Page:BirdWatcherShetlands.djvu/155

Rh 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