292 ZOOLOGY. 



restricted and noisy flight ; while in the ostrich, emu [pi. 94, 95), and some 

 swimming birds, they are only useful to assist in running or swimming. In 

 all cases, however, there appears to be ample compensation in enlarged powers 

 of the two latter methods of locomotion. 



All birds are protected from external injury by a more or less densely 

 arranged growth of feathers, Avhich are their peculiar clothing. In the 

 smaller birds these are frequently highly ornamental : while in the tribes 

 inhabiting the Avater or found only within the limits of excessive cold, in 

 which utility is more strictly an object, this clothing is remarkably compact, 

 and adapted in the most wonderful manner to the retention of warmth. In 

 the swimming birds it is absolutely impervious to water, and is constantly 

 lubricated by an oily secretion from a gland near the rump, more or less 

 developed in all birds. 



The periodical renewal of this plumage is termed moulting, and usually 

 takes place in all birds once in a year. It usually occurs several times 

 before the mature plumage of the species is attained. The plumage alters 

 also considerabl}^ in the course of a year, the brightest colors and otherwise 

 most complete plumage being observable during the season of courtship and 

 incubation ; after which it fades and in some species a moult takes place, 

 during which the male bird assumes a plumage resembling that of the female, 

 only attaining his OAvn again in the spring. 



The digestive power of all birds is very powerful, and the demand for 

 food is accordingly of frequent recurrence. All birds are great eaters, and 

 seem necessarily to require a comparatively large quantity of food to 

 sustain their active habits and peculiar organization. The food is at first 

 temporarily deposited in a sac, which is merely an extension of the gullet, 

 and termed the craw, from which it is generally transferred to the stomach 

 proper. This is composed of two apartments, in one of which the food is 

 moistened by a secretion from peculiar glands, and then transferred to the 

 second part or gizzard, in which the digestion is completed. The gizzard 

 is furnished with enormously strong muscles in birds which feed upon 

 grains, as the common fowl, but in those which subsist upon animal food 

 the muscles are of but ordinary development. Many birds swallow particles 

 of sand or gravel, for the purpose, it is believed, of assisting the powers of 

 digestion. The craw is wanting in some birds which feed upon soft fruits. In 

 parrots and pigeons it has the function of secreting a milk-like substance with 

 which the food of the parent is mixed, and afterwards disgorged for the purpose 

 of feeding the young. 



Of the senses possessed by birds that of sight is the most acute, and 

 exceeds in powder that of all other animals and even of man. The rapa- 

 cious vultures and eagles have this sense in the highest degree, but all 

 birds have it very acutely developed. The sense of smell does not seem to 

 be at all remarkable in any birds, not with standin^the opinion formerly 

 entertained that by this sense vultures were attracted to their prey. This 

 opinion has been completely disproved, and the fact fully established that 

 they depend upon sight alone. Hearing is usually acute, and is very 

 delicate in the owls, some of which are the only birds which have external 

 496 



