It seems now well made out that the great throat-pouch of the European male bustard (_Otis tarda_), and of at least four other species, does not serve, as was formerly supposed, to hold water, but is connected with the utterance during the breeding-season of a peculiar sound resembling “ock.” The bird whilst uttering this sound throws himself into the most extraordinary attitudes. It is a singular fact that with the males of the same species the sack is not developed in all the individuals.[101] A crow-like bird inhabiting South America (_Cephalopterus ornatus_, fig. 40) is called the umbrella-bird, from its immense top-knot, formed of bare white quills surmounted by dark-blue plumes, which it can elevate into a great dome no less than five inches in diameter, covering the whole head. This bird has on its neck a long, thin, cylindrical, fleshy appendage, which is thickly clothed with scale-like blue feathers. It probably serves in part as an ornament, but likewise as a resounding apparatus, for Mr. Bates found that it is connected “with an unusual development of the trachea and vocal organs.” It is dilated when the bird utters its singularly deep, loud, and long-sustained fluty note. The head-crest and neck-appendage are rudimentary in the female.[102] [Illustration: Fig. 40. The Umbrella-bird or Cephalopterus ornatus (male, from Brehm).] The vocal organs of various web-footed and wading birds are extraordinarily complex, and differ to a certain extent in the two sexes. In some cases the trachea is convoluted, like a French horn, and is deeply embedded in the sternum. In the wild swan (_Cygnus ferus_) it is more deeply embedded in the adult male than in the female or young male. In the male Merganser the enlarged portion of the trachea is furnished with an additional pair of muscles.[103] But the meaning of these differences between the sexes of many Anatidæ is not at all understood; for the male is not always the more vociferous; thus with the common duck, the male hisses, whilst the female utters a loud quack.[104] In both sexes of one of the cranes (_Grus virgo_) the trachea penetrates the sternum, but presents “certain sexual modifications.” In the male of the black stork there is also a well-marked sexual difference in the length and curvature of the bronchi.[105] So that highly important structures have in these cases been modified according to sex.