Just above the end of the tail of a plucked pigeon is to be seen a small conical body, the oil-gland, from which the bird obtains a fluid which it applies to the feathers during the preening process.
The feathers.—Feathers do not grow upon all parts of a bird’s body, but are restricted to certain definite areas or “feather tracts” which, however, differ in arrangement in different species.
Fig. 183.—Pigeon. A, portion of a wing feather; cal, quill; rch, shaft; B, filoplume; C, nestling-down.
One of the large feathers of the wing of the pigeon is illustrated in [Fig. 183], A. It consists of a hollow, horny quill (cal.), prolonged into a solid shaft (rch.) which supports the web or vane. The outer face of the feather is slightly convex and smooth; while the inner surface is somewhat concave and rough, and carries a little tuft of down at the junction of quill and shaft. The growing feather is nourished by a conical projection of the deeper part of the skin, which fits into a small hole (inf. umb.) at the base of the quill. It is easily shown that the vane of the feather is composed, on each side, of a large number of parallel laths which spring from the shaft. These laths are called barbs. Considerable resistance is felt when an attempt is made to pull the barbs apart, but the manner of their connection cannot be clearly seen without the aid of the microscope. A fairly low magnifying power, however, shows ([Fig. 184]) that each barb bears extremely delicate threads—barbules—on each side, arranged on the barb much as the barbs are arranged on the shaft. The barbules on one side of each barb (the side furthest from the quill) are seen to carry tiny hooks; while the barbules of the other side of the barb are furnished with flanges. The hooked barbules of one barb cross the flanged barbules of the next and interlock with them; so that relatively great force is required to pull the barbs apart and destroy the continuity of the web. The quill-feathers of the tail, and the coverts, have webs of similar structure.
Fig. 184.—Structure of a Feather. A, small portion of a feather with pieces of two barbs, each having to the left three hooked barbules, and to the right a number of flanged barbules; B, hooklet of one barbule interlocking with flange of another barbule; C, two adjacent flanged barbules; D, a hooked barbule. (After Pycraft.)
The contour-feathers are used for protection and warmth rather than for flight, and hence have less-perfectly interlocking barbs. The hair-like structures seen on the skin of the plucked bird are extremely simple feathers called filoplumes ([Fig. 183], B). Each consists of a long, slender axis, bearing at the end a few barbs which do not interlock.