It is difficult to give an adequate idea of the strange, lackadaisical appearance which these birds present while acting in the above-described manner. With widely-gaping bills, swelled throats, necks stretched out, and heads moving slowly all about, now up, now down, now to this side, now to that, they look sometimes "sick of love," like Solomon, and sometimes as though about to be sick indeed—in fact, on the point of vomiting. All the bird's actions are peculiar, but none more so than this wide gaping distension of the mandibles, with the full view that it offers of the whole interior cavity of the mouth. This last is not indeed brilliant, as is that of the kittiwake, but, for all that, it is very pleasing, of a delicate mauvy blue, æsthetic in its appearance, and in harmony with the soft and delicate tinting of the plumage. There is no reason to suppose that the latter beauty is unappreciated by the bird itself, when seen in the opposite sex. Why, then, should the pale mauve or blue of the inside of the mouth—this purple chamber flung open for inspection during the season of courtship or of nuptial dalliance—be not appreciated too?
The razorbill's mouth, inside, is of a conspicuous light yellow, which, when exposed suddenly to view, contrasts very forcibly with the black of the beak and upper plumage. In dalliance these birds throw the head straight up into the air, and, opening their clean-cut bills, so that one sees the gay interior like a line of bright gamboge, utter a deep guttural note, which is prolonged and has a vibratory roll in it, like the cry of the gorilla when angry—si parva licet componere magnis—as described by Du Chaillu. It is not loud, however, and so is easily lost amidst elemental sounds and the cries of other birds. The vibratory character of the note becomes more marked under the influence of excitement, and the mandibles themselves vibrate as they are opened at intervals, somewhat widely. In the midst of their duet the pair toss their heads about, catch hold of each other's beaks, and give quick little emotional nibbles at the feathers of their throats or breasts. If we can suppose that the birds are interested in each other's appearance whilst thus acting—that they admire or are sexually excited by one another—then it would be strange if the bright flashing yellow so constantly exhibited did not play its full part in producing this result. Imagine ourselves razor-bills, and thus acting. Could we be blind to such revelations? I think not.
The pretty little black guillemot—the dabchick of ocean—may often be seen sitting in a niche of the cliffs, and calling to another—its mate presumably—either above or below it. The cry is, for the most part, a weak, twittering sound, but occasionally rises into a very feeble little wail or scream. All the while the bird is uttering it he keeps raising and again depressing his head and opening his beak so as to show conspicuously the inside of his mouth, which is of a very pretty rose or blush-red hue, almost as vivid as that of the feet. The beak is opened more widely than would seem to be necessary for the production of the sound, as if to show this coloration, even though, for the moment, there may be no other bird there to see it. If, however, the rosy inward complexion were in any way an attraction, it would be natural for a bird, wanting its mate, to associate the wish with the action of opening the beak, just as a lonely dove in a cage will coo and bow as to a partner. As a matter of fact, the crying bird very soon flies to the other one (or vice versâ), and, standing beside her, utters his little twitter as a greeting. She, being couched down, responds by raising her head, so that the tip of her beak touches, or nearly touches, his. Then he couches also, and sitting thus, side by side—comfy on the sheer edge of the precipice—the two turn, from time to time, their heads towards each other, open their bills, and twitter together. Every time they open them the pretty rose tapestry of the mouth-chamber must be plain to each or either, and the more so that they are vis-a-vis.
In all these four birds, therefore, we have a nuptial habit of distending the jaws, side by side with a brilliant or pleasing coloration of the region which, by such action, is exhibited. Moreover, in the case of one of them, more particularly—viz. the fulmar petrel—this distension may be unaccompanied with any note, though it always is with the odd gestures and lackadaisical expression which I have tried to describe. In other words, the beak is sometimes opened as a part of the bird's nuptial actions, and not merely with a view to the production of sound. That originally this alone would have been the motive of its being so can hardly, I suppose, be doubted, but may it not be possible that the eye has gradually come to share in a pleasure which was, at first, communicated through the ear alone, and that a process of selection, founded, perhaps, on some initial freshness of colouring, has in time produced a special kind of adornment? If this were so, we might expect that some of the birds so adorned would have the habit of opening the bill in this manner without uttering any note at all, or, at least, that they would very frequently do so. Such an instance we have in the shag, that smaller and more adorned variety of the cormorant, which is much more common on our northern coasts than the so-called common one. One of the most ordinary nuptial actions of these birds is to throw the head into the air, and open and shut the beak several times in succession; and sometimes they hold it wide open for several seconds together. Each time, as the jaws gape, a splendid surface of bright gamboge yellow is exhibited, which the human eye, at any rate, has to admire, and which exactly matches with the naked yellow skin at the base of the two mandibles on either side, where they become lost in what may be called the bird's cheek. This exterior brightly-tinted surface is continuous with the interior and much larger one, and my view is that the colour of the latter represents an extension of that of the former, by a similar process of sexual selection. There is no doubt whatever that this outward adornment largely adds to the handsome appearance of the shag, and probably those naturalists who believe in sexual selection at all will think it as much due to that agency as the crest and the sheeny green plumage. But if the closely similar colouring of the adjacent interior region is to be looked on as merely fortuitous (we escape here, thank heaven, from the all-pervading protective theory), why should the other be thought to be anything more? If the shag had not this habit of opening its mouth and thus displaying what is, in itself, so very striking, it would be difficult, I think, to accept sexual selection as an explanation even of the facial adornment, since, if the one effect were nonsignificant, so might the other be. As it is, I can see no reason why it should not have brought about both.
I have often watched shags thus throwing up their heads and opening and shutting their jaws at one another, and though I have generally been fairly close to them I have never heard them utter a note whilst so doing. I consider these actions—together with other still more peculiar ones, which they indulge in during the breeding season—to be of a sexual character, and, if so, this silent and oft-repeated distension of the jaws must have some kind of meaning. The large and brilliantly-coloured surface which is thus displayed supplies this meaning, as I am inclined to think.
The fact that some birds—I have not the knowledge to say how many—which do not open the beak in this way, have yet the inside of the mouth brightly or conspicuously coloured, may seem to throw doubt on the theory here advanced; but of course sexual selection is not the only power which may have produced such coloration, and the likelihood of its having done so is decreased if there is no outer facial adornment to match that within. The cuckoo is one such example, for—I speak on the strength of young ones which I have seen in the nest—the whole of its inner mouth is of a really splendid salmon colour. When approached, the nestling cuckoo assumes a most threatening attitude, alternately dilating and drawing itself in, now receding into the nest, now rising up in it as though to strike, having all the while its mouth wide open and hissing violently. Its feathers are ruffled, and altogether it has a quite terrifying aspect, to which the triangular flaming patch that seems to burst out of the centre of it—for the head is drawn right back upon the body—very largely contributes. Especially is this so when, as is mostly the case, there is considerable shadow in the recess of the nest, amidst the surrounding foliage. If it can be supposed that the large false head and painted eyes of the puss or elephant hawk-moth caterpillars have been acquired as a protection against enemies—as to which see Professor Poulton's interesting suggestion[6]—then it certainly seems to me more than possible that the flame-like throat of the young cuckoo has been developed in the same manner, pari passu with the loud, snake-like hiss and intimidating gestures. In conclusion, I would suggest that the bright or pleasingly-coloured mouth-cavity which some birds possess may have a distinct meaning, and be the product either of natural or sexual selection.
[6] The Colours of Animals (International Scientific Series).
AERIAL PIRACY