Birds

What are they?

Birds are vertebrates of the class aves. Birds have much in common irrespective of their species, as far as behaviour is concerned. . Wings, legs, and tail-stretching movements are generally universal in all species. They often have a strong territorial sense and defended areas are maintained during breeding season. The bird "song," a signal, is used to attract mates, warn off rivals, alert other birds to danger, and, in the case of young birds with their parents, ask for food.

Characteristics

Body features:

The big flying birds have evolved skeletons that are highly pneumatic, with part of the bone replaced by air spaces, a form of modification for reducing weight. Because of forelimb modification for flight, birds have developed a bipedal gait.

Feathers, the feature unique to birds, are made of a protein called keratin. The contour feathers serve two functions - giving a streamline shape and making the body of the bird waterproof with the basal part of the feathers serving as insulation. The contour feathers are shed in a process called molting. Coloration in birds is caused by pigments, structure, or both. This coloration present in birds hel in mating of birds.

Structural modifications of birds features include feet covered with scales and occasionally vestigial claws on wing tips, a bill covered with keratin, and an oil gland for dressing the plumage.

Strength and lightness of skeleton, necessary for flying , are due to pneumatic features and to the fusion of elements. Birds can move both their upper and lower jaws, an ability that is missing in mammals. The modifications in the wing skeleton include feathers, the restricted motion of elbow and wrist joints to one plane, loss of functional claws, and hollow wing bones. In large flying birds the pneumatic bones are strengthened by struts at stress points.

The muscles of the viscera and the heart muscles of birds are similar to those of mammals and reptiles. Feather muscles in the skin can raise or depress the feathers when necessary. A pectoralis muscle lowers the wing, opposed by a supracoracoideus, which raises it with a pulley-like action through a tendon.

Like mammals, and unlike reptiles, birds have a circulatory system that separates the lung circulation from that of the body (Systemic and Pulmonary Circlulations are seperate ). Birds' hearts are large in proportion to body weight. The lung differs from that of other land vertebrates in having capillaries for passage of air and gas exchange.

The crop, an enlarged part of the esophagus used for temporary food storage, is an adaptation for flight.

As with reptiles, male birds have internal testes. The sexual organs of both sexes enlarge and regress according to season. The temperature of birds ,who are warm-blooded creatures, is approximately 41 C (106 F). Insulation is provided by feathers, together with layers of subcutaneous fat in some water birds. In cold temperatures, heat loss from the exposed, unfeathered feet is minimized by a reduction of blood flow to that part of the bird by contraction of the blood vessels and by a network of heat-exchange in the blood vessels of the upper leg. Rapid panting dissipates heat when necessary, since birds do not have sweat glands.

From where did they evolve and when?

Birds evolved from reptiles in the Jurassic period (about 150 million years ago). It's ancestor was the Archaeoptryx.