Fregatidae – Frigatebirds

The Fregatidae or Frigatebirds are a family of seabirds, 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 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 and squid, caught when chased to the water surface by large predators such as tuna. They 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 only able to breed 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; the Magnificent Frigatebird, Great Frigatebird and Lesser Frigatebird. Two are endangered the Christmas Island Frigatebird and Ascension Island Frigatebird, and restrict their breeding habitat to one small island each. The oldest fossils date to the early Eocene, around 50 million years ago.

Having the largest wing-area-to-body-weight ratio of any bird, frigatebirds are essentially aerial. 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. Like swifts they are able to spend the night on the wing, but they will also return to an island to roost in trees or on cliffs. Field observations in the Mozambique Channel found that Great Frigatebirds could remain on the wing for up to 12 days while foraging. Highly adept, they use their forked tails for steering during flight and make strong deep wing-beats, 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. Conversely, they do not swim and with their short legs cannot walk well or take off from the sea easily.

The average life span is unknown but in common with seabirds such as the wandering albatross and Leach’s storm petrel, frigatebirds are long-lived. In 2002, 35 ringed Great Frigatebirds were recovered on Tern Island in the Hawaiian Islands. Of these ten were older than 37 years and one was at least 44 years of age.

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. They also place their heads in the shade of their wings, and males frequently flutter their gular pouches.

Frigatebirds’ feeding habits are pelagic, and they may forage up to 500km from land. They do not land on the water but snatch prey from the ocean surface using their long, hooked bills. They mainly catch small fish such as flying fish, particularly the genera Exocoetus and Cypselurus, that are driven to the surface by predators such as tuna and dolphinfish, but they will also eat cephalopods, particularly squid. Menhaden of the genus Brevoortia can be an important prey item where common, and jellyfish and larger plankton are also eaten. Frigatebirds have learned to follow fishing vessels and take fish from holding areas. Conversely tuna fishermen fish in areas where they catch sight of frigatebirds due to their association with large marine predators. Frigatebirds also at times prey directly on eggs and young of other seabirds, including boobies, petrels, shearwaters and terns, in particular the sooty tern.Vp>They will rob other seabirds such as boobies, particularly the red-footed booby, tropicbirds, shearwaters, petrels, terns, gulls and even ospreys 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.