2992 ZOOLOGY. 
restricted and noisy flight; while in the ostrich, emu (pl. 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, which are their peculiar clothing. Im the 
smaller birds these are frequently highly ornamental; while in the tribes 
inhabiting the water 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 considerably in the course of a year, the brightest colors and otherwise 
most complete plumage being observable durmg 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 own 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 power 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, notwithstanding 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 
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