Feather

A feather is one of the epidermal growths that forms the distinctive outer
covering on a bird.

Feathers, which originate from the scales of reptiles, are the most
complicated integumentary structure among the vertebrates. Like hair, nails
and scales, feathers are integumentary appendages; skin organs that form by
controlled proliferation of cells in the epidermis, or outer skin layer,
that produce keratin proteins. They insulate birds from water and cold
temperatures and provide colour which is sometimes used as camouflage
against predators and sometimes as a means of visual communication. Although
individual feathers are very light, a bird's plumage weighs two or three
times more than its skeleton.

There are two basic types of feather; vaned feathers which cover exterior of
the body and down feathers which are underneath the vaned feathers providing
an insulating layer. The vaned feathers include the remiges
(flight-feathers), the rectrices (tail-feathers) and the contour feathers
which are distributed over the whole body. A typical feather features a main
shaft, called the rachis. Fused to the rachis are a series of branches, or
barbs, the barbs themselves are also branched and form the barbules. At the
base of the feather, the rachis expands to form the hollow tubular calamus,
or quill, which inserts into a follicle in the skin.

A birdŐs feathers are replaced periodically during its life through molt,
new feathers are formed through the same follicle from which the old ones
were fledged.