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The nature of feather construction
Each feather grows out of the dermal tissue from a follicle in similar fashion to hair in mammals. Some feathers can be moved by muscles attached to the follicles. For example, tail and wing feathers can be adjusted to aid in flight. Body feathers can be erected independently or in groups for the purpose of body temperature adjustment as well as for display. Most feather follicles are well supplied with nerves, so it appears that feathers may serve as organs of touch. During development the feather is a living structure well supplied with blood, but once matured the feather itself is a dead structure. After a period of use it is shed or molted, and then replaced by a new feather from the same follicle.
There are two basic types of feathers from which others are derived; down feathers and vaned feathers. Down feathers are essentially random fluff having no barbicels on the barbules to interlock their barbs. In nestling birds down feathers consist of a tuft of barbs without a rachis. The juvenile and adult bird have down feathers that include a rachis. Vaned feathers include all feathers with a flat expanse of barbs extending parallel out from the shaft. Contour and flight feathers are pennaceous vaned feathers and are accepted as vaned feathers, where that plumulaceous feathers generally are not. Technically speaking, as discussed under Feather Anatomy 101, a marabou feather, though strictly a plumulaceous feather, is also a vaned feather. Down feathers, though plumulaceous, have a random arrangement of barbs, and thus would not be considered vaned.
Other feather types similar in some respects to down and vaned feathers while unique in others include filoplumes, bristles, and semiplumes. A filoplume (thread feather) is a hair-like feather with barbs at the end of the shaft. They are distributed to all feather types, are always intimate to other feathers (from one to twelve adjacent a feather,) but grow out of their own follicles. Their purpose seems to have something to do with subtle detection of movement of the adjacent feather such that they may, for example, aid in adjustment of feathers when in flight. (Filoplumes are sometimes incorrectly referred to as pinfeathers. A pinfeather is any feather that is immature.) Virtually all bristles are found on bird's heads. They are stiff with a tapered shaft having barbs only on the proximal portion of the shaft (i.e., Crown Crane crest feathers.) Often they are mistaken for filoplumes which differ by having barbs at the distal end of the shaft. A semiplume is a down-like (plumulaceous) feather having a rachis, barbs, and barbules, but no barbicels (i.e., marabou.)