General Characteristics Class Aves

General Characteristics:

1. General Overview: The class Aves includes about 10,052 species of birds, ranging from parrots, pigeons, crows, to ostriches and penguins. Birds are flying vertebrates, although some, like the ostrich and penguin, are flightless.
2. Body Structure: Birds have a streamlined body divided into head, neck, trunk, and tail. The exoskeleton consists of light feathers that help with insulation and flight. The skin is dry, with no glands except for the oil gland at the base of the tail. Endoskeleton is ossified (bony), composed of light, delicate bones to facilitate flight. Forelimbs are modified into wings for flight, while hindlimbs are adapted for walking, swimming, or clasping branches. Hindlimbs are covered with scales.
3. Digestive System: Birds have a well-developed digestive system with additional chambers like the crop (for storing food) and the gizzard (for grinding food). The alimentary canal opens into a three-chambered cloaca.
4. Respiratory System: Lungs are spongy and inelastic, connected to large air sacs that aid in respiration and help reduce body weight for flight. The syrinx, located at the base of the trachea, is responsible for sound production.
5. Excretory System: Metanephric kidneys serve as excretory organs, producing uric acid as the main excretory product, making bird excreta semisolid.
6. Circulatory System: Birds possess a four-chambered heart with two auricles and two ventricles. Only the right aortic arch persists. RBCs are oval, biconvex, and nucleated.
7. Temperature Regulation: Birds are warm-blooded (homeothermic), maintaining a constant body temperature regardless of external conditions.
8. Nervous System: The nervous system includes 12 pairs of cranial nerves.
9. Reproduction: Birds exhibit sexual dimorphism with separate sexes. Internal fertilization occurs, and birds are oviparous (egg-laying). Cleidoic eggs with a calcareous shell and extra-embryonic membranes are laid. In females, only the left ovary is functional, while the right ovary is absent. Development is direct, with no larval stage.

Examples: Flying birds: Corvus (crow), Passer (sparrow), Columba (pigeon), Psittacula (parrot), Anas (duck), Pavo (peacock), Milvus (kite), Neophron (vulture). Flightless birds: Emu, Kiwi, Rhea, Struthio (ostrich), Aptenodytes (penguin). Fossil bird: Archaeopteryx (link between reptiles and birds). Peacock (Pavo cristatus), Vulture (Neophron), Penguin (Aptenodytes), Ostrich (Struthio), Crow (Corvus splendens), Rock Pigeon (Columba livia), Parrot (Psittacula)