The =feathers= are perhaps the most characteristic feature of birds. The large feathers of the wings and tail are called _quill feathers_. A quill feather (Fig. 291) is seen to consist of two parts, the _shaft_, or supporting axis, and the broad _vane_ or web. What part of the shaft is round? Hollow? Solid? Is the shaft straight? Are the sides of the vane usually equal in width? Can you tell by looking at a quill whether it belongs to the wing or the tail, and which wing or which side of the tail it comes from? Do the quills overlap with the wide side of the vane above or beneath the next feather? Can you cause two parts of the vane to unite again by pressing together the two sides of a split in the vane? Does the web separate at the same place when pulled until it splits again? [Illustration: FIG. 292.—I, CONTOUR FEATHER. II, III, PARTS OF QUILL FEATHER, enlarged. ] The hollow part of the shaft of a quill feather is called the _quill_. The part of the shaft bearing the vane is called the _rachis_ (rā-kis). The vane consists of slender _barbs_ which are branches of the shaft (II, Fig. 292). As the name indicates (see dictionary), a barb resembles a hair. The barbs in turn bear secondary branches called _barbules_, and these again have shorter branches called _barbicels_ (III, Fig. 292). These are sometimes bent in the form of hooklets (Fig. 292, III), and the hooklets of neighbouring barbules interlock, giving firmness to the vane. When two barbules are split apart, and then reunited by stroking the vane between the thumb and the finger, the union may be so strong that a pull upon the vane will cause it to split in a new place next time. [Illustration: FIG. 293.—A DOWN FEATHER, enlarged. ] There are =four kinds= of feathers, (1) the _quill_ feathers, just studied; (2) the _contour_ feathers (I, Fig. 292), which form the general surface of the body and give it its outlines; (3) the _downy_ feathers (Fig. 293), abundant on nestlings and found among the contour feathers of the adult but not showing on the surface; (4) the _pin_ feathers, which are hair-like, and which are removed from a plucked bird by singeing. The contour feathers are similar in structure to the quill feathers. They protect the body from blows, overlap so as to shed the rain, and, with the aid of the downy feathers, retain the heat, thus accounting for the high temperature of the bird. The downy feathers are soft and fluffy, as they possess few or no barbicels; sometimes they lack the rachis (Fig. 293). The pin feathers are delicate horny shafts, greatly resembling hairs, but they may have a tuft of barbs at the ends. [Illustration: FIG. 294.—DORSAL AND VENTRAL VIEW OF PLUCKED BIRD, showing regions where feathers grow. ]