The Feathered Bipeds
The class of animals that consists of the birds, which are warm blooded, they lay eggs and are vertebrates, primarily adapted for flying.
Class Aves - The Birds
There seems to be little doubt that birds evolved from the ancient reptiles. There are still some marked resemblance is between the two groups: general features of the skull, especially having one occipital condyle, the scales on the hind limbs of birds, shelled eggs and embryonic development, are all similar. In the transition of birds from reptiles, the most important feature was the modification of forelimbs into wings and the development of features, which must be connected with efforts on the part of those reptiles that used to have long leaps and then acquired the adaptation to flight. Unfortunately, many fossils of the transitional stages have not yet been found. However, the important discovery of the imprint Archaeopteryx in 1861, which lies in the Berlin Museum, and another specimen evidence of transition. The former, which may be called lizard-tailed to birds, was about the size of the crow. It had a long tape on both sides of which feathers were present. Each had elongated jaws in the form of beak having teeth in them and claws were present on the wings. All such features point to the transitional changes.
The development of feathers , not only served or flight, but also as a non-conducting layer for heat consideration, which enabled animals to maintain a constant and high body temperature thus making them warm-blooded or homoeothermic. Flight increased the need of many other structural and physiological muscles. Other changes resulted in the four –chambered heart with complete separation of oxygenated and deoxygenated blood. An efficient breathing system evolved, consisting of lungs, supplemented by a number of air sacs, occupying the spaces between the internal organs. This insured double passage of air through the lungs. The jaws are modified to form a beak, with no teeth but provided with a horny sheath. The nervous system and sense organs got highly developed so that birds show highly organized behavior in the form of nest-building, protection of the young, courtship, and migration which come very close to the behavior of highest apes and man. Birds also possess an acute hearing, a good physical equilibrium and extremely keen vision.