The duck.—The birds of the duck order—which includes also the geese and swans—differ in many important respects from all our previous types, and it is interesting to observe how perfectly these differences are adapted to the manner of life. On land, the ungainly waddle, which is entailed by the insertion of the hind limbs so far back on the body (Fig. 212), shows that the duck is not in an entirely congenial element; in the water the bird is a model of graceful movement, perfectly balanced, progressing smoothly by alternate strokes of its webbed feet, and altering its course to any desired direction with the utmost ease. The duck feeds largely upon the small animals which abound in the water of ponds, and in the mud of the sides, and the beak is beautifully fitted for the duties it has to perform. It is [Pg 328] covered on the outside by a soft and highly sensitive skin, which enables the bird to detect with certainty the presence of its prey in the mud; the inner edge of the beak is provided with horny plates which, with similar plates situated on the edge of the thick, fleshy tongue, form a very efficient strainer, by means of which the useless water and thin mud can be forced out at the sides of the mouth and separated from the worms, etc., which were taken into the mouth at the same time. A thick coat of down-feathers, which lies next the skin and contains a great deal of entangled air, forms a non-conducting layer which prevents the undue escape of the heat of the body, and saves the bird from becoming chilled when in the water. Ducks are, moreover, careful to keep their feathers well oiled, and may often be seen preening themselves—applying the bill alternately to the oil-gland on the tail and to the feathers. The completeness with which water flows off a duck’s back is proverbial. Fig. 212.—Ducks. [Pg 329] During the greater part of the year the brilliant plumage of the drake forms a striking contrast to the sober brown and grey feathers of the female. Another point of difference is that in the male the four middle tail-feathers are curled upwards. From July to October, however, before the moult takes place, the two sexes are very similar in appearance. Birds of the duck order moult in a somewhat different manner from most other birds, in that all the quill-feathers are shed at once, instead of in pairs. Until the new feathers develop, flight is of course out of the question, and the birds remain as secluded as possible in the meantime.