In the feathers of a large portion of birds there is a plumiform formation, or small feather or plumule. This plumule is conspicuous in gallinaceous birds--for instance, the Pheasants (Fig. 53); it springs from the fore part of the tube, just at the commencement of the shafts; it gradually narrows, and is continued in the form of a very delicate, thread-like fibre; from its side proceed two series of barbs, and from the barbs two series of barbules, extremely fine, entirely disunited, and very loose. This plumule seldom exists among aquatic birds, but in gallinaceous fowls it attains the length of two-thirds of the feather, and in the Emu and Cassowary it equals it in length. Feathers may be divided into those specially employed as the means of locomotion and those intended to protect the bird from extreme cold. The former are much stronger, more compact, and more elongated than the others. The row of feathers bordering the wing behind is known as the _alar quills_, or wing-quills, and those terminating the extremity of the tail, as _caudal quills_. From the head, backwards to the tail, the feathers increase in strength and size; those on the face, or round the base of the bill, being smallest, the tail-coverts longest. Immediately covering the base of the wing-quills are a row of feathers on both surfaces of the wing; these are the quill-coverts. The most brilliant feathers are found in birds of warm climates, and the more tropical the climate the more dazzling and brilliant is the plumage. In many species the brilliant plumage is confined to the males, while that of the females is dark and sombre. In other cases it is the same in both sexes. The young of some species attain the adult appearance after the first moult; others take several years to acquire their full splendour. Birds cast their feathers at least once a year, in order to put on a fresh dress. This is called moulting--a change which usually occurs in the autumn, but sometimes both in spring and autumn. During the moulting season birds are dull, retiring, and silent; but when they emerge from this state they proudly display their lively colours, which now rival the gayest flowers that surround them. Among the gallinaceous birds, and especially among the aquatic species, there exist over the coccyx certain receptacles from which is distilled the oily substance with which they lubricate their plumage. These receptacles are known as the uropygial glands. On the lower surface is a layer of cellular tissue containing a similar fluid, which seems to be connected with the growth of the feathers. [Illustration: Fig. 56.] [Illustration: Fig. 57.] [Illustration: Fig. 58.] [Illustration: Fig. 59.] [Illustration: Fig. 60.]