As in previous instances, observing that the interloper had a voice fully capable of making his wants known, I gave the comfortable little beast ample room to spread himself on the ground, and let the lone little starveling survivor of the rightful brood have his cot all to himself.
And yet, as I left the spot, I confess to a certain misgiving, as the pleading chirrup of the ousted fledgling followed me faintly and more faintly up the hill, recalling, too, the many previous similar acts of mine—and one in particular, when I had slaughtered in cold blood two of these irresponsibles found in a single nest. But sober second thought evoked a more philosophic and conscientious mood, the outcome of which leading, as always, to a semi-conviction that the complex question of reconciliation of duty and humanity in the premises was not thus easily disposed of, considering, as I was bound to do, the equal innocence of the chicks, both of which had been placed in the nest in obedience to a natural law, which in the case of the cow-bird was none the less a divine institution because I failed to understand it. Such is the inevitable, somewhat penitent conclusion which I always arrive at on the cow-bird question; and yet my next cow-bird fledgling will doubtless follow the fate of all its predecessors, the reminiscent qualms of conscience finding a ready philosophy equal to the emergency; for if, indeed, this parasite of the bird home be a factor in the divine plan of Nature's equilibrium, looking towards the survival of the fittest and the regulation of the sparrow and small-bird population, which we must admit, how am I to know but that this righteous impulse of the human animal is not equally a divine, as it is certainly a natural institution looking to the limitations of the cow-bird? One June morning, a year or two ago, I heard a loud squeaking, as of a young bird in the grass near my door, and, on approaching, discovered the spectacle of a cow-bird, almost full-fledged, being fed by its foster-mother, a chippy not more than half its size, and which was obliged to stand on tiptoe to cram the gullet of the parasite.
The victims of the cow-bird are usually, as in this instance, birds of much smaller size, the fly-catchers, the sparrows, warblers, and vireos, though she occasionally imposes on larger species, such as the orioles and the thrushes. The following are among its most frequent dupes, given somewhat in the order of the bird's apparent choice: song-sparrow, field-sparrow, yellow warbler, chipping-sparrow, other sparrows, Maryland yellow-throat, yellow-breasted chat, vireos, worm-eating warbler, indigo-bird, least-flycatcher, bluebird, Acadian flycatcher, Canada flycatcher, oven-bird, king-bird, cat-bird, phœbe, Wilson's thrush, chewink, and wood-thrush.
But one egg is usually deposited in a single nest; the presence of two eggs probably indicates, as in the case of the European cuckoo, the visits of two cow-birds rather than a second visit from the same individual—the presence of two cow-bird chicks of equal size being rather a proof of this than otherwise, in that kind Nature would seem to have accommodated the bird with an exceptional physiological resource, which matures its eggs at intervals of three or more days, as against the daily oviposition of its dupes, thus giving it plenty of time to make its search and take its pick among the bird-homes. Whether the process of evolution has similarly equipped our cow-bird I am not aware; but the vicious habits of the two birds are so identical that the same accommodating functional conditions might reasonably be expected. It is, indeed, an interesting fact well known to ornithologists that our own American cuckoos, both the yellow-billed and black-billed, although rudimentary nest-builders, still retain the same exceptional interval in their egg-laying as do their foreign namesake. The eggs are laid from four days to a week apart, instead of daily, as with most birds, their period of perilous nidification on that haphazard apology of a nest being thus possibly prolonged to six weeks. Thus we find, in consequence, the anomalous spectacle of the egg and full-grown chick, and perhaps one or two fledglings of intermediate stages of growth, scattered about at once, helter-skelter, in the same nest. Only two years ago I discovered such a nest not a hundred feet from my house, containing one chick about two days old, another almost full-fledged, while a fresh-broken egg lay upon the ground beneath. Such a household condition would seem rather demoralizing to the cares of incubation, and doubtless the addled or ousted egg is a frequent episode in our cuckoo's experience.
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.