Write the characteristic features of class $Aves$.

(N/A) The characteristic features of class $Aves$ (birds) are as follows:
$1$. Presence of feathers: Most of them can fly,except for flightless birds (e.g.,$Struthio$ or Ostrich).
$2$. Beak: They possess a beak.
$3$. Forelimbs: Modified into wings.
$4$. Hind limbs: Generally have scales and are modified for walking,swimming,or clasping tree branches.
$5$. Skin: Dry without glands,except for the oil gland at the base of the tail.
$6$. Endoskeleton: Fully ossified (bony) and the long bones are hollow with air cavities (pneumatic bones).
$7$. Digestive tract: Contains additional chambers,the crop and gizzard.
$8$. Heart: Completely four-chambered.
$9$. Thermoregulation: They are warm-blooded $(homoiothermous)$ animals,meaning they maintain a constant body temperature.
$10$. Respiration: By lungs. Air sacs connected to the lungs supplement respiration.
$11$. Reproduction: Sexes are separate,and fertilization is internal.