Why are birds adapted to aerial mode of life?

{1}The forelimbs of all the birds are modified in to wings which is useful for them to fly.

{2}The bones of birds are hollow which decrises the weight hence it's help to fly.

{3}There shap is like streamlined and spindle.

this reduces the resistance while flying.

{4}The exoskeleton is of feathers which make them lighter.

{5}There are air sacsin their body which give buoyancy.

Hence......................................................