Systematics
Following the development of research techniques in molecular biology in the late 20th century, in particular methods for studying DNA-DNA hybridization, 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 (tube-nosed 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.
Some official bodies have adopted the proposed Sibley-Ahlquist taxonomy almost entirely; however, a more common approach worldwide has been to retain the traditional groupings, and modify rather than replace them in the light of new evidence as it comes to hand. The family listing here follows this more conservative practice. Bird taxonomy has been in a state of flux for some years, and it is reasonable to expect that the large differences between different classification schemes will continue gradually to resolve themselves as more evidence becomes available.
Recently a DNA study found that the families Ardeidae, Balaenicipitidae, Scopidae and the Threskiornithidae belong to the Pelecaniformes. This would make Ciconiidae the only group.
Storks were distinct and possibly widespread by the Oligocene. Like most families of aquatic birds, storks seem to have arisen in the Palaeogene, maybe 40-50 million years ago (mya). For the fossil record of living genera, documented since the Middle Miocene (about 15 mya) at least in some cases, see the genus articles.
Though some storks are highly threatened, no species or subspecies are known to have gone extinct in historic times. A Ciconia bone found in a rock shelter on the island of Réunion was probably of a bird taken there as food by early settlers; no known account mentions the presence of storks on the Mascarene Islands.
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