Feather Anatomy and Function

Feathers are unique to birds. They are a defining characteristic of the group, meaning simply that if an animal has feathers, then it is a bird. Feathers serve many functions in birds but most notable is the critical role feathers play in enabling birds to fly. Unlike feathers, flight is not a characteristic restricted to birds—bats fly with great agility and insects fluttered through the air several million years before birds joined them. But feathers have enabled birds to refine flight to an art form matched by no other organism alive today.

In addition to helping to enable flight, feathers also provide protection from the elements. Feathers provide birds with waterproofing and insulation and even block harmful UV rays from reaching birds' skin.

Feathers are made up of keratin, an insoluble protein that is also found in mammalian hair and reptilian scales. In general, feathers consist of the following structures:

• calamus - the hollow shaft of the feather that attaches it to the bird's skin
• rachis - the central shaft of the feather to which the vanes are attached
• vane - the flattened part of the feather that is attached on either side of the rachis (each feather has two vanes)
• barbs - the numerous branches off the rachis that form the vanes
• barbules - tiny extensions from barbs that are held together by barbicels
• barbicels - tiny hooks that interlock to hold the barbules together

Birds have several different types of feathers and each type is specialized to serve a different function. In general, feather types include:

• primary - long feathers located at the tip of the wing
• secondary - shorter feathers located along the trailing edge of the inner wing
• tail - feathers attached to the bird's pygostyle
• contour (body) - feathers that line the bird's body and provide streamlining, insulation, and waterproofing
• down - fluffy feathers located under the contour feathers that serve as insulation
• semiplume - feathers located under the contour feathers that serve as insulation (slightly larger than down feathers)
• bristle - long, stiff feathers around the bird's mouth or eyes (the function of bristle feathers is not known)

Feathers suffer wear and tear as they are exposed to the elements. Over time, the quality of each feather deteriorates and thus compromises its ability to serve the bird in flight or to provide insulation qualities. To prevent feather deterioration, birds shed and replace their feathers periodically in a process called molting.

Sources:

• Attenborough D. 1998. The Life of Birds. London: BBC Books.
• Sibley D. 2001. The Sibley Guide to Bird Life & Behavior. New York: Alfred A. Knopf.
• Museum of Paleontology (University of California, Berkely)