Characteristics of Birds

A number of unique anatomical, physiological, and behavioral adaptations enable birds to meet the aerodynamic requirements of flight. Natural selection has favored a lightweight body and powerful wing muscles that give birds their strength.

Feathers are modified scales that serve two primary functions: providing lift for flight and conserving body heat. Soft, fluffy down feathers cover the body of very young birds and provide an insulating undercoat in adults. Contour feathers give adult birds their streamlined shape and provide coloration and additional insulation. Flight feathers are specialized contour feathers on the wings and tail. Birds also have dust-filtering bristles near their nostrils.

The structure of a feather combines maximum strength with minimum weight. Feathers develop from tiny pits in the skin called follicles. A shaft emerges from the follicle, and two vanes, pictured in Figure 42-3a, develop on opposite sides of the shaft. At maturity, each vane has many branches, called barbs. The barbs, in turn, have many projections, called barbules, equipped with microscopic hooks, as shown in Figure 42-3b. The hooks interlock and give the feather its sturdy but flexible shape. Feathers are made of keratin, an insoluble protein that is highly resistant to decomposition. Keratin is also the protein that makes up fingernails, claws, hair, and scales in animals.

Feathers need care. In a process called preening, birds use their beaks to rub their feathers with oil secreted by a preen gland, located at the base of the tail. Birds periodically molt, or shed and regrow their feathers. Birds living in temperate climates usually replace their flight feathers during the late summer.

figure 42-4

Ulna

Radius -

Pelvic girdle (fused ischium, ilium, and pubis)

Maxilla

Mandible

Pectoral girdle

Pelvic girdle (fused ischium, ilium, and pubis)

Pygostyle

Furcula Scapula'

Coracoid Rib

Sternum

-Tarsometatarsus

Tibiotarsus

Furcula Scapula'

Coracoid Rib

Sternum

-Tarsometatarsus

The avian skeleton is well adapted for flight. The bones are air filled, making them light but strong. The skeleton is arranged in such a way that it supports the large muscles necessary for flight.

Pygostyle

Tibiotarsus

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