It is remarkable that only small birds properly sing. The Australian genus Menura, however, must be excepted; for the _Menura Alberti_, which is about the size of a half-grown turkey, not only mocks other birds, but “its own whistle is exceedingly beautiful and varied.” The males congregate and form “_corroborying_ places,” where they sing, raising and spreading their tails like peacocks and drooping their wings.[97] It is also remarkable that the birds which sing are rarely decorated with brilliant colours or other ornaments. Of our British birds, excepting the bullfinch and goldfinch, the best songsters are plain-coloured. The kingfisher, bee-eater, roller, hoopoe, woodpeckers, &c., utter harsh cries; and the brilliant birds of the tropics are hardly ever songsters.[98] Hence bright colours and the power of song seem to replace each other. We can perceive that if the plumage did not vary in brightness, or if bright colours were dangerous to the species, other means would have to be employed to charm the females; and the voice being rendered melodious would offer one such means. [Illustration: Fig. 39. Tetrao cupido; male. (From Brehm.)] In some birds the vocal organs differ greatly in the two sexes. In the _Tetrao cupido_ (fig. 39) the male has two bare, orange-coloured sacks, one on each side of the neck; and these are largely inflated when the male, during the breeding-season, makes a curious hollow sound, audible at a great distance. Audubon proved that the sound was intimately connected with this apparatus, which reminds us of the air-sacks on each side of the mouth of certain male frogs, for he found that the sound was much diminished when one of the sacks of a tame bird was pricked, and when both were pricked it was altogether stopped. The female has “a somewhat similar, though smaller, naked space of skin on the neck; but this is not capable of inflation.”[99] The male of another kind of grouse (_Tetrao urophasianus_), whilst courting the female, has his “bare yellow œsophagus inflated to a prodigious size, fully half as large as the body;” and he then utters various grating, deep hollow tones. With his neck-feathers erect, his wings lowered and buzzing on the ground, and his long pointed tail spread out like a fan, he displays a variety of grotesque attitudes. The œsophagus of the female is not in any way remarkable.[100]