Traditionally, the order Ciconiiformes has included a variety of large, long-legged wading birds with large bills: storks, herons, egrets, ibises, spoonbills, and several others. Ciconiiformes are known from the Late Eocene. At present the only family retained in the order is the storks, Ciconiidae.
Taxonomic issues with Ciconiiformes
Following the development of research techniques in molecular biology in the late 20th century, in particular methods for studying DNA-DNA hybridisation, a great deal of new information has surfaced, much of it suggesting that many birds, although looking very different from one another, are in fact more closely related than was previously thought. Accordingly, the radical and influential Sibley-Ahlquist taxonomy greatly enlarged the Ciconiiformes, adding many more families, including most of those usually regarded as belonging to the Sphenisciformes (penguins), Gaviiformes (divers), Podicipediformes (grebes), Procellariiformes (tubenosed seabirds), Charadriiformes, (waders, gulls, terns and auks), Pelecaniformes (pelicans, cormorants, gannets and allies), and the Falconiformes (diurnal birds of prey). The flamingo family, Phoenicopteridae, is related, and is sometimes classed as part of the Ciconiiformes.
However, morphological evidence suggests that the traditional Ciconiiformes should be split between two lineages, rather than expanded, although some non-traditional Ciconiiformes may be included in these two lineages.
The exact taxonomic placement of New World Vultures remains unclear. Though both are similar in appearance and have similar ecological roles, the New World and Old World Vultures evolved from different ancestors in different parts of the world and are not closely related. Just how different the two families are is currently under debate, with some earlier authorities suggesting that the New World vultures belong in Ciconiiformes. More recent authorities maintain their overall position in the order Falconiformes along with the Old World Vultures or place them in their own order, Cathartiformes. The South American Classification Committee has removed the New World Vultures from Ciconiiformes and instead placed them in Incertae sedis, but notes that a move to Falconiformes or Cathartiformes is possible.
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