BLUE MOUNTAIN-PARROTS.
A honey-eating species.
Photo by J. Peat Millar] [Beith.
YOUNG CUCKOO EJECTING EGG.
The egg is held in position by the head and wings.
Let us imagine that this home so lately threatened is that of the modest little hedge-sparrow, and take a peep during the absence of the owners, after quiet has established itself once again. Lying side by side with the tiny sky-blue eggs of the hedge-sparrow we should find the relatively large, greyish-green or reddish-grey egg of the cuckoo. What a contrast! If the hedge-sparrows notice this too, they evidently do not mind, for they invariably hatch it with their own.
But some birds are not so accommodating as this, and would ruthlessly destroy or reject any egg surreptitiously introduced into the nest. Consequently more deception has to be practised. The hawk-like garb still serves its purpose to draw off the intended dupes from the nest; but this is not enough, for to deposit an egg of the normal cuckoo type would be worse than useless, since it would meet with instant destruction on the return of the owners of the nest. But the cuckoo, strange to say, has proved equal to the occasion, and meets the difficulty by laying an egg to match those in the nest. The Redstart, Wagtail, Sedge-warbler, Red-backed Shrike, and Meadow-pipit may be cited as instances of—shall we say exclusive?—birds which must be circumvented by "colourable imitations." Perhaps the most wonderful of the cuckoo successes in this direction is the imitation of the redstart's egg, which is blue.
Naturally these facts have given rise to much speculation, but even now we cannot regard the discussion as finally settled. Some ornithologists held that the egg of every individual cuckoo was subject to great variations, and that the place of deposit of each egg was determined only after the bird had ascertained its colour. If this were true, surely we should find blue cuckoos' eggs in hedge-sparrows' as well as redstarts' nests. But we don't! Others have sought to explain the existence of mimicking eggs to the influence of the food peculiar to the foster-parent upon the germ of the young female cuckoo, which, through this channel, became transmitted to all its descendants. To support this hypothesis it was necessary to throw overboard the old individual variability explanation, and to adopt one that is certainly nearer the truth—to wit, that each cuckoo chooses the nest of that species in which itself was reared as a depository, in turn, for its own egg, and only when such is not available will it select some other species, and trust to luck for its adoption. This would certainly account for many anomalies; but as it seems that there are more eggs unlike than like those of the selected foster-parents, it cannot be a perfect explanation.
A third explanation takes that part of the second for granted which assumes that cuckoos select nests of the species which served them as foster-parents, and explains the mimicry, when this occurs, as due to the results of natural selection.