The majority of birds are designed for flight. Because of the extremely restrictive physiological and anatomical demands of flight, birds are finely tuned metabolic machines with very high metabolic rates and constrained aerodynamic morphologies. Birds are in fact the most structurally uniform of all vertebrate groups.
However, it is difficult to define the birds (Class Aves) on the basis of characters. Recent fossil evidence shows that many characters which are normally considered "avian" evolved in the theropod lineage. Not only that, but some of the features that we might define modern birds by, have not been present throughout the avian lineage. As a consequence of this, birds are better defined phylogenetically (i.e. on evolutionary relationships), rather than morphologically.
Author: Robert Davis
Last updated: 21/11/05
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