests may disintegrate before the end of the season.<p>A single dull chalky-white egg measuring 68 mm × 48 mm (2.7 in × 1.9 in) is laid during each breeding season. If the egg is lost the pair bond breaks; females may acquire a new mate and lay again in that year. Both parents incubate the egg in shifts that last between 3–6 days; the length of shift varies by location, although female shifts are longer than those of males. Incubation can be energetically demanding, birds have been recorded losing between 20 and 33% of their body mass during a shift.<p>Incubation lasts for around 55 days. Great frigatebird chicks begin calling a few days before hatching and rub their egg tooth against the shell. The altricial chicks are naked and helpless, and lie prone for several days after hatching. Chicks are brooded for two weeks after hatching after which they are covered in white down, and guarded by a parent for another fortnight after that. Chicks are given numerous meals a day after hatching, once older they are fed every one to two days. Feeding is by regurgitation, the chick sticks its head inside the adults mouth.<p>Parental care is prolonged in great frigatebirds. Fledging occurs after four to six months, the timing dependent on oceanic conditions and food availability. After fledging chicks continue to receive parental care for between 150 and 428 days; frigatebirds have the longest period of post-fledging parental care of any bird. The length of this care depends on oceanic conditions, in bad years (particularly El Niño years) the period of care is longer. The diet of these juvenile birds is provided in part by food they obtained for themselves and in part from their parents. Young fledglings will also engage in play; with one bird picking up a stick and being chased by one or more other fledglings. After the chick drops the stick the chaser attempts to catch the stick before it hits the water, after which the game starts again. This play is thought to be important in developing the aerial skills needed to fish.<p>Great frigatebirds take many years to reach sexual maturity and only breed once they have acquired the full adult plumage. This is attained by female birds when they are eight to nine years of age and for male birds when they are 10 to 11 years of age. The average lifespan is unknown but is assumed to be relatively long. As part of a study conducted in 2002 on Tern Island in Hawaii, 35 ringed great frigatebirds were recaptured. Of these 10 were 37 years or older and one was at least 44 years old.

Conservazione

Tendenza della popolazione

declining