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 (Sibley & Ahlquist 1990). 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). 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 subdivided hierarchical Linnaean classification; no cladistic methods were used in producing it.
Basal divergences of modern birds
based on Sibley-Ahlquist taxonomy
The resulting arrangement differs greatly from the more traditional approach used in list of birds. 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) 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 various clades of songbirds, to name a few. Even DNA-DNA hybridization studies produced more recently 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).
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).
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; that is, penguins are closer to those birds than herons are to storks. This revolutionary reordering has been widely accepted by North American ornithologists, and the American Ornithologists' Union has adopted some of its provisions, but in other parts of the world its adoption has been more deliberative: it has been a respected major influence on existing classification schemes. 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 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. The ratites and tinamous will now be followed by the ducks and their allies, and then the pheasants and their allies. Penguins, grebes and divers are placed with other groups that were traditionally considered more modern. This latter research has been accepted by the British Ornithologists' Union, and the British list will now start with Anseriformes and Galliformes.
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).
