The structure of feathers
Feathers are unique to birds, but were derived from scales and arise to form papillae. A papilla consists of a projection of vascularized dermaltissue that grows out of an epidermal pit, called the feather follicle. A typical feather consists of a stiff axial rod, or shaft. The proximal portion of the shaft, the quill is hollow whereas the distal end is solid. The shaft bears two rows of branches, or barbs, which in turn support two rows of smaller, numerous barbules. The feathery vane is composed of a double series of barbs and barbules. The barbules on the side of the barb towards the tip of the feather bear hooklets or barbicels, that form bridges with ridges on the adjacent proximal barbules. The vane is thus lightweight and pliable, but also extremely strong and resilient. At least once a year each feather is shed and a new feather develops from the same papilla. Birds usually shed, or moult, their old feathers during late summer. There may be partial or complete moult in spring when the bird assumes a more colourful breeding plumage. The acquisition of breeding plumage may also result from wear or the breaking-off of feather tips, thus exposing different colours beneath.