Sibley-Ahlquist Taxonomy of Birds - Characteristics

Characteristics

The classification appears to be an early example of cladistic classification 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. However the classification study did not employ modern cladistic methods, as it relies strictly on DNA-DNA hybridization as the sole measure of similarity.

The Sibley-Ahlquist 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. However, later studies failed to support many of the rearrangements in the Sibley-Ahlquist classification, such as the monophyly of the Corvida.

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 inclusion 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
• New 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.

The Galloanseres (waterfowl and landfowl) has found widespread acceptance. The DNA evidence of Sibley-Ahlquist for the monophyly of the group is supported by the discovery of the fossil bird 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.

In the light of more recent studies, the AOU, starting in the late 1990s, moved away from advocating the Sibley-Ahlquist taxonomy as originally published and today advocates the Howard-Moore taxonomy as baseline.