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The Inner Bird

The inner bird refers to the bird beneath the feathers. It includes the bird’s unique body and skeletal architecture in which the muscles are concentrated near the body’s core and depend on exceptionally long tendons to move the bones in the extremities. Muscles on the breast move the wings and muscles in the thigh curl the toes. It is this architecture that distinguishes modern birds from their early ancestors and from feathered dinosaurs.

Any evolutionary story for birds must track the changes in the pectoral girdle that produce the powerful and sophisticated flight techniques seen in modern species. However, it must also account for other important aspects of avian biology such as the exceptional levels of parental care exhibited by most birds. The level of parental investment in reproduction is also reflected in dramatic changes to the primitive reptilian skeleton. The narrow hips of the dinosaur’s pelvic girdle have been replaced by the bird’s broad abdominal vault. That vault is the key to avian reproductive biology that is based on exceptionally large eggs and the production of hatchlings close to adult size. Its peculiar shape is also linked to the development of the bird’s peculiar three-part leg and the specialized tail fan that offers a high degree of control during aerial flight.

Flight also plays an important part in parental care. In most species, the heavy egg and a steady supply of provisions for the young must be carried to an elevated or distant nest site. In some birds that role depends on truly exceptional flight techniques.