Class Aves

Class Aves (Latin avis = bird) includes animals such as Columba (pigeon), Psittacula (parrot), and flightless birds like Struthio (ostrich), Kiwi and Aptenodytes (penguin), as well as Corvus (crow), Neophron (vulture) and Passer (sparrow).

The body is differentiated into head, neck, trunk and tail. The skin is thin, dry and non-glandular, except for an oil-secreting gland at the base of the tail, the uropygial gland. The bones are hollow (pneumatic), containing air cavities that reduce overall body weight for flight. The jaws lack teeth and are instead modified into a beak. The digestive system includes a crop (for temporary storage) and a gizzard (for grinding food).

Blood is red due to the presence of red blood cells, which — unlike in mammals — are biconvex and nucleated. The heart is perfectly four-chambered, giving birds efficient double circulation. Respiration occurs through the lungs, aided by air sacs that also increase buoyancy. The brain is enlarged, with a well-developed cerebellum for balance and coordination during flight.

Sexes are separate with prominent sexual dimorphism. Birds are oviparous, fertilisation is internal, and parental care of eggs and young is very well developed. Some species undertake seasonal migration. …