The Pelecaniformes are an order of medium-sized and large waterbirds found worldwide. As traditionally—but erroneously—defined, they encompass all birds that have feet with all four toes webbed. Hence, they were formerly also known by such names as totipalmates or steganopodes. Most have a bare throat patch (gular patch), and the nostrils have evolved into dysfunctional slits, forcing them to breathe through their mouths. They also have a pectinate nail on their longest toe. This is shaped like a comb and is used to brush out and separate their feathers. They feed on fish, squid, or similar marine life. Nesting is colonial, but individual birds are monogamous. The young are altricial, hatching from the egg helpless and naked in most. They lack a brood patch.
The pelicans, shoebill and hamerkop form a clade within the order, with their next closest relatives being a clade containing the herons, ibises and spoonbills. The Fregatidae (frigatebirds), Sulidae (gannets and boobies), Phalacrocoracidae (cormorants and shags), Anhingidae (darters), and Phaethontidae (tropicbirds) were traditionally placed in the Pelecaniformes, but molecular and morphological studies indicate they are not such close relatives. They have been placed in their own orders, Suliformes and Phaethontiformes, respectively.