Page:Popular Science Monthly Volume 77.djvu/88

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HE cyclical instincts of birds, present, as we have seen, a well ordered series, rising and waning in due course, until the reproductive cycle is complete. Nevertheless, the order and harmony which commonly prevail are subject to many disturbances of a transient, or of a more lasting character. When variations in the cycle, whatever their nature, become regular and permanent, any consequent loss or injuiryinjury [sic] to the species seems to be counterbalanced by the rise of new instincts in both young and adult, which may involve marked structural changes, as shown in the parasitic cuckoos of the old world and their non-parasitic relatives of the new. If transient merely, there is more or less individual loss, according to the nature and extent of the disturbance.

We shall now consider some of these variations in the cyclical series, and we may assume, though with little exact knowledge, that when any character of the sort to be described has become general or permanent this has been effected through a gradual process of selection, with or without environmental influence and other unknown agencies. We may further assume that all modern birds originally built proper nests, and there can be little doubt that many either falter or fail in this work at present through the loss of an instinct which they once possessed; but this question aside, we can be reasonably assured that all originally concealed or guarded their eggs.

The nest, in the first instance, tends to secure a more equable distribution of warmth and moisture for eggs or young; incidentally it may conceal and therefore protect both young and adult, and add to the comfort of the whole family. There would seem to be a vast difference between digging a hole in the warm, moist sand, as we see the turtle, or the moleomaleo [sic], one of the brush turkeys, doing, and weaving through the unremitted efforts of many days, a beautiful pouch like the oriole's, so admirably adapted for protection, both by its form and by its position. Yet it is by no means certain that the fundamental nest building instinct is entirely wanting in the moleomaleo [sic], the peculiar habits of which will be later considered.

Nest-building of one kind or another is found in all classes of vertebrates, and the guarding and fighting instincts at nesting-time are as strong in some of the fishes as in birds, but while the practise is