Aves
The Class Aves (the birds) are characterized by being feathered, winged, bipedal tetrapods. The forelimbs are modified into wings for flight (although some birds are seondaryly flightless), with specialized flight feathers. Specialized tail feathers attach onto the short, plate-like pygostyle (fused caudal vertebrae). The trunk is short and stiff, with a large, bony, keeled sternum, a stout pectoral girdle, and a pelvic girdle fused to a synsacrum. The pubis is directed backward. A long flexible neck is present. The head is dominated by a large brain and eyes. The jaws lack teeth and are covered with a horhy ramphotheca; the upper jaw is kinetic (moves relative to the braincase). All birds are homeothermic, with a four-chambered heart and right aortic arch. The lungs are small and inflexible, with a complex system of air sacs and connecting bronchi. The brain has well developed cerebral hemispheres, optic lobes, and cerebellum. Birds have excellent vision, hearing and balance. They are oviparous, with large yoked, amniotic, shelled eggs. The adults incubate the eggs and care for the young, which reach adult size quickly (in weeks or months). Many birds are social, and all have elaborate courtship involving songs, displays, and bright plumage colours. They are distributed over the entire earth, almost from pole to pole; land birds inhabit almost every oceanic island.
The practical importance of birds is large. Poultry, such as ducks, geese, turkeys, chickens, forms a source of food in some cultures. Furthermore, birds are kept often as pets, e.g. canaries, thus supporting an entire industry ranging from the captive breeding of to the production of food for these animals. In addition to these more primitive uses, there is also a still-growing portion of humankind that is interested in and derives great pleasure from birdwatching, i.e. observing birds in the wild. When done in a way that neither harms the birds or their habitat the growing number of birdwatchers appears to have a positive effect on the protection of birds and their habitats (bird sanctuaries with observation huts etc, bird safaries).