In the foregoing cases sounds are made by the aid of structures already present and otherwise necessary; but in the following cases certain feathers have been specially modified for the express purpose of producing the sounds. The drumming, or bleating, or neighing, or thundering noise, as expressed by different observers, which is made by the common snipe (_Scolopax gallinago_) must have surprised every one who has ever heard it. This bird, during the pairing-season, flies to “perhaps a thousand feet in height,” and after zig-zagging about for a time descends in a curved line, with outspread tail and quivering pinions, with surprising velocity to the earth. The sound is emitted only during this rapid descent. No one was able to explain the cause, until M. Meves observed that on each side of the tail the outer feathers are peculiarly formed (fig. 41), having a stiff sabre-shaped shaft, with the oblique barbs of unusual length, the outer webs being strongly bound together. He found that by blowing on these feathers, or by fastening them to a long thin stick and waving them rapidly through the air, he could exactly reproduce the drumming noise made by the living bird. Both sexes are furnished with these feathers, but they are generally larger in the male than in the female, and emit a deeper note. In some species, as in _S. frenata_ (fig. 42), four feathers, and in _S. javensis_ (fig. 43), no less than eight on each side of the tail are greatly modified. Different tones are emitted by the feathers of the different species when waved through the air; and the _Scolopax Wilsonii_ of the United States makes a switching noise whilst descending rapidly to the earth.[110] [Illustration: Fig. 42. Outer tail-feather of Scolopax frenata.] [Illustration: Fig. 43. Outer tail-feather of Scolopax javensis.] In the male of the _Chamæpetes unicolor_ (a large gallinaceous bird of America) the first primary wing-feather is arched towards the tip and is much more attenuated than in the female. In an allied bird, the _Penelope nigra_, Mr. Salvin observed a male, which, whilst it flew downwards “with outstretched wings, gave forth a kind of crashing, rushing noise,” like the falling of a tree.[111] The male alone of one of the Indian bustards _(Sypheotides auritus_) has its primary wing-feathers greatly acuminated; and the male of an allied species is known to make a humming noise whilst courting the female.[112] In a widely different group of birds, namely the Humming-birds, the males alone of certain kinds have either the shafts of their primary wing-feathers broadly dilated, or the webs abruptly excised towards the extremity. The male, for instance, of _Selasphorus platycercus_, when adult, has the first primary wing-feather (fig. 44), excised in this manner. Whilst flying from flower to flower he makes “a shrill, almost whistling, noise;”[113] but it did not appear to Mr. Salvin that the noise was intentionally made.