Fregata minor

Geographic Range

Great frigatebirds are found in tropical waters globally. They occur between 25 degrees north and 25 degrees south. Nesting colonies are known from offshore islands throughout the tropical Pacific, western Atlantic, and south Indian oceans. Most of what is known about great frigatebirds is known from nesting colonies and there is little information about their range and movements outside of the breeding season. Males and females may occupy separate ranges outside of the breeding season. Great frigatebirds are considered sedentary, although individuals disperse from nesting areas to broader ranges when not breeding.

• Biogeographic Regions
• indian ocean
  • native native
• atlantic ocean atlantic ocean
  • native native
• pacific ocean pacific ocean
  • native native

Habitat

Great frigatebirds are found over open, tropical ocean waters and near offshore, oceanic nesting islands. Males and females may occupy different ranges outside of the breeding season, which may be influenced by their different wing loading characteristics and the nature of winds over different areas of the ocean. When not breeding, great frigatebirds wander widely to feed on fish and squid in areas with high concentrations of prey, such as at ocean upwellings, divergences, and convergences. Great frigatebirds breed on islands without predators. They nest in trees and shrubs, such as beach naupaka ( Scaevola sericea ), beach heliotrope ( Tournefortia argentea ), pisonia ( Pisonia grandis ), and mangroves ( Bruguiera and Rhizophora species). Nests are usually above 0.5 m and may be several kilometers inland on larger islands. Great frigatebirds are superb soaring birds and do not need to come to land frequently to roost. They can soar for long period of time, including overnight.

• Habitat Regions
• tropical tropical
• saltwater or marine saltwater or marine

• Aquatic Biomes
• pelagic pelagic

Physical Description

Great frigatebirds have highly specialized for life in flight. They have the highest ratio of wing are to body mass of any bird. They have exceptionally long wings for soaring, long, forked tails for maneuverability in flight, and very small legs and feet. Their legs and feet are so small that they cannot walk on them, only perching between flights. Great frigatebirds are also one of few seabird species that are sexually dimorphic in both size and plumage. Males are smaller than females and are entirely black, with a greenish-purple sheen dorsally. Females are larger, with a black head and black feathers dorsally, but with a white chin and chest that merges into their white belly. Males have a large, red, inflatable gular sac that becomes enlarged during the breeding season and is used in courtship displays. Male gular sacs become smaller and fades in color outside of the breeding season. Immature individuals are similar to females in plumage, but with light rufous feathers on the head and between the grey chin and chest and white belly. Great frigatebirds are distinctive birds, most often seen soaring above the water, where their long, forked tail and long, pointed wings held in a "W" shape make them easy to identify. They have long bills with a strongly hooked tip. They are from 85 to 105 cm in body length, from 205 to 230 cm in wingspan, and from 1 to 1.8 kg mass.

Great frigatebirds are likely to be confused only with other frigatebirds , especially in their immature or juvenile plumages. Incomplete understanding of regional variation in plumage patterns, vocalizations, and soft body parts in widespread species, such as great, lesser , and magnificent frigatebirds may also complicate identifications.

There are 5 recognized subspecies of great frigatebirds. Subspecies vary in body size, plumage, eye ring color, and bill color, but patterns of variation have not been well described. Subspecies are defined geographically and banding studies suggest that there may be little migration of individuals among regions. Subspecies are: F. m. minor in the eastern Indian Ocean and Australia, F. m. aldabra in the western Indian Ocean, F. m. palmerstoni throughout western and central Pacific, F. m. ridgwayi in the eastern Pacific, and F. m. nicolli in the western Atlantic.

Great frigatebirds seem to maintain body temperatures of about 40 degrees Celsius, shivering at lower temperatures. Nestlings are dependent on parents to protect them from the heat of the tropical sun. They use a variety of body postures to help radiate or absorb heat.

• Other Physical Features
• endothermic endothermic
• homoiothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger
• sexes colored or patterned differently
• male more colorful
• ornamentation ornamentation

Reproduction

Great frigatebirds form monogamous, mated pairs yearly. If a pair is unsuccessful in mating, then they may divorce and select a new mate to attempt breeding. Extra-pair copulations are frequent and males often attempt to copulate with mated females when their mates are absent.

Males display in tight groups on a shrub or tree, often only 1 to 1.5 meters apart. They display continuously for several days until they acquire a mate. Females soar above display sites to assess males. Courtship displays involve a male inflating his bright red gular sac, pointing his head and bill upwards, vibrating the wings while they are extended, and using a warble vocalization and bill-rattling. Males orient themselves towards females that are soaring above. Courtship display then proceeds to reeling vocalization and rolling the head from side to side. Once a female chooses a male, they spend several days close to each other and occasionally engaging in mutual head waving.

• Mating System
• monogamous monogamous

Great frigatebird females breed only every 2 years or less often. Males occasionally breed yearly, but typically breed only as often as every 2 years. The timing of breeding varies substantially with region and the breeding season is extended. Breeding is seasonal in a region, but breeding is recorded from December through September throughout their range. Eggs are typically laid in a 5 to 6 month period, but eggs have been observed throughout the year. Seasonality of breeding in a region is probably linked to regional food availability. In nest clusters within a colony, egg-laying and hatching may be fairly synchronous. Once a pair bond has formed, a nest has been prepared, and an egg is laid, mates do not interact much, even when they exchange caretaking responsibilities. Time from courtship to nest building may be as little as a few days or as long as 4 weeks. Nests are generally platforms built of twigs, sticks, and other collected materials on the same trees or bushes that were used by males for courtship displays, resulting in clustered nesting colonies of 3 to 50 nests and 0.6 to 1.4 m between nests. Nests are generally sheltered from the wind but in full sun. Occasional nests are built on the ground. Generally a single, white egg is laid, but rare nests with 2 eggs or nestlings have been observed. It is possible that the eggs were laid by more than one female. Females may lay a second egg in a season if the first fails or is destroyed. Eggs are incubated immediately after laying and are never left unattended. Young begin to fly at 150 days old. They remain on the nest for 150 to 428 days after fledging, where they continue to be fed and protected by their parents. Fledglings remain near the nest for 10 to 16 months after hatching, at which point they disperse to the ocean. Great frigatebirds have an extended period of adolescence and attain sexual maturity between 5 and 7 years old. Occasionally individuals with immature plumage have been observed breeding.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• oviparous oviparous

Once a mated pair is formed, one or the other of the parents stays at the nest site until the nestling is 4 to 6 weeks old. Both parents incubate the egg and brood the nestling. Parents take turns incubating the egg from 3 to 18 days at a time. Typical incubation shift lengths are from 4.1 to 6.4 days long, but they vary regionally and are probably related to the distance the other parent has to travel to be able to forage. Females incubate for longer than males, in general. Young are altricial at hatching, naked and with their eyes closed. They grow very slowly, possibly as an adaptation to low or variable food availability. Growth rates vary with the availability of food. They are fed 2 to 4 times a day in their first few weeks and only every 1 to 2 days later in their deelopment. Nestlings are not left unattended by a parent until they are about 1 month old. By 14 days old nestlings are covered in white down and they develop flight plumage and begin flying by 150 days after hatching. They remain in the nest for 150 to 428 days after fledging and continue to be cared for by their parents during that time.

• Parental Investment
• altricial altricial
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • male
    • female
• pre-weaning/fledging
  • provisioning
    • male
    • female
  • protecting
    • male
    • female
• pre-independence
  • provisioning
    • male
    • female
  • protecting
    • male
    • female

Lifespan/Longevity

There is little available information on longevity in great frigatebirds. The oldest banded bird lived to 37 years and a maximum age of 40 years is estimated and from 25 to 30 years is considered a typical lifespan. At one site, during a productive year, between 60 and 70% of eggs laid were successfully raised to fledging. But complete failure, or failure rates of up to 81% are reported from El Niño years when food is scarce.

Behavior

Great frigatebirds rarely land on the ground or on the water, except perhaps to pick up nesting material or by accident. Their short legs and small feet make them ungainly on the ground and make it difficult to propel themselves off of the water. Their feet are not webbed. However, they can soar for many hours without flapping, taking advantage of updrafts over water. They will soar over nesting colonies rather than roost, when the winds are right. Great frigatebirds are more likely to be active and soar along the fronts of storm systems or in high wind, they are more likely to roost in calm winds. Great frigatebirds have exceptionally low wing loading, which differs between sexes because of their differences in size. This may explain the fact that sexes are segregated geographically outside of the breeding season. There is some evidence that great frigatebirds return to the same area to breed.

Great frigatebirds are active during the day, often roosting at night, although they will soar throughout the night as well. They forage in flocks, sometimes with multiple seabird species, and roost in groups of just a few to thousands. Nesting colonies are large and often include other species, such as red-footed boobies ( Sula sula ), black noddies ( Anous minutus , and white terns ( Gygis alba ).

• Key Behaviors
• flies
• diurnal diurnal
• motile motile
• nomadic nomadic
• territorial territorial
• social social

Home Range

While roosting and in nesting colonies they defend small display and nesting sites with bill-snapping, vocalizations, lunging, and gular displays among males. These territories are very small, so that individuals are able to touch each other. There is no firm data on home range, but estimates suggest that great frigatebirds feed from 80 to 500 km from their colony.

Communication and Perception

Great frigatebirds are generally quiet, but they make a variety of sounds in different contexts, especially at breeding colonies. They snap their bills and squawk at birds that are too close to them in breeding colonies. Hatchlings use a harsh begging call, along with bobbing of their heads and spread wings. Adults call to young as they return also, to let them know they are coming with food. Males call more than females but there are no songs. There are 3 kinds of calls: landing calls, warbling, and reeling calls. Landing calls occur when adults are returning to the breeding colony, although females are usually quiet. Warbling and reeling calls are used by males during courtship displays, along with bill-rattling. These calls may be used when flying over potential female mates or when engaging in mutual head-waving as part of courtship. Great frigatebirds also use bill-snapping or rattling and vibrations of their mandibles to make sounds. Bill-snapping is used in agressive interactions, rattling and vibrations are used in courtship.

• Communication Channels
• visual visual
• acoustic acoustic

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

Great frigatebirds are best known for their kleptoparasitic habits; they frequently steal food from other sea birds by harassing them until they drop their prey or regurgitate a recent meal. They pursue other birds, especially near nesting colonies, diving at them and grabbing them until they release their food. Great frigatebirds then dive rapidly to catch the released prey or regurgitate before it hits the water. Bird species commonly harassed by great frigatebirds are boobies ( Sula ), tropicbirds ( Phaethon ), and petrels ( Pterodroma ). However, great frigatebirds capture most of their food themselves, by grabbing fish at or just below the water's surface.

Great frigatebirds eat mainly flying fish ( Exocoetidae ) and squid ( Ommastrephidae ) found within 15 cm of the ocean surface. Most foraging occurs over deep, ocean waters in areas where upwelling, divergence, or convergence brings nutrient rich water close to the surface. They may also feed over schools of large, predatory fish ( Katsuwonus and Euthynnus species ) or dolphins ( Stenella , Delphinus , and Steno species) that drive smaller fish to the surface. Great frigatebirds will also feed opportunistically in coastal areas on turtle hatchlings, fish scraps from commercial fishing operations, and on seabird nestlings in breeding colonies, including great frigatebird nestlings from their own nesting colonies. They are often seen foraging in large, mixed-species flocks, especially flocking with sooty terns ( Sterna fuscata ) and wedge-tailed shearwaters ( Puffinus pacificus ). Great frigatebirds occasionally drink fresh water by dipping their bill into water while in flight.

• Primary Diet
• carnivore carnivore
  • piscivore piscivore
  • molluscivore molluscivore

• Animal Foods
• birds
• reptiles
• fish
• mollusks

Predation

There are no reported natural predators of adults, although humans will capture adults, eggs, and young to eat. Eggs and nestlings may be preyed on by other frigatebirds ( Fregata ), owls ( Strigiformes ), and introduced predators such as rats ( Rattus ) and domestic cats ( Felis catus ). Bristle-thighed curlews ( Numenius tahitiensis ) have been reported eating their eggs.

Ecosystem Roles

Great frigatebirds commonly forage and nest with other species of seabirds. They will steal food from other seabirds as well, especially from boobies, tropicbirds, and petrels. Parasites reported include feather lice ( Phthiraptera ), including the species Colopocephalum angulaticeps , Fregatiella aurifasciata , and Pectinopygus gracilicornis , and hippoboscid flies ( Olfersia spinifera ).

Economic Importance for Humans: Positive

Great frigatebirds are important members of pelagic ocean ecosystems. Adults, nestlings, and eggs are collected for food in some areas. On islands in the Pacific young frigatebirds were sometimes raised as pets and used to convey messages from traveling islanders to their homes. Nesting colonies contribute to guano deposits.

• Positive Impacts
• food food
• produces fertilizer

Economic Importance for Humans: Negative

There are no adverse effects of great frigatebirds on humans. They live on remote, offshore islands and over open ocean. They may take fish scraps from commercial fishing operations or steal small fish from nets or baited hooks.

Conservation Status

Great frigatebirds are considered "least concern" by the IUCN because of their large population sizes and range. Populations have historically declined, primarily because of disturbance at historical breeding colonies and destruction of nesting habitats. In addition, introduced predators can seriously impact nesting populations. Populations may still be declining, but there is little data to understand the pattern of decline. They are protected by the U.S. Migratory Bird Treaty Act. Great frigatebird adults and young are captured and eaten by humans and die when they collide with man-made structures.

Other Comments

Great frigatebirds are also sometimes known as "man o'-war birds," a reference to their aggressiveness towards other birds and an analogy to the fast "frigate" or "man o'war" ships used by pirates. Great frigatebirds are also known as "iwa" in Hawai'ian, a name which means "thief."

Additional Links

Encyclopedia of Life

Contributors

Tanya Dewey (author), Animal Diversity Web.

References

Gauger, M., V. Schreiber, E. Schreiber. 2002. Great Frigatebird (Fregata minor). The Birds of North America Online , 681: 1-20. Accessed June 16, 2009 at http://bna.birds.cornell.edu/bna/species/681 .