It fills us with fascination and envy.
Any pilot knows weight is an important consideration in flying.
Birds have jettisoned extra weight from their design. Their slender bones are hollow and filled with air, but rigid and strong too. Some of the bones, like those of the pelvis, have fused to strengthen and simplify the skeleton. The breastbone has a deep keel that allows the attachment of large flight muscles. Feathers are finely engineered tools, designed to meet the flight needs of each species' lifestyle-stiff ones for rapid flight and maneuverability, soft ones for silent hunting.