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

Birds have undergone many structural adaptations to suit their aerial life. Some of these adaptations are as follows.
 Streamlined body for rapid and smooth movement
 Covering of feathers for insulation
 Forelimbs modified into wings and hind limbs used for walking, perching, and swimming
 Presence of pneumatic bones to reduce weight
 Presence of additional air sacs to supplement respiration

 Following are the modifications in birds that help them fly:

 • Pneumatic or hollow bones reduce weight. 

• Forelimbs are modified into wings. 

• Streamlined body helps in flying. 

• Uricotelic excretion helps in minimizing the need for carrying water and thus helps in weight reduction.

The following modification helps birds to fly

• presence of feathers
• presence of pneumatic bones; pneumatic bones are long hollow bones (have air cavities)
• Fore limbs are modified into wings to help in flight
• hind limbs generally have scales helping them to clasp the prey in flight or a tree branch
• Excretion of urine and faeces is through single opening and excretion has very less amount of water this is to reduce body weight
• Aerodynamically built body helps in flying.

Following modification in birds help them fly: 

1. Pneumatic or hollow bones make for a light weight skeleton. 

2. Fore limbs are modified into wings to assist in flight. 

3. Excertion of urine and faeces is through single opening facilitating weight reduction. 

4. Aerodynamic body helps in flying.