Birds: Class Aves
Birds are bipedal, tetrapod vertebrates with feathers. Feathers alone distinguish birds from all other living vertebrates. Birds are derived from early reptiles. The avian skeleton retains many reptilian features but is highly modified for flight. The bones and feathers of the forelimb, particularly, are transformed into wings capable of powerful, flapping flight. Birds maintain a high body temperature through metabolic heat production and have a large, four-chambered heart that supports the demands of sustained activity and high metabolism.
DNA-based resolution of the phylogenetic relationships among world birds has increased dramatically in the past few years, and will continue to accelerate. Many of the latest results are available on the web as part of the Tree of Life (TOL) project.
We now have the ability to to distinguish taxa of similar rank that are reciprocally monophyletic. This lets us redefine some groups of species that in hindsight were paraphyletic. Changes in taxonomy follow, especially splitting of orders, families, and genera to create monophyletic taxa that can be reconnected to their closest relatives in the classification. These adjustments help us build a working framework for continued improvements.
In addition, DNA sequences clarify (or erase) some of the connections between the deepest historical branches of avian evolution (Hackett et al 2008). Accordingly, we can revisit the compositions of the classical Orders of birds and the relationships among them. Predictably some groups (clades) of bird species are not as closely related to each other as previously thought.
The Class Aves includes fossil groups of birds as well as modern taxa. The Subclass Neornithes includes all extant birds. The earliest divergence within Neornithes splits the Superorder Paleognathae (ratites and tinamous) and the Superorder Neognathae. In turn, the Neognathae include two primary major groups of birds. The orders of most modern birds constitute the infraclass Neoaves. The waterfowl (Order Anseriformes) and the chicken-like birds (Order Galliformes) constitute the infraclass Galloanserae.
Following the IOC World Bird List the complete classification of modern birds is a hierarchical arrangement of 40 orders and 231 families, which include 2268 genera and over 10,496 extant species.
References
Braun, E. L., and R. T. Kimball. 2002. Examining basal avian divergences with mitochondrial sequences: model complexity, taxon sampling, and sequence length. Syst. Biol. 51: 614-625.
Cracraft, J., F. K. Barker, and A. Cibois. 2003. Avian higher level phylogenetics and the Howard and Moore Checklist of Birds. Pp. 16-21 in The Howard and Moore Complete Checklist of the Birds of the World (E. C. Dickinson, ed.). Princeton, NJ, Princeton University Press.
Ericson, P. G. P., T. J. Parsons, and U. S. Johansson. 2001. Morphological and molecular support for nonmonophyly of the Galloanserae. Pp. 157-168 in New Perspectives on the Origin and Early Evolution of Birds (J. Gauthier and L. F. Gall, eds.). New Haven, CT, Peabody Museum of Natural History, Yale University.
Groth, J. G., and G. F. Barrowclough. 1999. Basal divergences in birds and the phylogenetic utility of the nuclear RAG-1 gene. Mol. Phylogenet. Evol. 12: 115-123.
Hackett, S.J., R.T. Kimball, S. Reddy, R.C.K. Bowie, E. L. Braun, M. J. Braun, J. L. Chojnowski, W. A. Cox, K.-L. Han, J. Harshman, C. J. Huddleston, B. D. Marks, K. J. Miglia, W. S. Moore, F. H. Sheldon, D. W. Steadman, C. C. Witt, and T. Yuri. 2008. A phylogenomic study of birds reveals their evolutionary history. Science 320: 1763-1768
Hedges, S. B., P. H. Parker, C. G. Sibley, and S. Kumar. 1996. Continental breakup and the ordinal diversification of birds and mammals. Nature 381: 226-229.
Mayr, G., and J. Clarke. 2003. The deep divergences of neornithine birds: A phylogenetic analysis of morphological characters. Cladistics 19: 527-553.
Mindell, D. P., and J. W. Brown. 2005. Galloanserae. Version 14 December 2005. http://tolweb.org/Galloanserae/15840/2005.12.14 in Tree of Life Web Project, http://tolweb.org
Mindell, D. P., J. W. Brown, and J. Harshman. 2005. Neoaves. Version 14 December 2005. http://tolweb.org/Neoaves/26305/2005.12.14 in Tree of Life Web Project, http://tolweb.org
Mindell, D. P., M. D. Sorenson, D. E. Dimcheff, M. Hasegawa, J. C. Ast, and T. Yuri. 1999. Interordinal relationships of birds and other reptiles based on whole mitochondrial genomes. Syst. Biol. 48: 138-152.
Mindell, D. P., M. D. Sorenson, C. J. Huddleston, H. C. Miranda, Jr., A. Knight, S. J. Sawchuk, and T. Yuri. 1997. Phylogenetic relationships among and within select avian orders based on mitochondrial DNA. Pp. 213-247 in Avian Molecular Evolution and Systematics (D. P. Mindell, ed.). San Diego, Academic Press.
Tree of Life Web Project. 1999. Aves. Birds. Version 01 January 1999. http://tolweb.org/Aves/15721/1999.01.01 in Tree of Life Web Project, http://tolweb.org
van Tuinen, M., C. G. Sibley, and S. B. Hedges. 2000. The early history of modern birds inferred from DNA sequences of nuclear and mitochondrial ribosomal genes. Mol. Biol. Evol. 17: 451-457.