Pelecaniformes

The Pelecaniformes is a (possibly invalid) 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 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.

In the all-encompassing "steganopode" circumscription, the Pelecaniformes had some 50–60 living species. However, modern opinion considers the apparent similarities the result of convergent evolution, and based on a wealth of evidence splits the classically defined "Pelecaniformes" into several groups. Most lineages—frigatebirds, gannets, cormorants and anhingas—constitute indeed a natural group, for which the name Phalacrocoraciformes has been proposed. Tropicbirds are of unclear relationships, but appear to be a quite distinct lineage; they are typically placed in their own order. The pelicans (Pelecanidae), meanwhile, are linked to the storks (Ciconiidae) by two bizarre monotypic families, the Hammerkop (Scopidae) and the Shoebill (Balaenicipitidae). Indeed, they may be more closely related to storks than these are to herons. To overcome this confusion, it has been proposed to merge the "core" Pelecaniformes into the Ciconiiformes.

Systematics and evolution
Sibley and Ahlquist's landmark DNA-DNA hybridisation studies (see Sibley-Ahlquist taxonomy) led to them placing the families traditionally contained within the Pelecaniformes together with the grebes, cormorants, ibises and spoonbills, New World vultures, storks, penguins, albatrosses, petrels, and loons together as a sub-group within a greatly expanded order Ciconiiformes, a radical move which by now has been all but rejected: their "Ciconiiformes" merely assembled all early advanced land- and seabirds for which their research technique delivered insufficient phylogenetic resolution.

Recent research strongly suggests that the similarities between the Pelecaniformes as traditionally defined are the result of convergent evolution rather than common descent, and that the group is paraphyletic. All families in the traditional or revised Pelecaniformes except the Phalacrocoracidae have only a few handfuls of species at most, but many were more numerous in the early Neogene. Fossil genera and species are discussed in the respective family or genus accounts; one little-known prehistoric pelecaniforms, however, cannot be classified accurately enough to assign them to a family. This is "Sula" ronzoni from Early Oligocene rocks at Ronzon (France), which was initially believed to be a sea-duck and possibly is an ancestral pelecaniform.

The "pelecaniform" lineages appear to have originated around the end of the Cretaceous. Monophyletic or not, they appear to belong to a close-knit group of "higher waterbirds" which also includes groups such as penguins and Procellariiformes. It is interesting to note that there are quite a lot of fossil bones from around the K–T boundary which cannot be firmly placed with any of these orders and rather combine traits of several of them. This is of course only to be expected, if the theory that most if not all of these "higher waterbird" lineages originated around that time is correct. Of those apparently basal taxa, the following show some similarities to the traditional Pelecaniformes:


 * Lonchodytes (Lance Creek Late Cretaceous of Wyoming, USA)
 * Torotix (Late Cretaceous)
 * Tytthostonyx (Late Cretaceous/Early Palaeocene)
 * Cladornis (Deseado Early Oligocene of Patagonia, Argentina)
 * "Liptornis"—a nomen dubium

The proposed Elopterygidae—supposedly a family of Cretaceous Pelecaniformes—are neither monophyletic nor does Elopteryx appear to be a modern bird.

List of "pelecaniform" families

 * Pelecanidae: pelicans. Very large birds with throat pouches in which they catch and store fish while hunting.
 * Pelagornithidae: pseudotooth birds. An extinct family of gigantic seabirds that looked similar to albatrosses, but had a large bill with tooth-like projections that enabled them to pick up slippery prey like fish or squids more easily. They may actually be Galloanserae closely related to waterfowl, not Neoaves like the other "pelecaniform" families.
 * Plotopteridae: plotopterids or diving-"boobies". An extinct group of penguin-like seabirds. Possibly link penguins and (some?) pelecaniforms. Depending on how the remaining Pelecaniformes would be split up, the plotopterids might have to be placed in a monotypic order, as some similarities with penguins are possibly synapomorphies.
 * Fregatidae: frigatebirds. A group of five closely related large birds with black and white plumage, very long wings, and parasitical hunting habits. Red throat patches are inflated in display. They are usuually placed in a monotypic suborder Fregatae, and this seems to be appropriate. If split off in the Phalacrocoraciformes, it may also be simply treated as a basal lineage thereof.

The following four families can be united as suborder Sulae (Sulides in older sources), and would make up the core of the Phalacrocoraciformes:
 * Sulidae: gannets and boobies. Medium to large species which hunt by diving from the air into the sea (plunge diving). Long wings and bills, often coloured feet.
 * Phalacrocoracidae: cormorants and shags. Medium to large with hooked bills and usually black or similar dark plumage. Plumage is not fully waterproof.
 * Anhingidae: darters. Another small closely related group of four species, with long bills, snake-like necks and the ability to swim with their body submerged. Plumage is not fully waterproof.
 * Protoplotidae: an extinct family which apparently is derived from the same ancestor as the darters, but is very poorly known.

The tropicbirds (Phaethontidae) and their prehistoric relatives Prophaethontidae were traditionally placed in the Pelecaniformes, but molecular and morphological studies indicate they are not that close relatives. They have been placed in their own order Phaethontiformes. They are medium-sized birds, adapted to a marine lifestyle similar to frigatebirds. They are also noted for their aerobatic capabilities, appearing somewhat like large, slow, white hummingbirds in courtship flight. Adults have two long central tail feathers, no gular patch and normal nostrils. Hatchlings are covered in down. They have been included in the "Metaves" a proposed clade that is likely not monophyletic however; most evidence points towards a fairly close relationship with Procellariiformes and/or Charadriiformes.