Class Aves

Class Aves

Birds are the chordate class most extensively and visibly modified for flight, and nearly every major organ system shows some adaptation serving this single overriding function.

Skeletal modifications include: bones that are pneumatic (hollow, air-filled, and internally reinforced with thin bony struts) to reduce body weight without sacrificing strength; a greatly enlarged, keeled sternum (breastbone), which provides the necessary surface area for the attachment of the powerful flight muscles; fusion of several vertebrae and other bones (for example, the posterior vertebrae fusing into a single pygostyle supporting the tail feathers) to make the flying body more rigid and stable in the air; and forelimbs modified into wings.

Muscular and physiological modifications include very large, powerful pectoral (breast) flight muscles anchored to the keeled sternum, a highly efficient respiratory system involving air sacs connected to the lungs (which allows a continuous, one-way flow of air through the lungs, supporting the very high oxygen demand of flight), and a four-chambered heart with a high heart rate supporting a high, warm-blooded (endothermic) metabolic rate, sufficient to fuel sustained, energetically demanding flight. …