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The Pelecaniformes are an order of medium-sized and large Water bird found worldwide. As traditionally (but ) defined, they encompass all birds that have feet with all four Webbed foot. Hence, they were formerly also known by such names as totipalmates or steganopodes. Under this obsolete definition, the Fregatidae (frigatebirds), Sulidae (gannets and boobies), Phalacrocoracidae (cormorants and shags), Anhingidae (darters), and Phaethontidae (tropicbirds) were included in the Pelecaniformes. Subsequent molecular and morphological studies indicate they are in fact not close relatives to "true" Pelecaniformes, and they are now placed in their own orders, Suliformes and Phaethontiformes, respectively.
Pelecaniformes as currently defined comprise the pelicans, shoebill and hamerkop, which form a clade within the order (suborder Pelecani), along with herons (Ardeidae) and ibises and spoonbills (Threskiornithidae); the relationships between Pelecani and the other two families are still being debated by .
Most have a bare throat patch (gular skin), and the have evolved into dysfunctional slits, forcing them to breathe through their mouths. They also have a (comb-like) nail on their longest toe, which is used Self-grooming their feathers. They feed on fish, squid, and other . Nesting is Bird colony, but the individual birds are monogamous, pairing up to rear their respective clutches. These birds lack a brood patch. The young are altricial, hatching from the egg helpless and naked in most species.
Research from the beginning of the 21st century strongly suggested that the similarities between this traditional definition of Pelecaniformes are the result of convergent evolution rather than common descent, being adaptations that were converged upon by birds living in and near aquatic environments; the traditional definition of the group would thus be polyphyletic.
Charles Sibley and Ahlquist's landmark DNA–DNA hybridisation studies (now known as the Sibley–Ahlquist taxonomy) led to them placing the families traditionally contained within the Pelecaniformes together with the , , ibises and spoonbills, New World vultures, , penguins, , , and together as a subgroup 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.
Morphological study had suggested pelicans are sister to a gannet-cormorant clade, yet genetic analysis groups them with the hamerkop and shoebill, though the exact relationship between the three is unclear. Mounting evidence pointed to the shoebill as a close relative of pelicans. This also included microscopic analysis of eggshell structure by Konstantin Mikhailov in 1995, who found that the shells of pelecaniform eggs were covered in a thick microglobular material; importantly, the eggs of shoebills have these strucures, but not those of the tropicbirds. Reviewing genetic evidence to date, Cracraft and colleagues surmised that pelicans were sister to the shoebill with the hamerkop as the next earlier offshoot. (2004): Phylogenetic Relationships Among Modern Birds (Neornithes): Toward an Avian Tree of Life. In: : Assembling the Tree of Life: 468–489. Oxford University Press, New York. PDF fulltext Ericson and colleagues sampled five in a 2006 study spanning the breadth of bird lineages, and came up with pelicans, shoebill and hamerkop in a clade. Hackett and colleagues sampled 32 kilobases of nuclear DNA and recovered shoebill and hamerkop as sister taxa, pelicans sister to them, and herons and ibises as sister groups to each other, with the heron and ibis group a sister to the pelican/shoebill/hamerkop clade. The phylogenetic tree below illustrates this situation:
The current International Ornithological Committee classification has pelicans grouped with the shoebill (Balaenicipitidae), hamerkop (Scopidae), ibises and spoonbills (Threskiornithidae), and herons, egrets and bitterns (Ardeidae). The IOC considers Threskiornithidae and Ardeidae to from a clade, citing Hackett et al. 2008.
Another hypothesis is that Threskiornithidae is sister to the rest of Pelecaniformes, and Ardeidae and Pelecani form a clade, as can be seen below:
Fossil genera and species are discussed in the respective family or genus accounts; one little-known prehistoric Pelecaniforme, however, cannot be classified accurately enough to assign them to a family; "Sula" ronzoni from Early Oligocene rocks at Ronzon, France, which was initially believed to be a Mergus, is a possible ancestral Pelecaniform. The proposed Elopterygidae—supposedly a family of Cretaceous Pelecaniformes—are neither monophyletic nor does Elopteryx appear to be a modern bird.
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