Bock, Walter J. Department of Biological Sciences, Columbia University, New York, New York.
Last reviewed:August 2019
Show previous versions
- Other distinctive features
- Fossil record
- Links to Primary Literature
- Additional Readings
The class of animals consisting of the birds. Birds (Fig. 1) are vertebrate animals comprising the class Aves. (In some classification schemes, the term Neornithes may also be used when referring to modern birds.) Primarily adapted for flying, the members of the Aves are characterized by being feathered, warm-blooded (homeothermic), and bipedal (two-legged), with the forelimb modified into a wing that (together with the tail feathers attached to the short tail) forms the flight mechanism, and by having a very high metabolic rate. Such a characterization, however, as with any group of vertebrates, holds for the living forms and most fossil members of the class, but is blurred by the early fossil record, which contains species with characteristics closer to those of the reptilian ancestors of birds. The feathers of birds are lightweight modifications of the outer skin possessing remarkable aerodynamic qualities. They serve not only as surfaces to generate lift and thrust, and as a streamlined outer surface of the body, but also as insulation to maintain high body temperatures. In addition, birds have lightweight, hollow bones; a well-developed air-sac system and flow-through lungs; a wishbone or furcula (fused clavicles); and a hand reduced to three digits (comparable to digits 2, 3, and 4 of the human hand). Birds have most likely evolved from an ancestor within the large group of ancient diapsid reptiles known as archosaurs (including alligators, snakes and lizards, and dinosaurs, among others). However, debate still centers on whether birds are derived from a basal archosaurian stock or arose later directly from the later and more derived theropod dinosaurs (carnivores such as Allosaurus and Velociraptor). The scientific study of birds is known as ornithology. See also: Adaptation (biology); Animal flight; Archosauria; Avian evolution; Diapsida; Dinosauria; Evolution; Evolution of theropod dinosaurs; Feather; Neornithes; Reptilia; Vertebrata
The content above is only an excerpt.
for your institution. Subscribe
To learn more about subscribing to AccessScience, or to request a no-risk trial of this award-winning scientific reference for your institution, fill in your information and a member of our Sales Team will contact you as soon as possible.
to your librarian. Recommend
Let your librarian know about the award-winning gateway to the most trustworthy and accurate scientific information.
AccessScience provides the most accurate and trustworthy scientific information available.
Recognized as an award-winning gateway to scientific knowledge, AccessScience is an amazing online resource that contains high-quality reference material written specifically for students. Contributors include more than 9000 highly qualified scientists and 43 Nobel Prize winners.
MORE THAN 8700 articles covering all major scientific disciplines and encompassing the McGraw-Hill Encyclopedia of Science & Technology and McGraw-Hill Yearbook of Science & Technology
115,000-PLUS definitions from the McGraw-Hill Dictionary of Scientific and Technical Terms
3000 biographies of notable scientific figures
MORE THAN 19,000 downloadable images and animations illustrating key topics
ENGAGING VIDEOS highlighting the life and work of award-winning scientists
SUGGESTIONS FOR FURTHER STUDY and additional readings to guide students to deeper understanding and research
LINKS TO CITABLE LITERATURE help students expand their knowledge using primary sources of information