Updated by: Arabinda Naik
The class-Aves (avis - bird) or flying vertebrates include about 10,052 species. An immense variety in birds is found such as parrots, pigeons, crows, ostrich, penguins, ducks, sparrows, kingfishers, etc. Birds have aerial mode of life. They spend most of the time in air. The body is usually streamlined. It is divided into head, neck, trunk, and tail. Exoskeleton consists of light feathers. It acts as insulator and helps in flight. Most of them fly except the flightless birds (e.g., ostrich). The skin is dry without glands except the oil gland at the base of the tail. Endoskeleton is fully ossified (i.e., bony) and is made up of delicate and light bones. Locomotion occurs through wings. They have modified forelimbs, hindlimbs generally have scales and are modified for walking, swimming, or clasping the tree branches. The digestive system of birds has additional chambers, the crop and gizzard. Alimentary canal leads into three-chambered cloaca. Lungs are respiratory organs, which are spongy. in elastic and are connected to large air sacs. The pharynx is without voice box and voice is produced by special organs called syrinx present at the base of trachea. Excretory organs are kidneys that are metanephric in adults. Excretory matter is mainly uric acid and urine is semisolid. Heart is four-chambered with two auricles and two ventricles. Only right aortic arch persists. Red Blood Corpuscles (RBCs) are oval, biconvex, and nucleated. Birds are warm-blooded or homeothermal animals, i.e., they maintain the constant body temperature. Nervous system contains 12 pairs of cranial nerves. Sexes are separate with sexual dimorphism. Left ovary is present in females, while the right one is absent. Fertilisation is internal. Birds are oviparous. Eggs possess extra embryonic membrane and development is direct. Cleidoic eggs. are present and are covered by calcareous shell. e.g., Flight birds, Corvus (crow), Passer (sparrow), Columba (pigeon), Psittacula (parrot), Anas (duck), Pavo (peacock). Milvus (kite), Neophron (vulture), etc. Flightless Birds Emu, Kiwi, Rhea, Struthio (ostrich), Aptenodytes (penguin), etc. Fossil Bird Archaeopteryx.