Class Aves

Feathers And Hollow Bones
The hollow bones make the skeleton lighter for flying, and allow more oxygen to be absorbed from the air into the blood for the extra energy a bird needs in order to fly.The feathers insulate birds from water and cold temperatures. They may also provide insulation to the eggs and young. The individual feathers in the wings and tail play important roles in controlling flight.

Reproduction
Birds reproduce by internal fertilization,which is when the egg is fertilized inside the female. The male brings his sperm to the female cloaca which fertilizes the egg.

Circulatory System
The birds circulatory system consist of a four-chambered heart and blood                                                                   vessel.

Endothermic
Most birds are endothermic. An endotherm is an animal that can control its internal body temperature.