PELECANIFORM

order (Pelecaniformes) of relatively large, aquatic birds that share the common characteristic of webbing between all four toes. The three suborders are the Phaethontes (including the tropic bird), the Fregatae (the frigate bird), and the Pelecani (including the pelican, booby, gannet, cormorant, and snakebird). The three species of tropic birds, confined to the tropics, are the most pelagic (free-flying over open ocean) of the pelecaniforms. They range far out to sea in search of small fish and squid, which they detect while hovering over the sea, and then plunge into a powerful dive to capture prey just below the surface of the water. Their courtship display is conducted in midair and often ends with one or both birds flying to the nest site, which is located on open ground under the shelter of a bush or rock, where a single egg is eventually laid. Frigate birds are also found solely in the tropics. They are distinguished by their long powerful wings, deeply forked tail, and long, hooked beak. Frigate birds use their mastery of flight to advantage in pursuing other seabirds, which they harry until the seabirds disgorge their prey. The frigate birds then catch the food in the air or retrieve it from the surface. They also catch their own prey, particularly flying fish, in midair and can snatch fish from the surface without alighting. Courtship behaviour of the frigate bird includes a display in which the male, while leaning back and extending his wings, inflates his scarlet throat (gular) sac, vibrates his wings, and claps his bill. This display is also used in individual recognition. As with the tropic birds, a single egg is laid. Five living species of frigate birds are known. Pelicans are found in both freshwater and marine habitats from the tropics to the temperate zones. These large, buoyant birds tend to fish while swimming, thrusting their large beaks and long necks below the surface to scoop up fish, which they store in their distensible throat pouches. Their buoyancy is due to pneumatic bone structure and water-repellent plumage. Groups of pelicans will cooperate to herd fish into the shallows. The brown pelican (Pelecanus occidentalis) of the Pacific coast of Central and South America, unlike the other species, is known to forage some distance offshore, plummeting into the water to seize its prey. Pelicans build nests on the ground, large but untidy constructions of twigs, grass, algae, and feathers. They lay one to three eggs. Boobies are found from the tropics to the temperate zones in marine habitats. Both gannets and boobies are strong fliers, capturing prey by diving from great heights to spear fish underwater. The blue-footed booby (Sula nebouxii) exhibits cooperation when hunting in a group. Boobies are colonial nesters. Their courtship display consists of raising the head and tail and partially lifting the wings. Most boobies lay their eggs in small depressions on open ground, though some species nest on cliff ledges. Gannets and boobies lay one or two eggs. The cormorants and snakebirds (also called darters, or anhingas) are long-necked, underwater swimmers who pursue and lie in wait for prey below the surface of the water. Snakebirds are found in freshwater habitats from the tropics to the temperate zones. The cormorants inhabit both freshwater and marine environments and are dispersed from the Equator to the polar regions. While cormorants pursue fish and sometimes mollusks on their underwater dives, snakebirds lie in wait underwater and stab passing fish. The cormorants can engage in cooperative fishing activity. Adaptations for diving in these species include closed external nostrils. Both cormorants and snakebirds are colonial nesters; male courtship display consists of raising the head and tail and lifting and waving their wings. The clutch consists of two to five eggs. While few external features unite the pelecaniforms, certain anatomical features are common to species with similar habits. For example, the more aerial species have pneumatized skeletons. Those types that dive from great heights, such as boobies, tropic birds, and brown pelicans, have air sacs under the skin, forming a spongy, protective mattress that aids in buoyancy. In cormorants and snakebirds the large contour feathers are wettable and trap little air. This keeps buoyancy low and enables them to submerge. Because most pelecaniform birds nest in exposed situations, they and their young are subject to overheating. A common method of cooling is by rapid panting of the throat pouch (absent in tropic birds) to maximize evaporative cooling. In all species, both parents share the feeding and incubating of the young. Great frigate bird (Fregata minor) Brown pelican (Pelecanus occidentalis). Pied cormorant (Phalacrocorax varius) (order Pelecaniformes), any of the relatively large and diverse group of aquatic birds that share the common characteristic of webbing between all four toes. There are three main groups (suborders): Phaethontes, the tropic birds; Fregatae, the frigate birds (see photograph); and Pelecani, the pelicans (see photograph), boobies (including the gannet), cormorants (see photograph), and snakebirds (anhingas or darters). All are relatively large birds: they range in length from about 40 centimetres or 16 inches (excluding the elongated central tail feathers) in the white-tailed tropic bird (Phaethon lepturus) to 1.8 metres (six feet) in the Dalmatian pelican (Pelecanus crispus). Additional reading G.F. Van Tets, A Comparative Study of Some Social Communication Patterns in the Pelecaniformes (1966), discusses the behaviour and relationships of the different groups. Bryan Nelson, Galpagos: Islands of Birds (1968), contains a readable and authoritative account of the ecology and behaviour of tropical pelecaniforms, especially boobies and frigatebirds, and his The Sulidae (1978), is an extensive treatment of gannets and boobies. N. Philip Ashmole The Editors of the Encyclopdia Britannica