feather

A feather consists of a central shaft (rachis) bearing barbs that branch into barbules, which interlock via hooklets (barbicels) to form a continuous vane. The proximal hollow portion (calamus) anchors the feather in the follicle. Down feathers lack interlocking barbules, creating a fluffy insulating structure.

Feathers develop from cylindrical follicles in the skin, which are invaginations of the epidermis lined by a germinative layer. The follicle produces keratinocytes that differentiate and keratinize to form feather components. Feathers grow cyclically and are replaced during molt.

Feather types include contour feathers (outer covering, flight and display), flight feathers — primaries and secondaries on wings, rectrices on tail (all for aerial locomotion), down feathers (insulation beneath contour feathers), semiplume feathers (transitional insulation), and filoplumes (sensory).