Classification Until recently, the Pelecaniforme order was characterized as the only group of birds with all four toes connected with webs. Actually, this morphologic similarity was the result of convergence rather than descent from a recent common ancestral progenitor. Instead, genetic analyses have shown that the progeny of the Pelecaniforme ancestor comprises the five following families of aquatic birds.

Threskiornithidae (34 species) include ibises and spoonbills. When flying, ibises and spoonbills extend the neck and legs. They breed in colonies, frequently with herons. Spoonbills feed by sweeping their long bill from side to side in the mud.

Ardeidae (60 species) include herons (subdivided into typical herons, night herons, and tiger herons), egrets and bitterns. Herons commonly stand and fly with the neck bent in an S shape and fly with the legs trailing loosely. They have powder downs; the latter are areas of feathers that continually disintegrate to a fine powder which is used for preening (absorbing and removing fish oil, scum, and slime from the plumage).

Scopidae (Hamerkop, 1 species) is a wading bird which bears a conspicuous horizontal crest on the back of its large head. Active especially at twilight, the bird sits beside a stream with its head down or wades slowly. It builds an enormous nest of sticks, sometimes 1.8 m (6 feet) across and 1.2 m (4 feet) high, on a rocky ledge or in a tree. The nest is dome shaped with an entrance on the side and a narrow tunnel leading to a central chamber lined with mud.

Pelicanidae (8 species) generally catch their prey while swimming, thrusting their long bills and long necks below the surface to scoop up fish in their distensible throat pouches. The pouch is not used to store the fish, which are swallowed immediately. When fishing in shallow water, pelicans often cooperate to form a kind of living net. Each bird keeps station until the fish panic and can be captured as they try to escape between the birds. Pelicans have also been observed herding and capturing ducklings in a similar way. The brown pelican, unlike the other species, often forages some distance offshore and habitually fishes by plunging from the air.With some species reaching a wingspan of 3 m (10 feet), and weighing up to 13 kg (30 pounds), Pelicans are among the largest of living birds.

(A fifth family, Balaenicipitidae with a single species, the Shoebill, is not illustrated).

Articles on ibis, spoonbill, heron, hammerhead and pelican (2012) Encyclopædia Britannica Ultimate Reference Suite.  Chicago: Encyclopædia Britannica

Hackett SJ et al (2008): Phylogenomic Study of Birds