When the female returns, and finds in her nest an egg which she immediately perceives to be different from her own, she leaves the nest, and perches on a branch near it, returns and retires several times in succession, flies off, calling loudly for her mate, who soon makes his appearance, manifesting great anxiety at the distress of his spouse. They visit the nest together, retire from it, and continue chattering for a considerable time. Nevertheless, the obnoxious egg retains its position, the bird continues to deposit its eggs, and incubation takes place as usual. The egg of the Cow Bird is of a regular oval form, pale greyish-blue, sprinkled with umber-brown dots and short streaks, which are more numerous at the larger end.
Incubation has been continued for nearly a fortnight, and the young Cow Bird bursts the shell. Another remarkable occurrence now takes place. The eggs of the foster-bird are yet unhatched, and soon after disappear. In every case the Cow Bird's egg is the first hatched, and herein also is manifested the wisdom of Nature; for the parent-birds finding a helpless object, for whose subsistence it behoves them to provide, fly off to procure food for it. The other eggs are thus neglected, and the chicks which they contain necessarily perish. Birds have probably the means of knowing an addle egg, for, when any such remain after the hatching of the others, they always remove them from the nest; and, in the present case, the remaining eggs are soon removed, and may sometimes be seen strewn about in the vicinity of the nest. In the case of the Cuckoo matters are differently managed, for the young bird of that species very ungratefully jostles out of the nest all his foster-brothers and sisters, that he may have room enough for himself. If we are fond of admiring the wisdom of Nature, we ought to mingle reason with our admiration; and here we might be tempted to suspect her not so wise as we had imagined, for why should the poor Yellow-throat have been put to the trouble of laying all these eggs, if they are, after all, to produce nothing? This is a mystery to me; nevertheless, my belief in the wisdom of Nature is not staggered by it.
As the young Cow-Bird grows up, its foster-parents provide for it with great assiduity, and manifest all the concern and uneasiness at the intrusion of a stranger, that they would do were their own offspring under their charge. When fully fledged, the young bird is of a sooty-brown colour. Long after it has left the nest, it continues to be fed by its affectionate guardians, until it is at length able to provide for itself.