IN THE SHETLANDS 131 
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 char- 
acter, 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 
