The Sibley-Ahlquist taxonomy is a radical bird taxonomy proposed by Charles Sibley and Jon Edward Ahlquist. It is based on DNA-DNA hybridization studies conducted in the late 1970s and throughout the 1980s.

DNA-DNA hybridization is among a class of comparative techniques in molecular biology that produce distance data (versus character data) and that can be analyzed to produce phylogenetic reconstructions only using phenetic tree-building algorithms. In DNA-DNA hybridization, the percent similarity of DNA between two species is estimated by the reduction in hydrogen bonding between nucleotides of imperfectly complemented heteroduplex DNA, (i.e., double stranded DNAs that are experimentally produced from single strands of two different species) compared with perfectly matched homoduplex DNA (both strands of DNA from the same species).

This revolutionary reordering was initially widely accepted by North American ornithologists, and the American Ornithologists' Union adopted some of its provisions. In other parts of the world its adoption has been more deliberative: it has been a respected major influence on existing classification schemes but hardly any authority adopted it in its entirety. Today, the Sibley-Ahlquist taxonomy has fallen out of use; cladistic analyses have questioned its very methodological foundations and most of the proposed major changes (with one notable exception) are today considered to be false.

Characteristics

The classification appears to be cladistic because it codifies many intermediate levels of taxa: the "trunk" of the family tree is the class Aves, which branches into subclasses, which branch into infraclasses, and then "parvclasses", superorders, orders, suborders, infraorders, "parvorders", superfamilies, families, subfamilies, tribes, subtribes and finally genera and species. In fact, the Sibley-Ahlquist taxonomy is merely a highly structured result of phenetic studies; no cladistic methods were used in producing it. Though the strong subdivision into taxonomic ranks suggests accuracy, the "parv"-ranks are simply considered unnecessary as of 15 years later for example.

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Basal divergences of modern birds
in the Sibley-Ahlquist taxonomy

The resulting arrangement differs greatly from the more traditional approach used in the Clements taxonomy. More recently published phylogenetic reconstructions based on cladistic and maximum likelihood analyses of DNA sequences lend credence to some of the DNA-DNA hybridization-based taxonomy, e.g. the recognition of palaeognathous birds as monophyletic and sister to all others, and the recognition that fowl and waterfowl (Galloanserae, initially named "Galloanseri") are one anothers' closest relatives and sister to the remainder of all birds, Neoaves. However, new studies also categorically reject many of the rearrangements in the Sibley-Ahlquist classification, e.g., the all-encompassing order Ciconiiformes, placement of buttonquails at the base of Neoaves, and the monophyly of the Corvida as well as the Passerae, to name a few.

Even DNA-DNA hybridization studies produced later on in other laboratories disagree with some of Sibley and Ahlquist's results, e.g., inclusion of the limpkin in the sungrebe family, and the placement of flamingos near storks rather than next to grebes.

The major changes at order level are as follows:

• Enlarged Struthioniformes replaces the ratite orders Rheiformes (rheas), Casuariiformes (cassowaries), and Apterygiformes (kiwis) and Struthioniformes (ostriches).
• Tinamiformes (tinamous) is unchanged.
• A new, greatly enlarged Ciconiiformes includes the previous Sphenisciformes (penguins), Gaviiformes (divers), Podicipediformes (grebes), Procellariiformes (tubenoses), Pelecaniformes (pelicans and allies), Ciconiiformes (storks and allies), Falconiformes (birds of prey), Charadriiformes (waders, gulls, terns, and auks), and the family Pteroclidae (sandgrouse).
• Anseriformes (ducks and allies) is unchanged.
• New Craciformes chachalacas etc. Previously Galliformes
• New Ralliformes rails and crakes (this was eventually changed back to the traditional inclusioin in Gruiformes)
• New Gruiformes Cranes
• New Turniciformes button-quails etc. Previously Gruiformes
• Columbiformes doves. Sandgrouse moved to Ciconiiformes.
• Psittaciformes cockatoos and parrots unchanged
• New Musophagiformes turacos. Previously Cuculiformes.
• New Cuculiformes rest of cuckoos
• New Strigiformes owls enlarged to include Caprimulgiformes nightjars
• New Apodiformes swifts
• New Trochiliformes hummingbirds. Previously Apodiformes.
• Coliiformes mousebirds unchanged
• Trogoniformes trogons unchanged
• New Coraciiformes rollers
• New Upupiformes Hoopoe, previously Coraciiformes
• New Bucerotiformes hornbills, previously Coraciiformes
• New Galbuliformes jacamars and puffbirds, previously Piciformes
• Piciformes woodpeckers
• Passeriformes perching birds unchanged.

Some of these changes are minor adjustments. For instance, instead of putting the swifts, treeswifts, and hummingbirds in the same order that includes nothing else, Sibley and Ahlquist put them in the same superorder that includes nothing else, consisting of one order for the hummingbirds and another for the swifts and treeswifts. In other words, they still regard the swifts as the hummingbirds' closest relatives.

Other changes are much more drastic. The penguins were traditionally regarded as distant from all other living birds. For instance, Wetmore put them in a superorder by themselves, with all other non-ratite birds in a different superorder. Sibley and Ahlquist, though, put penguins in the same superfamily as divers (loons), tubenoses, and frigatebirds. According to their view, penguins are closer to those birds than herons are to storks.

The new research suggested that the ducks and gallinaceous birds are each other's closest relatives and together form the basal lineage of neognathous (non-ratite) birds, distinct from the others which are collectively called Neoaves. The ratites and tinamous are followed by the ducks and their allies and the pheasants and their allies. Penguins, grebes and divers are placed with other groups that were traditionally considered more modern.

A more recent paper by van Tuinen, Sibley, and Hedges looked in more detail at the early ancestry of bird groups. The traditional view of avian evolution places ratites and tinamous at the base of the tree of modern birds (Neornithes), followed by old marine groups such as the penguins, grebes and divers.

The Galloanseres (waterfowl and landfowl) have found widespread acceptance and roundabout support. The ancientness was splendidly confirmed with the 2005 report on Vegavis iaai, an essentially modern but most peculiar waterfowl that lived near Cape Horn some 66-68 million years ago, still in the age of the dinosaurs.

On the other hand, penguins, grebes, divers, and so on (colloquially sometimes called "higher waterbirds") are still considered very ancient neoavian orders - quite possibly together with the shorebirds (waders) which seem a bit older still, the most ancient ones. The supposed distinctness of the storks and herons as well as at least the supposed degree of closeness of penguins to frigatebirds have been refuted. They, as well as the "Ciconiiformes" assemblage, appear to be due to the shortcomings, both methodological and analytical, of DNA-DNA hybridization.

Today, no major ornithological organization uses the Sibley-Ahlquist taxonomy anymore. The AOU, starting in the late 1990s, moved away from advocating the Sibley-Ahlquist taxonomy and today advocates the Howard-Moore taxonomy as baseline. Compared to the main competitor, the Clements taxonomy (which at that time was based on simple qualitative analyses of morphology), it can be stated that most of the changes suggested by Sibley and Ahlquist - with the notable exception of the Galloanserae - are erroneous and caused by the flawed methodology.

Classification

Palaeognathae

Ratitae	Struthioniformes	Struthionidae Rheidae Casuariidae Apterygidae
	Tinamiformes	Tinamidae

Neognathae

Galloanserae

Galloanserae	Gallomorphae	Craciformes	Cracidae Megapodiidae
		Galliformes	Phasianidae Numididae Odontophoridae
	Anserimorphae	Anseriformes	Anhimidae Anseranatidae Dendrocygnidae Anatidae

Turnicae

Turnicae	Turniciformes	Turnicidae

Picae

Picae	Piciformes	Indicatoridae Picidae Megalaimidae Lybiidae Ramphastidae

Coraciae

Coraciae	Galbulimorphae	Galbuliformes	Galbulidae Bucconidae
	Bucerotimorphae	Bucerotiformes	Bucerotidae Bucorvidae
		Upupiformes	Upupidae Phoeniculidae Rhinopomastidae
	Coraciimorphae	Trogoniformes	Trogonidae
		Coraciiformes	Coraciidae Brachypteraciidae Leptosomidae Momotidae Todidae Alcedinidae Halcyonidae Cerylidae Meropidae

Coliae

Coliae	Coliiformes	Coliidae

Passerae

Passerae	Cuculimorphae	Cuculiformes	Cuculidae Centropodidae Coccyzidae Opisthocomidae Crotophagidae Neomorphidae
	Psittacimorphae	Psittaciformes	Psittacidae
	Apodimorphae	Apodiformes	Apodidae Hemiprocnidae
		Trochiliformes	Trochilidae
	Strigimorphae	Musophagiformes	Musophagidae
		Strigiformes	Tytonidae Strigidae Aegothelidae Podargidae Batrachostomidae Steatornithidae Nyctibiidae Eurostopodidae Caprimulgidae
	Passerimorphae	Columbiformes	Raphidae Columbidae
		Gruiformes	Eurypygidae Otididae Gruidae Aramidae Heliornithidae Psophiidae Cariamidae Rhynochetidae Rallidae Mesitornithidae
		Ciconiiformes	Pteroclidae Thinocoridae Pedionomidae Scolopacidae Rostratulidae Jacanidae Chionidae Pluvianellidae Burhinidae Charadriidae Glareolidae Laridae Accipitridae Sagittariidae Falconidae Podicipedidae Phaethontidae Sulidae Anhingidae Phalacrocoracidae Ardeidae Scopidae Phoenicopteridae Threskiornithidae Pelecanidae Ciconiidae Fregatidae Spheniscidae Gaviidae Procellariidae
		Passeriformes	Acanthisittidae Pittidae Eurylaimidae Philepittidae Tyrannidae Thamnophilidae Furnariidae Formicariidae Conopophagidae Rhinocryptidae Climacteridae Menuridae Ptilonorhynchidae Maluridae Meliphagidae Pardalotidae Petroicidae Irenidae Orthonychidae Pomatostomidae Laniidae Vireonidae Corvidae Callaeatidae Picathartidae Bombycillidae Cinclidae Muscicapidae Sturnidae Sittidae Certhiidae Paridae Aegithalidae Hirundinidae Regulidae Pycnonotidae Hypocoliidae Cisticolidae Zosteropidae Sylviidae Alaudidae Nectariniidae Melanocharitidae Paramythiidae Passeridae Fringillidae

See also

• Charles Sibley
• The Sibley-Monroe checklist, multiple parts:
• part 1 - Struthionidae to Odontophoridae
• part 2 - Anhimidae to Picidae
• part 3 - Megalaimidae to Todidae
• part 4 - Alcedinidae to Psittacidae
• part 5 - Apodidae to Musophagidae
• part 6 - Tytonidae to Columbidae
• part 7 - Eurypygidae to Jacanidae
• part 8 - Chionididae to Falconidae
• part 9 - Podicipedidae to Hydrobatidae
• part 10 - Acanthisittidae to Tyrannidae
• part 11 - Thamnophilidae to Rhinocryptidae
• part 12 - Climacteridae to Vireonidae
• part 13 - Corvidae
• part 14 - Callaeidae to Sturnidae
• part 15 - Sittidae to Cisticolidae
• part 16 - Zosteropidae
• part 17 - Alaudidae to Passeridae
• part 18 - Fringillidae
• List of birds (based on the Clements taxonomy)

References

• (2005): Definitive fossil evidence for the extant avian radiation in the Cretaceous. Nature 433: 305-308. DOI:10.1038/nature03150 PDF fulltext Supporting information
• Sibley, Charles Gald & Ahlquist, Jon Edward (1990): Phylogeny and classification of birds. Yale University Press, New Haven, Conn.
• On the Phylogeny and Classification of Living Birds, by Charles G. Sibley
• The Early History of Modern Birds Inferred from DNA Sequences of Nuclear and Mitochondrial Ribosomal Genes, by Marcel van Tuinen, Charles G. Sibley, and S. Blair Hedges
• Sibley's Classification of Birds, by Eric Salzman, Birding, December 1993. The Web version lacks the illustrations, which show parts of the family tree, and includes only a partial bibliography, but adds a sequence down to the tribe level with detail on intermediate taxa (especially for the passerines).