(a) A wing is a remodeled version of the tetrapod forelimb. (b) The bones of many birds have a honeycombed internal structure and are filled with air. (c) A feather consists of a central air–filled shaft, from which radiate the vanes. The vanes are made up of barbs, which bear small branches called barbules. Birds have contour feathers and downy feathers. Contour feathers are the stiff ones that contribute to the aerodynamic shapes of the wings and body. Their barbules have hooks that cling to barbules on neighboring barbs. When a bird preens, it runs the length of each contour feather through its bill, engaging the hooks and uniting the barbs into a precise shape. Downy feathers lack hooks, and the free–form arrangement of their barbs produces a fluffiness that provides insulation by trapping air. 1