It is an interesting question which the contrast of the American and European cuckoo thus presents. Is the American species a degenerate or a progressive nest-builder? Has she advanced in process of evolution from a parasitical progenitor building no nest, or is the bird gradually retrograding to the evil ways of her notorious namesake?
The evidence of this generic physiological peculiarity in the intervals of oviposition, taken in consideration with the fact of the rudimentary nest, would seem to indicate the retention of a now useless physiological function, and that the bird is thus a reformer who has repudiated the example of her ancestors, and has henceforth determined to look after her own babes.
With the original presumed object of this remarkable prolonged interval in egg-laying now removed, the period will doubtless be reduced through gradual evolution to accommodate itself to the newly adopted conditions. The week's interval, taken in connection with the makeshift nest or platform of sticks, is now a disastrous element in the life of the bird. Such of the cuckoos, therefore, as build the more perfect nests, or lay at shortest intervals, will have a distinct advantage over their less provident fellows, and the law of heredity will thus insure the continual survival of the fittest.
The cuckoo is not alone among British birds in its intrusion on other nests. Many other species are occasionally addicted to the same practice, though such acts are apparently accidental rather than deliberate, so far as parasitical intent is concerned. The lapse is especially noticeable among such birds as build in hollow trees and boxes, as the woodpeckers and wagtails. Thus the English starling will occasionally impose upon and dispossess the green woodpecker. In the process of nature in such cases the stronger of the two birds would retain the nest, and thus assume the duties of foster-parent. Starting from this reasonable premise concerning the prehistoric cuckoo, it is not difficult to see how natural selection, working through ages of evolution by heredity, might have developed the habitual resignation of the evicted bird, perhaps to the ultimate entire abandonment of the function of incubation. Inasmuch as "we have no experience in the creation of worlds," we can only presume.
Indeed, the similarities and contrasts afforded by a comparison of the habits of all these birds-European cuckoo, American cuckoo, and cow-bird-afford an interesting theme for the student of evolution. What is to be the ultimate outcome of it all? for the murderous cuckoo must be considered merely as an innocent factor in the great scheme of Nature's equilibrium, in which the devourer and the parasite would seem to play the all-important parts, the present example being especially emphasized because of its conspicuousness and its violence to purely human sentiment. The parasite would often seem to hold the balance of power.
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Jonathan Swift's epitome of the subject, if not specifically true, is at least correct in its general application:
"A flea Has smaller fleas that on him prey; And these have smaller still to bite 'em; And so proceed
Even the tiny egg of a butterfly has its ichneumon parasite, a microscopic wasp, which lays its own egg within the larger one, which ultimately hatches a wasp instead of the baby caterpillar.
But who ever heard of anything but good luck falling to the lot of cow-bird or cuckoo, except as its blighting course is occasionally arrested by the outraged human? They always find a feathered nest.
In this connection it is interesting to note certain developments in bird life upon the lines of which evolution might work with revolutionary effect. Most of our birds are helpless and generally resigned victims to the cow-bird, but there are indications of occasional effective protest among them. Thus the little Maryland yellow-throat, according to various authorities, often ousts the intruded egg, and its broken remains are also occasionally seen on the ground beneath the nests of the cat-bird and the oriole. The red-eyed vireo, on the other hand, though having apparently an easier task than the latter, in the lesser depth of her pensile nest, commonly abandons it altogether to the unwelcome speckled ovum-always, I believe, if the cow-bird has anticipated her own first egg.
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