semiplumes

Semiplumes are a category of feather in birds that lie between down and contour feathers in both structure and function. They possess a central shaft (rachis) and a vane, but the vane is loose and plumaceous toward the base, with longer barbs toward the distal end that do not interlock as tightly as in contour feathers. The basal portion is fluffy, providing insulation.

Unlike down feathers, which are very soft and lack a prominent rachis, semiplumes have a more defined rachis and a discernible but soft vane. Compared with contour feathers, semiplumes are not designed for flight or aerodynamic stability; they primarily insulate and cushion the body and help smooth the plumage. Semiplumes are one of the several feather types described in avian anatomy, alongside down, contour, filoplumes, and bristles.

Semiplumes are widespread among birds and are typically located beneath the outer contour feathers, on the body and around the neck. They contribute to insulation, add bulk to the plumage for shaping, and can play a role in display or camouflage, depending on color and texture.

Development and variation: Semiplumes develop from follicles like other feathers and are moulted with the rest of the plumage. In some species they are reduced or replaced by other feather types in certain body regions; juveniles often have more noticeable semiplumes that decrease as adults reach maturity.

Etymology: The term semiplume derives from semi- "half" and plume "feather."