bank, and then conveyed the egg in her bill to a wagtail’s nest.
Cuckoos, too, have been not unfrequently shot as they were
carrying a cuckoo’s egg, presumably their own, in their bill,
and this has probably given rise to the vulgar, but seemingly
groundless, belief that they suck the eggs of other kinds of birds.
More than this, Mr G. D. Rowley, who had much experience of
cuckoos, declares (Ibis, 1865, p. 186) his opinion to be that traces
of violence and of a scuffle between the intruder and the owners
of the nest at the time of introducing the egg often appear,
whence we are led to suppose that the cuckoo ordinarily, when
inserting her egg, excites the fury (already stimulated by her
hawk-like appearance) of the owners of the nest by turning out
one or more of the eggs that may be already laid therein, and thus
induces the dupe to brood all the more readily and more strongly
what is left to her. Of the assertion that the cuckoo herself
takes any interest in the future welfare of the egg she has foisted
on her victim, or of its product, there is no good evidence.
But a much more curious assertion has also been made, and one that at first sight appears so incomprehensible as to cause little surprise at the neglect it long encountered. To this currency was first given by Salerne (L’Hist. nat. &c., Paris, 1767, p. 42), who was, however, hardly a believer in it, and it is to the effect, as he was told by an inhabitant of Sologne, that the egg of a cuckoo resembles in colour that of the eggs normally laid by the kind of bird in whose nest it is placed. In 1853 the same notion was prominently and independently brought forward by Dr A. C. E. Baldamus (Naumannia, 1853, pp. 307–325), and in time became known to English ornithologists, most of whom were naturally sceptical as to its truth, since no likeness whatever is ordinarily apparent in the very familiar case of the blue-green egg of the hedge-sparrow and that of the cuckoo, which is so often found beside it.[1] Dr Baldamus based his notion on a series of eggs in his cabinet,[2] a selection from which he figured in illustration of his paper, and, however the thing may be accounted for, it seems impossible to resist, save on one supposition, the force of the testimony these specimens afford. This one supposition is that the eggs have been wrongly ascribed to the cuckoo, and that they are only exceptionally large examples of the eggs of the birds in the nests of which they were found, for it cannot be gainsaid that some such abnormal examples are occasionally to be met with. But it is well known that abnormally large eggs are not only often deficient in depth of colour, but still more often in stoutness of shell. Applying these rough criteria to Dr Baldamus’s series, most of the specimens stood the test very well.
There are some other considerations to be urged. For instance, Herr Braune, a forester at Greiz in the principality of Reuss (Naumannia, tom. cit. pp. 307, 313), shot a hen cuckoo as she was leaving the nest of an icterine warbler (Hypolais icterina). In the oviduct of this cuckoo he found an egg coloured very like that of the warbler, and on looking into the nest he found there an exactly similar egg, which there can be no reasonable doubt had just been laid by that very cuckoo. Moreover, Herr Grunack (Journ. für Orn., 1873, p. 454) afterwards found one of the most abnormally coloured specimens, quite unlike the ordinary egg of the cuckoo, to contain an embryo so fully formed as to show the characteristic zygodactyl feet of the bird, thus proving unquestionably its parentage.
On the other hand, we must bear in mind the numerous instances in which not the least similarity can be traced—as in the not uncommon case of the hedge-sparrow already mentioned, and if we attempt any explanatory hypothesis it must be one that will fit all round. Such an explanation seems to be this. We know that certain kinds of birds resent interference with their nests much less than others, and among them it may be asserted that the hedge-sparrow will patiently submit to various experiments. She will brood with complacency the egg of a redbreast (Erithacus rubecula), so unlike her own, and for aught we know to the contrary may even be colour-blind. In the case of such a species there would be no need of anything further to ensure success—the terror of the nest-owner at seeing her home invaded by a hawk-like giant, and some of her treasures tossed out, would be enough to stir her motherly feelings so deeply that she would without misgiving, if not with joy that something had been spared to her, resume the duty of incubation so soon as the danger was past. But with other species it may be, and doubtless is, different. Here assimilation of the introduced egg to those of the rightful owner may be necessary, for there can hardly be a doubt as to the truth of Dr Baldamus’s theory as to the object of the assimilation being to render the cuckoo’s egg “less easily recognized by the foster-parents as a substituted one.” It is especially desirable to point out that there is not the slightest ground for imagining that the cuckoo, or any other bird, can voluntarily influence the colour of the egg she is about to lay. Over that she can have no control, but its destination she can determine. It would seem also impossible that a cuckoo, having laid an egg, should look at it, and then decide from its appearance in what bird’s nest she should put it. That the colour of an egg-shell can be in some mysterious way affected by the action of external objects on the perceptive faculties of the mother is a notion too wild to be seriously entertained. Consequently, only one explanation of the facts can here be suggested. Every one who has sufficiently studied the habits of animals will admit the influence of heredity. That there is a reasonable probability of each cuckoo most commonly putting her eggs in the nest of the same species of bird, and of this habit being transmitted to her posterity, does not seem to be a very violent supposition. Without attributing any wonderful sagacity to her, it does not seem unlikely that the cuckoo which had once successfully foisted her egg on a reed-wren or a titlark should again seek for another reed-wren’s or another titlark’s nest (as the case may be), when she had another egg to dispose of, and that she should continue her practice from one season to another. It stands on record (Zoologist, 1873, p. 3648) that a pair of wagtails built their nest for eight or nine years running in almost exactly the same spot, and that in each of those years they fostered a young cuckoo, while many other cases of like kind, though not perhaps established on so good authority, are believed to have happened. Such a habit could hardly fail to become hereditary, so that the daughter of a cuckoo which always put her egg into a reed-wren’s, titlark’s or wagtail’s nest would do as did her mother. Furthermore it is unquestionable that, whatever variation there may be among the eggs laid by different individuals of the same species, there is a strong family likeness between the eggs laid by the same individual, even at the interval of many years, and it can hardly be questioned that the eggs of the daughter would more or less resemble those of her mother. Hence the supposition may be fairly credited that the habit of laying a particular style of egg is also likely to become hereditary. Combining this supposition with that as to the cuckoo’s habit of using the nest of the same species becoming hereditary, it will be seen that it requires only an application of the principle of natural selection to show the probability of this principle operating in the course of time to produce the facts asserted by the anonymous Solognot of the 18th century, and by Dr Baldamus and others since. The particular gens of cuckoo which inherited and transmitted the habit of depositing in the nest of any particular species of bird eggs having more or less resemblance to the eggs of that species would prosper most in those members of the gens where the likeness was strongest, and the other members would (ceteris paribus) in time be eliminated. As already shown, it is not to be supposed that all species, or even all individuals of a species, are duped with equal ease. The operation of this kind of natural selection would be most needed in those cases where the species are not easily duped—that is, in those cases which occur the least frequently. Here it is we find it, for observation shows that eggs of the cuckoo deposited in nests of the red-backed shrike (Lanius collurio), of the bunting (Emberiza miliaria), and of the icterine warbler approximate in their colouring to eggs of those species—species in whose nests the cuckoo rarely (in comparison with others) deposits eggs.
