Great Frigatebird, Fregata minor

The Great Frigate bird (fregata minor) is a big dispersive seabird in the frigatebird family. Their major nesting populations are found in the Pacific, including the Galapagos Islands and the Indian Oceans, plus a population in the South Atlantic.

This bird is a lightly built large seabird up to 105 cm in length with feathers that are mostly black. This species shows sexual dimorphism; the female bird is bigger than the adult male with a white throat and breast, and the male’s scapular feathers have a green-purple sheen. During breeding season the male can swell up its striking red gular sac. They feed on fish that are taking during flight from the oceans surface, and indulges in kleptoparasitism less often than other frigatebirds. They feed in pelagic waters within 50 miles of their breeding colony or roosting areas.

It has a wide distribution throughout the world’s tropical seas. The most northern extent of their range in the Pacific Ocean is Hawaii, with around 10,000 pairs nesting mostly in the Northwestern Hawaiian Islands. In Central and South Pacific, colonies are found on most islands Groups from Wake Island to the Galapagos Islands to New Caledonia with a few pairs nesting on Australian possessions in the Coral Sea. Some colonies are also found on numerous Indian Ocean islands including Aldabra, Christmas Island, Maldives, and Mauritius. The small populations found in the Western Atlantic Ocean might still persist but are considerably small if they do. Great Frigatebirds take on regular migrations across their range, both standard trips and more infrequent widespread dispersals. The birds that are marked with wing tags on Tern Island in French Frigate Shoals were found to regularly travel to Johnston Atoll; one was reported in Quezon City in the Philippines. Although their far ranging birds also exhibit philopatry, breeding in their natal colony even if they travel to other colonies.

The scientific name of Fregata minor came to be because when it was first discovered, it was thought to be a small pelican, and so named Pelecanus minor by the German naturalist Johann Friedrich Gmelin in 1789. Due to the rules of taxonomy, its species name of minor was held on to despite being placed in a separate genus. This has led to the inconsistency between minor, Latin for ‘smaller’ in contrast with its common name. This species is also known as the lwa or ‘thief’ in Hawaiian in Hawaii. It is one of the 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 (F. andrewsii).

The Great Frigatebird is a bulky seabird and, despite its name, it is the second largest frigatebird, after the Magnificent Frigatebird. The Great Frigatebird measures 33 to 41 inches in length with long pointed wings of 80.5 to 90.5 inches and long forked tails. This bird weighs from 1.4 to 3.4 pounds. The frigatebirds hold the highest ratio of wing area to body mass, and the lowest wing loading of any bird. This has been hypothesized to make the birds able to utilize marine thermals created by small differences between tropical air and water temperatures. The male birds are smaller than females, but the extent of the variation varies with the geography. The plumage of the males is black with scapular feathers that have a green-purple iridescence when they refract sunlight. The females are black with a white breast and throat and have a red ring around their eye. The immature birds are black with a rust-tinged white face, head and throat.

The Great Frigatebird searches for food in pelagic waters within 50 miles of the breeding colony or roosting areas. Flying fish from the family Exoceotidae are the most frequent item in their diet; other fish species and squid may be eaten as well. Their prey is caught while in flight, either from just below the surface or from the air in the case of flyingfish flushed from the water. The bird will make use of school of predatory tuna or pods of dolphins that push schooling fish to the surface. Just like all frigatebirds, they will not alight on the water surface and are usually incapable of taking off should accidentally do so. Great Frigatebirds will also hunt seabird chicks at their breeding colonies, taking mostly the chicks of Sooty Terns, Spectacled Terns, Brown Noddies and Black Noddies. Studies have shown that only females (adults and immatures) hunt this way, and only a few individuals account for most of the kills. Great Frigatebirds will also attempt kleptoparasitism, chasing other nesting seabirds (boobies and tropic birds in particular) in order to make them regurgitate their food. This behavior is not thought to play an important part of their diet, and is instead a supplement to food obtained through hunting. A study of Great Frigatebirds stealing from Masked Boobies estimated that the frigatebirds could obtain at most 40% of the food needed, and on average obtained only 5%.

These 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 then vegetation is absent) in colonies of up to several thousand pairs. Nesting bushes are usually shared with other species, especially Red-footed Boobies and other species of frigatebirds. Both males and females have a patch of red skin at the throat called the gular sac; in the male Great Frigatebirds this is inflated in order 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 twenty minutes into a startling red balloon. As the 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 choose the display site, or may seek another site, to form a nesting site; once the nesting site has been established both sexes will defend their territory which is usually the area surrounding the nest from other frigate birds.

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. The males collect loose nesting material (twigs, vines, and flotsam) from around the colony and off of 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 snatches off the nesting site or stolen from other birds themselves searching for nesting material. Great Frigatebird nests are large platforms of loosely woven twigs that quickly become encrusted with guano. There isn’t much attempt to maintain the nests during the breeding season and nests may disintegrate before the end of the season.

A single dull chalky white egg measuring about 68 x 48 mm is laid during each breeding season. If the egg is lost, the pair bond breaks; females may acquire a new mate and lay another egg again in that same year. Both of the parents incubate the egg in shifts that last between 3 to 6 days; the length of the shift varies by location, although the females shift are longer than those of males. The incubation can be energetically demanding; birds have been recorded losing between 20 to 33% of their body mass during a shift. Incubation lasts for around 55 days. Great Frigatebird chicks start 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. The 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 they are older they are fed ever one to two days. Feeding is by regurgitation, the chick sticks its head inside the adults’ mouth.

Parental care is prolonged in Great Frigatebirds. Fledging occurs after 4 to 6 months, the timing dependent on oceanic conditions and food availability. After fledging chicks continue to receive parental care for between 150 to 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 Nino 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 around by one or more other fledglings. After the chick drops their stick, the chaser attempts to catch the stick before it hit’s the surface of the water, after which the game starts again. This type of play is thought to be important in the developing the aerial skills needed to fish.

Image Caption: Male greater frigate bird displaying. Photograph taken on Genovesa Island (El Barranco) in the Galapagos Islands. Credit: Charlesjsharp/Wikipedia(CC BY-SA 3.0)