Contour feathers cover the bird's body. They are close fitting, yet separated from the skin to help isolate the body from outside humidity and temperature. With the assistance of follicle muscles, the contour feathers can be erected, then lowered to adjust the depth of the protective layer. They help to smooth and streamline the body for flight. In some species they may be greatly modified for purposes of display or some other ornamental purpose.
Many contour feathers have afterfeathers attached at the base.
Usually a contour feathers' afterfeather is no more than half the length of the attached contour feather, yet exceptions always seem to occur in nature. Two birds, the Emu and the Cassowary, have afterfeathers as long as their contour feathers, while some birds such as the pigeon and ostrich have no afterfeathers. Tail feathers are usually large, stiff in texture, asymmetric, have vanes that are almost entirely pennaceous, and lack afterfeathers. The outermost feather's lateral vane is narrow, stiff, and convex compared to the softer, longer, concave barbs of the inner vane. This effect is digressive as the feathers work toward the center pair, such that the center pair's vane is fairly symmetric right to left.