What are the modifications that are observed in birds that help them fly?

(N/A) Birds have undergone many structural adaptations to suit their aerial life. Some of these adaptations are as follows:
$1$. Streamlined body for rapid and smooth movement.
$2$. Covering of feathers for insulation.
$3$. Forelimbs modified into wings and hind limbs used for walking,perching,and swimming.
$4$. Presence of pneumatic (hollow) bones to reduce body weight.
$5$. Presence of additional air sacs to supplement respiration.