The characteristic features of Aves (birds) are the presence of feathers, and most of them can fly except flightless birds such as the ostrich. They possess a beak. The forelimbs are modified into wings. The hind limbs generally have scales and are modified for walking, swimming or clasping tree branches. The skin is dry without glands except the oil gland at the base of the tail. The endoskeleton is fully ossified (bony) and the long bones are hollow with air cavities, which makes them pneumatic. The digestive tract has additional chambers, the crop and gizzard. The heart is completely four-chambered. Birds are warm-blooded (homoiothermous), that is, they can maintain a constant body temperature. Respiration is by lungs, and air sacs connected to the lungs supplement respiration. Sexes are separate. Fertilisation is internal. They are oviparous and development is direct. Examples include Corvus (crow), Columba (pigeon), Psittacula (parrot), Struthio (ostrich), Pavo (peacock), Aptenodytes (penguin) and Neophron (vulture). Exam questions frequently ask about the modifications that help birds fly, namely feathers, wings, pneumatic bones, air sacs and a four-chambered heart, and about the oil gland as the only skin gland.