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. The down-feathers (Fig. 183, C), which form the temporary covering of nestling pigeons, also bear barbs which do not interlock. At regular intervals, either once or twice a year, birds moult, [Pg 278] that is, shed most or all of their feathers and grow new ones. The moulting usually takes place gradually and symmetrically: a flight feather from each wing, for example, being dropped at the same time. Flight.—The outstretched wing of a pigeon has a relatively great area; and is markedly convex on the upper surface and concave on the lower, resembling in this respect an open umbrella. The great difference in the resistance which the inside and outside of an open umbrella respectively oppose to the wind is familiar to everyone, and illustrates the advantage of the dome-shape of the outstretched wing. The force which the great breast-muscles put into the down-stroke of the wing is enormous in comparison with the weight of the body, and is sufficient to push the bird upwards and forwards in the air. The gently tapering shape and smooth surface of the feathered bird diminish the resistance of the air. When the wing is raised again for the next stroke, the quill-feathers are turned a little edgewise, so that the air slips between them; just as an oarsman “feathers” his oar to lessen the resistance to the blade in the return-stroke. The direction of the wing-stroke can be altered in accordance with the direction of air currents, and the fan of tail-feathers is capable of being opened or closed, raised or lowered, and turned at various angles to act as a rudder.