Food Habits

Main Foods Taken

Mostly flying fish (Exocoetidae) and squid (Ommastrephidae; Eisenmann 1962, Diamond 1973, del Hoyo et al. 1992), or discards from fisheries (Calixto-Albarran and Osorno 2000). Young turtles, crabs taken where available; also plankton, jellyfish, scraps discarded by boats, offal from slaughter houses and sewage outlets, young of other seabirds (Howell 1932, Eisenmann 1962, Calixto-Albarran and Osorno 2000).

Microhabitat For Foraging

Restricted to narrow zone (few centimeters) immediately above and below the ocean surface, except during piracy when victims may be pursued ≤150 m above land or water (Gibbs and Gibbs 1987). Takes airborne flying fish within a few centimeters of the surface by aerial-dipping. Often feeds over tuna or other predatory fish which drive smaller fish to the surface where they are available to frigatebirds. Satellite-tracking data needed to determine feeding locations and relationship to oceanographic features, and to test suggestion by Diamond (1972, 1973) that this species feeds closer to coast than other frigatebirds.

Time Of Feeding

Diurnal; generally begins leaving roost at dawn or soon thereafter, climbing on thermals and soaring off; leaves individually and in groups over several hours (AWD, EAS). Generally arrives back at roost or colony after noon; may soar overhead for hours before landing. Not known to catch food after dark, but may arrive back at colony to feed young after dark (Diamond 1973, Trivelpiece and Ferraris 1987).

Food Capture And Consumption

Much time spent soaring effortlessly, head drawn into shoulders, tail-forks closed, often at great height, probably searching for food at considerable distance (EAS, AWD). Two distinct methods of capturing food: (1) primary method, self-feeding, by gliding, dropping, or diving down to ocean surface, rapidly swinging bill down to snatch prey from just above, on, or just below the surface, without settling on water (Fig. 2); floating food (and nest material) picked delicately from surface; (2) piracy (kleptoparasitism) by aerial- chasing of variety of species (including own) to force them to disgorge food, which is then caught in flight as it falls, often before it hits ground or water (EAS, AWD). Adult male uses same techniques (self-feeding and piracy) to gather nest material. Activity patterns at colonies suggest that feeding is confined to daylight hours (Diamond 1973, Trivelpiece and Ferraris 1987). Patterns of piracy differ between sites and thought to be carried out by a small portion of population (EAS, AWD): at breeding colonies in Mexico (Isla Isabel; Osorno et al. 1992), Belize (J. Verner in Eisenmann 1962), and Little Cayman, West Indies (AWD), piracy carried out mostly by adult females and immatures (exclusively at Little Cayman); but males made more attacks than females or immatures at a nonbreeding roost in Galápagos Is. (Gibbs and Gibbs 1987) and in Campeche, Mexico (Gochfeld and Burger 1981). Success rate also varies between sites (4% in Mexico, 8.3% Galápagos, 38% Little Cayman) and between species of victim (e.g., in Galápagos, 22% on Swallow-tailed Gulls [Larus furcatus], 8% on Red-billed Tropicbirds [Phaethon aethereus]; but in Mexico, same success rate against Brown [Sula leucogaster] and Blue-footed [S. nebouxii] boobies). May attack singly or in groups; of 91 attacks at Little Cayman, success rate of 18 groups was 83% compared with 27% for single attackers (AWD); and at Isla Isabel, w. Mexico, groups were 3 times as successful as singletons (Osorno et al. 1992). Pirating frigatebirds may themselves be robbed, e.g., by Heermann’s Gulls (Larus heermanni) at Isla Isabel (Osorno et al. 1992) and Laughing Gulls (L. atricilla) at Barbuda (AWD). Victims of piracy include Brown Pelicans (Pelecanus occidentalis), Brown, Blue-footed, and Red-footed (Sula sula) boobies, Neotropic Cormorants (Phalacrocorax brasilianus), gulls, terns, tropicbirds, and Osprey (Pandion haliaetus). Frigatebird often seizes wing-tip, tail, or foot of victim and may up-end it in flight (AWD, EAS).

Apparently feeds closer inshore than other species of frigatebird (Diamond 1972, 1973), but this needs to be confirmed by detailed observations in more areas. Feeding rate of 2.2 feeds/chick/d for Magnificent Frigatebird in Galápagos Is., compared with 0.7 feeds/chick/d for Great Frigatebird there (Cepeda and Cruz 1994) supports view of this species as feeding inshore (Diamond 1972, 1973; Norton 1988) rather than pelagically as in other frigatebirds (Diamond 1972, Nelson 1975; see Breeding: parental care [rate of feeding], below); but rate of 0.29 feeds/chick/d on Isla Isabel, w. Mexico (Osorno 1996) contradicts this view. Suggestion by Carmona et al. (1995) that males feed further offshore than females is a misreading of Gochfeld and Burger (1981) and Diamond (1972, 1973); males must feed farther from colony after they have deserted it, but no evidence that they do so while still incubating or feeding young. Often feeds over schools of predatory fish that drive smaller fish to surface, but extent of reliance on such concentrations is unknown.

Major Food Items

Chiefly flying fish and squid on Barbuda, e. West Indies (Diamond 1973); clu-peid (herring) and scombroid (tuna) fish on Isla Santa Margarita, w. Mexico (Carmona et al. 1995); mainly discards from prawn-fishing boats at Isla Isabel, w. Mexico (Calixto-Albarran and Osorno 2000); also offal, young turtles, crabs, and seabird chicks taken from land or sea (Eisenmann 1962, Cramp and Simmons 1977, EAS). Only data on difference between seasons from Isla Isabel, w. Mexico, where change in proportions of major diet items (Perciforms, Scorpeaniforms, Clupeiforms, and Batrachoidiforms) between May and Jul 1991 attributed to seasonal changes in availability (Calixto-Albarran and Osorno 2000). Opportunistic: takes fish in freshwater ponds if nearby, or from fishing nets; eats small chicks of their own or other species (Eisenmann 1962, EAS). Learns new food sources: learned to take dynamited, stunned fish during building of Panama Canal (Hallinan 1924); Red-footed Booby eggs off Tobago (Brown 1947); fish from human hand (EAS).

Quantitative Analysis

Only quantitative information is from regurgitations from chicks, adults feeding chicks, or flying juveniles, at 3 colonies. In 10 samples from Barbuda, West Indies (Diamond 1973), squid occurred in 5, flying fish in 4, other fish in 4; of 51 identified food items, 55% squid (mainly Ommastrephidae), 29% flying fish. Squid 5–11 cm long, fish 3–17 cm. At Isla Santa Margarita, w. Mexico, diet was “sardines” belonging to 4 clupeid and 1 scombroid species (Carmona et al. 1995). At Isla Isabel, w. Mexico, 50 species of fish, one of squid and two of crustaceans, recorded in 158 regurgitations from adults, chicks, and juveniles (Calixto-Albarran and Osorno 2000). Adult males and females did not differ in diet, but juveniles consumed proportionately more surface-dwelling Clupeidae; this population as a whole is evidently consuming mostly discards from prawn-fishing boats (Calixto-Albarran and Osorno 2000). Diet samples may include items derived from other species by piracy, and are often biased by presence of indigestible but easily identified items such as pectoral spines of flying fish and squid beaks (Diamond 1973). Summary of qualitative information shows range of food varied, including several families of fish other than flying fish, jellyfish, large plankton, booby eggs, tern chicks, hatchling turtles, freshwater fish, and offal from slaughterhouses, fishing boats, and sewage outlets (Eisenmann 1962, Cramp and Simmons 1977). Likely takes almost anything available at surface; prone to capture on baited fishing lines (EAS). All data from breeding season; diet at other times unknown.

No data on how food is selected; probably takes prey that is most available near the surface (EAS, AWD). Food items may be stored in proventriculus for transport back to colony to feed chick.

No data. Male desertion of chick at 3 mo of age suggests this species may be less energy-limited than other frigatebirds in that both adults are not needed to complete raising of chicks (AWD, EAS). See also Breeding: phenology, below. Studies of energetics over a period of years are needed, especially in comparison with other species of frigatebird.

Metabolism

No data.

Diurnal Cycle Of Body Temperature

No quantitative data. Uses gular-fluttering during heat of day to dispel heat. May also: (1) sit with scapulars raised to allow air circulation and cooling under feathers; (2) sit upright with underwings exposed to sun, drooping wings and rotating (sometimes called “sunning”; see Fig. 3); (3) sit totally relaxed at perch, body down low, head and wings drooped straight down (AWD, EAS). Downy chicks sit with back to sun and head down in own shade during heat of day if not under parent; may elevate rump toward sun and may sit in “sunning” posture (EAS, AWD).

Drinks opportunistically from freshwater pools (A. Wetmore in Eisenmann 1962). Not known to cast pellets, but Great Frigatebird does cast pellets when small tern chicks consumed; Magnificent likely to also (EAS, AWD). Defecation on nests by chicks may contribute to reinforcement of nest structure during chick period.