The more recent philosophic sum marized ht• Chapman (Bird Life. New York, 18981. is that, apart from and above the various considerations already mentioned. the neeeskity for protection of the eggs and young from physical accidents, loss of heat, and seizure by enemies is the real motive; and the superior excellence as cradles of the nests of birds of the higher orders is explained by the fact that these orders are 'altriciar—that is, their young are horn in a helpless condition, must be cared for by the parents for a considerable time, and hence both old and young need much better and safer quarters than do the 'precocial' birds. whose young (e.g. chickens) run about at birth and have no need of a nursery.
Wallace also treated of the belief formerly prevalent that birds work by instinct and never make any improvement during their lifetime in nest-building. He asserted that the chief mental faculties so exhibited by birds are the same in kind as those manifested by mankind in the formation of their dwellings; that is, essen tially, imitation. and a slow and partial adapta tion to new conditions. In answer to the ob jection that it is not so much the material as the form and structure of nests that varies. Wallace replied that such diversities may be explained in a great measure by the general habits of the species, the nature of their tool:, the materials they can most easily obtain, and differences of habitat and needs that may have occurred within the period of existing species, due to changes in climate, the earth's surface, food. and so forth. Birds learn something. doubtless, in regard to the size, structure, and material of the nest of their own species before they leave it. Wallace quotes a windier of eases of birds reared in the nests of other birds that sang only the song of the foster parent, learned while in the nest. Then, too, young birds do not always mate with birds of their own age, and the young bird learns nest-building from its morn experienced mate. It is not unusual to see ono bird of a pair, say an English sparrow. redis posing the material that the other bird has just put in place. Several observers- have stated that young birds build less perfect nests titan old bird., and Wallace quotes one instance in Nvhich some young chaffinches were taken to New Zen. land and there set free. They built n most in the new home which showed "very little of that neatness of fabrication for• which this bird is noted in England." It is an oft-repented observa tion that the nests of the Baltimore oriole, when built near the habitations of man, differ in shape and structure from those in the wilds where twine and threads are not at hand, and where there is more necessity of concealment from hawks and snakes. The swallows and swifts of
all parts of the world are quick to change their nesting places from hollow trees and rocky cliffs or caverns to the porches. barns, and chitnnes of men's habitations, and changes in the style of their architecture follow. The nests of hon-e wrens and purple martins vary with the situa tion. chosen. The orchard oriole may build a shallow nest in stout branches or deep ones in swaying willows. 1any similar instances of change in form and material might be adduced. and savages imitate before they originate: birds, as well as all other animal.. do the same." so when the environment remains eonstant, the form and constructive material of birds' nests vary little.
Baomnso OF Bunts. The eggs of birds are hatched by the application of warmth for a sullivient time to mature the embryo to the stage when it breaks from the shell. This necessary warmth (about 105° F.) is secured by the bird covering the eggs with its body, 'sit ting' upon or 'incubating' them almost continu ously for a length of time which in a general way is proportioned to the bird's size. No very extended and accurate observations on this point have been recorded; the best are those by Evans in The Ibis (London) for 1891 and 1892. Broadly speaking, most of the small song-birds hatch their young in from 13 to 15 days, but the very smallest may take less time—the Inumning-bird, it is said, only 10 days. In canaries, it is from 15 to 18 days; in the common fowl, it is 21 days: in the duck, it is from 2S to 30 days; in the guinea-fowl, it is 28 or 29 days; in the turkey, 30 days; and in the swan. from 40 to -15 days. The men is said to sit 50 days. Small altricial birds usually begin sitting after the first egg is laid; but game-birds and water-fowl rarely begin to sit until the whole clutch is in the nest, so that the whole brood shall hatch simul taneously.
It is in most birds the function of the female to perform the duties of incubation, during which she is to a greater or less extent defended, fed, and cheered by her mate. Twice or oftener each day she leaves the nest for rest and to get food, and the male takes her place for an hour or two. It sometimes happens that if she is killed, the male concludes the process of incubation and cares for the young. In some groups he does the entire duty of sitting. This seems to be universally true of the ostrich and other ratite birds, and is the practice of the godwits, phalaropes, and certain other shore-birds. Both sexes join in the care of the young at first. but in most cases their education is gradually left entirely to the mother.