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 razorbills, and thus
