The Aves are the bird class of Craniates. They are similar to the reptiles, but are warm-blooded vertebrates (like the mammals) and have become adapted to flying, although some of the 8700 species cannot fly. Birds have light, hollow bones, a four-chambered heart (as do the mammals), the fore limbs are modified to form wings (a characteristic shared with the bats), and uniquely the body is covered with feathers which are moulted and replaced each year. Unlike mammals birds lay calcareous eggs, the number varying with the species from one to twenty or more. The Aves class is a very large class of animals and is subdivided into two subclasses, the Archaeornithes containing the most primitive birds which are closely related to the reptiles, and the Neornithes containing the more advanced birds. The Neornithes are again divided into four major divisions: the Odontognathae, or toothed birds; the Ichthyornithes which are all fossil forms with vertebrae like those of fish; the Impennes containing the penguins; and the Neognathae which a more specialized modern type of palatal structure. These divisions are further subdivided into orders, sub-orders and families. Research Aves
Abbreviations
Nature
Research Aves