There are some very strange birds around and one is the Great Frigatebird. Others are the hoatzin bird of South America and the helmeted hornbill of south-east Asia.
The great frigate bird has the scientific name of Fregata minor. While the species name ‘minor’ may seem somewhat paradoxical, the bird was originally thought to be some kind of small pelican hence ‘minor’ meaning ‘small’. The name ‘minor’ was retained when it was placed in a separate genus. It is also known as Iwa in Hawaii, meaning ‘thief’. There are five closely related species in the genus Fregata with the Christmas Island frigatebird being the closest relative to the great frigatebird.
The great frigatebird is widespread. It is found in the Pacific and Indian Oceans, on the Galapagos Islands and there may still be several small populations in the western Atlantic. The birds make regular migrations across their range, breeding mostly on isolated islands in tropical and subtropical waters.
The great frigatebird breeds in small bushes, in mangroves or on the ground.
The bird is lightly built and between 85 and 105 cm long. Weights range from 1,000 to 1,550 grams (males) and 1215 to 1640 grams (females). The angular wings are long and pointed and the forked tail long and scissor-like. Wingspan varies from 205 to 230 cm. Frigatebirds have the lowest wing loading and the highest ratio of wing area to body mass of any bird, allowing them to soar effortlessly over the ocean and engage in spectacular aerial manoeuvres.
The plumage is mostly black while the male’s shoulder plumage has a purple-green iridescence. Males have a faint pale brown bar on the wings. Females are larger than males, have a white throat and breast and a red eye-ring. Both sexes have a patch of red skin, a gular sac, at the throat.
The males distend their gular sac to enormous proportions during the breeding season. When fully distended the sac looks like a big scarlet balloon under the bill which is long and hooked.
This species, although it chases other seabirds until they disgorge their catch (at which point they seize it for themselves), engage in this behaviour less frequently than other frigatebirds. Studies have shown that on average only 5% of the frigatebird’s food requirements are gained in this way so it is a supplementary method of feeding. They also take fish from the surface of the sea while in flight and feed within 80 km (50 miles) of their breeding colony or roosting area.
The birds return to the birth colony to breed even if they have migrated to other colonies. These birds can sustain their flight over several days and nights and even sleep while on the wing. They use a minimum of energy to cover long distances by utilising thermals to soar to heights of up to 2,500 metres then gliding down and climbing again.
Frigatebirds are clumsy on land and at risk on the sea. On land, its short legs and very small feet make walking awkward. The plumage does not have a waterproof coating and the feet are not webbed. If the bird does happen to land on the water for any length of time the plumage becomes wet and it has great difficulty taking off.
Frigatebirds sun themselves by fully extending the wings to the sides and rotating the undersides of the wings upwards. By doing this, the chest and underwings are exposed to the sun.
Its main diet consists of flying fish together with other fish species and squid. Great frigatebirds will utilise schools of tuna or dolphin pods which push schooling fish to the surface, allowing the frigatebirds to take fish on the wing without needing to land on the water. They also take seabird chicks from breeding colonies.
Tern and noddy chicks are most at risk from frigatebird attack.
Great frigatebirds are monogamous over the breeding season. It may take two years before the youngsters are independent. Colonies of several thousand pairs may nest in bushes and trees near each other.
Before mating, groups of males congregate in bushes and trees and force air into the gular sac. Within 20 minutes, it is inflated to a huge red balloon. The females fly overhead and inspect the male groups. Males call to the females, clattering their bills, shaking their wings and waggling their heads from side to side. Once the female has selected a mate, they will snake their necks together and nibble at each other. They then choose a nesting site which will be defended against other birds.
Males collect (or steal) loose nesting material which the female fashions into a large platform of loosely woven twigs. The nests become encrusted with guano. Once built, the nest is not maintained and they often disintegrate before the end of the season.
A single, chalky-white egg is laid. Incubation is shared with shifts lasting from 3 to 6 days with females sitting for the bulk of the time. Up to a third of the bird’s body mass may be lost during a shift. After an incubation of some 55 days, the chick begins calling and rubs its egg tooth against the shell. The chick is naked and helpless. After two weeks, it is covered in white down and guarded for another fortnight.
At first the chick feeds on regurgitated food every few hours but once they are older, feeding drops to once every day or two. The chick sticks its head inside the adult’s mouth to gain its food. Fledging takes 4 to 6 months and care continues for another 150 to 428 days. At some time over this period the male is believed to leave to find another mate.
The chicks play ‘catch the stick’ with one bird flying off with a stick and the other chasing it. When the stick is dropped, the pursuer tries to catch it before it hits the water. Games such as this help prepare the young birds and develop their aerial skills for when they need to feed themselves. Even so, many chicks starve to death when first independent because of inefficient fishing skills. Nesting sites are often affected by El Nino events making the provision of sufficient food even harder.
The frigate bird is not considered at risk although some populations are declining to the point of being on the verge of extirpation. Severe storms can cause breeding failures and starvation of juveniles. The reproductive rate is very slow but drops in population may go unnoticed because of the long lifespan of the birds.