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

Birds exhibit several flight adaptations including a lightweight skeleton with hollow bones, the presence of feathers for aerodynamics, forelimbs modified into wings, a strong flight musculature especially the pectorals, a streamlined body to minimize air resistance, absence of urinary bladder and right ovary to reduce body weight, high metabolic rate, and efficient respiratory system with air sacs for continuous oxygen supply during flight.