Art. XLIV.—Remarks on the Long-tailed Cuckoo (Eudynamis taitensis).
[Read before the Wellington Philosophical Society, 3rd August, 1878.]
There is a remarkable phenomenon in the animal world known to naturalists as “mimicry,” or the law of protective resemblance. It is developed chiefly among insects, and particularly among the Lepidoptera. Mr. Wallace describes, at page 205 of his enchanting book on the “Malay Archipelago,” a butterfly which, when at rest, so closely resembles a dead leaf as almost to defy detection. The varied details of colouring combine to produce a disguise that so exactly represents a slightly curved or shrivelled leaf as to render the butterfly quite safe from the attacks of insectivorous birds, except when on the wing. The flight of the species, on the other hand, is so vigorous and rapid that it is well able then to protect itself. Mr. Wallace adds that in many specimens there occur patches and spots, formed of small black dots, so closely resembling the way in which minute fungi grow on leaves, that it is impossible not to believe that fungi have grown on the butterflies themselves! This protective imitation must obviously favour the species in the common struggle for existence, and may of itself be sufficient to save it from extinction. But there is another kind of “mimicry” where one insect which would, on discovery, be eagerly devoured, assumes for similar protective purposes a close resemblance to some other insect notoriously distasteful to birds and reptiles, and often belonging to a totally different family or order. Numberless instances might be given in illustration of this singular fact, every department furnishing examples of adaptation more or less complete, and all being explainable on the principle of variation under natural selection or the “survival of the fittest.” Mr. Wallace, when exploring in the Moluccas, was the first to discover similar instances of mimicry among birds, although the law of protective colouring had long been observed to exist in the case of birds' eggs. He gives two very curious examples of external resemblance, co-existing with very important structural differences, rendering it impossible to place the model and the copy near each other in any natural arrangement. In one of these a honeysucker has its colours mimicked by a species of oriole, and the reason is thus stated:—“They must derive some advantage from the imitation, and as they are certainly weak birds, with small feet and claws, they may require it. Now, the Tropidorhynchi are very strong and active birds, having powerful grasping claws, and long, curved, sharp beaks. They assemble together in groups and small flocks, and they have a very loud, bawling note, which can be heard at a great distance, and serves to collect
a number together in time of danger. They are very plentiful and very pugnacious, frequently driving away crows and even hawks, which perch on a tree where a few of them are assembled. It is very probable, therefore, that the smaller birds of prey have learnt to respect these birds, and leave them alone, and it may thus be a great advantage for the weaker and less courageous Mimetas to be mistaken for them. This being the case, the laws of Variation and Survival of the fittest, will suffice to explain how the resemblance has been brought about, without supposing any voluntary action on the part of the birds themselves; and those who have read Mr. Darwin's ‘Origin of Species’ will have no difficulty in comprehending the whole process.”
Among the many minor instances that have attracted notice, the English cuckoo (Cucubus canorus) is supposed to derive protection from the resemblance of its markings to those of the sparrow-hawk (Accipiter nisus), but the resemblance is far more striking between our long-tailed cuckoo (Eudynamis taitensis) and a North American species of hawk (Accipiter cooperi). In the fine specimens of the former which I exhibit this evening, it will be observed that the markings of the plumage are very pronounced, while the peculiar form of the bird itself distinguishes it very readily from all other New Zealand species. Beyond the general grouping of the colours there is nothing to remind us of our own bush-hawk, and that there is no great protective resemblance is sufficiently manifest from the fact that our cuckoo is persecuted on every possible occasion by the tui, which is timorous enough in the presence of a hawk. During a trip, however, on the Continent, in the autumn of 1871, I found in the Zoological Museum at Frankfort, what appeared to be the accipitrine model, in a very striking likeness to our bird. Not only has our cuckoo the general contour of Cooper's sparrow-hawk, but the tear-shaped markings on the under parts and the arrow-head bars on the femoral plumes are exactly similar in both. The resemblance is carried still further in the beautifully banded tail and marginal wing-coverts, and likewise in the distribution of colours and markings on the sides of the neck. On turning to Mr. Sharpe's description of the “young male” of this species in his Catalogue of the Accipitres in the British Museum (p. 137), it will be seen how many of the terms employed apply equally to our Eudynamis, even to the general words “deep brown above with a chocolate gloss, all the feathers of the upper surface broadly edged with rufous.”
The coincident existence of such a remarkable resemblance to a New World form, cannot of course be any protection to an inhabitant of New Zealand, and I do not pretend in this instance to apply the rule; but in the light of natural selection, to which at present no limit can be assigned, the fact itself is a suggestive one, the more so when we remember that this
cuckoo of ours is not a permanent resident, but migrates every winter to the Society Islands. Of this annual migration, across 1,500 miles of ocean, Captain Hutton has well remarked “there is nothing in the whole world so wonderful!”