Class Aves

Abstract

In many respects birds are the most highly specialised craniate class. Almost every part of their organisation is modified in accordance with aerial life. All birds possess feathers, and only birds possess them. The fore-limbs are modified as wings. The sternum and shoulder-girdles are strikingly altered and serve as origins for the great wing-muscles and the mechanical framework to support their activity. The pelvic girdle and hind-limbs have changed to support the entire weight of the body on the ground. The perfection of the respiratory system, enabling a more complete absorption of oxygen and the production of a higher constant temperature than in other animals, has come hand in hand with the evolution of incomparable powers of flight. Important negative characteristics of modern birds are an absence of teeth and the left aortic arch. The right ovary and oviduct have generally disappeared. The brain is of a highly specialised character. The elevation of the group from near the ground has allowed the olfactory apparatus in most species to degenerate, and this has been accompanied by an extraordinary enlargement of the eye, and enhanced efficiency in vision. Despite the general validity of the old assertion that birds are essentially glorified reptiles, the above series of strongly-marked characteristics are hardly equalled in distinction in any other class. Moreover, the organisation of existing birds is, in its essential features, singularly uniform.