● The characteristic features of Aves (birds) are the presence of `color{Violet}"feathers"` and most of them can fly except flightless birds (e.g., Ostrich).

● They possess `color{Violet}"beak"` .

● The `color{Violet}"forelimbs"` are modified into `color{Violet}"wings"`.

● The `color{Violet}"hind limbs"` generally have `color{Violet}"scales"` and are modified for walking, swimming or clasping the tree branches.

● Skin is `color{Violet}"dry"` without glands except the `color{Violet}"oil gland"` at the base of the tail.

● `color{Brown}"Skeletal System"`: Endoskeleton is fully `color{Violet}"ossified"` (bony) and the long bones are `color{Violet}"hollow with air cavities"` (`color{Violet}"pneumatic"`).



● `color{Brown}"Digestive System"`: The digestive tract of birds has additional chambers, the `color{Violet}"crop and gizzard"`.



● `color{Brown}"Circulatory System"`: Heart is completely `color{Violet}"four chambered"`.



● They are warm-blooded (`color{Violet}"homoiothermous"`) animals, i.e., they are able to maintain a constant body temperature.

● `color{Brown}"Respiratory System"`: Respiration is by `color{Violet}"lungs"`. `color{Violet}"Air sacs"` connected to lungs supplement respiration.

● Sexes are `color{Violet}"separate"`.

● Fertilisation is `color{Violet}"internal"`.

● They are `color{Violet}"oviparous"` and development is `color{Violet}"direct"`.



● `color{Brown}"Examples"` : Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).

● `color{Brown}"Habitat"`: They are found in a `color{Violet}"variety"` of habitats – polar ice caps, deserts, mountains, forests, grasslands and dark caves. Some of them have adapted to fly or live in water.

● They have `color{Violet}"two pairs"` of limbs, adapted for walking, running, climbing, burrowing, swimming or flying.

● Sexes are `color{Violet}"separate"` and fertilisation is `color{Violet}"internal"`.

● They are `color{Violet}"viviparous"` with few exceptions and development is `color{Violet}"direct"`.



● `color{Brown}"Examples"`: Oviparous-Ornithorhynchus (Platypus); Viviparous - Macropus (Kangaroo), Pteropus (Flying fox), Camelus (Camel), Macaca (Monkey), Rattus (Rat), Canis (Dog), Felis (Cat), Elephas (Elephant), Equus (Horse), Delphinus (Common dolphin), Balaenoptera (Blue whale), Panthera tigris (Tiger), Panthera leo (Lion).

● The most unique mammalian characteristic is the presence of `color{Violet}"milk producing glands"` (`color{Violet}"mammary glands"`) by which the young ones are nourished.



● The skin of mammals is unique in possessing `color{Violet}"hair."`



●External ears or `color{Violet}"pinnae"` are present.



● Different types of `color{Violet}"teeth"` are present in the jaw.

● Heart is `color{Violet}"four chambered"`.



● They are `color{Violet}"homoiothermous"`.

● Respiration is by `color{Violet}"lungs"`.