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

Explanation

Birds have undergone many structural adaptations to suit their aerial life Some of these adaptations are as follows. 
(i) streamlined body for rapid and smooth movement
(ii) covering of feathers for insulation.
(iii) Forelimbs modified into wings and hind limbs used for walking, perching and swimming.
(iv) Presence of pneumatic bones to reduce weight.
(v) Presence of additional air sacs to supplement respiration.