Final answer:
Contour feathers have a central shaft holding together inner and outer webs that help birds reduce drag and maintain smooth flight.
Step-by-step explanation:
Contour feathers consist of a central shaft with inner and outer webs. These feathers are essential for birds as they provide a smooth, aerodynamic surface, enabling air to flow efficiently over the bird's body, which reduces form drag and assists in flight. Moreover, the arrangement of the barbs and the interlocking barbules gives contour feathers a strong, flexible structure that is crucial for the bird's flight and protection.