Birds have hollow bones which make them light. They also have streamlined body which help them to fly in the air.
Birds have a proper body structure. There bones are light. They have feathers which helps them to fly. They use there tails as rudders to change direction.
Hollow bones to reduce body mass , feathers covering their body ,a stream lined body with powerful tail and wings with powerful muscles are the adaptations in birds for for flying.
ans is '30' because all other are following the rule of equation 2x2=4+1 = 5, 3x3=9+2=11, 4x4=16+3= 19, 5x5=25+4=29 (30 written in the series instead of 29), 6x6=36+5=41, 7x7=49+6=55