The avi-fauna is relatively much richer than the mammalia, owing to the large number of aquatic species, most of which are migratory with their "breeding" or "subsistence-areas" on the pampas. In more senses than one they constitute a "floating population," and their habits have in no way been modified by the conditions of the country. The order, including storks, ibises, herons, spoonbills, and flamingoes, counts about eighteen species; and the most noteworthy birds in it are two great ibises nearly as large as turkeys, with mighty resonant voices. The duck order is very rich, numbering at least twenty species, including two beautiful upland geese, winter visitors from Magellanic lands, and two swans, the lovely black-necked, and the pure white with rosy bill. Of rails, or ralline birds, there are ten or twelve, ranging from a small spotted creature no bigger than a thrush to some large majestic birds. One is the courlan, called "crazy widow" from its mourning plumage and long melancholy screams, which on still evenings may be heard a league away. Another is the graceful variegated _ypicaha,_ fond of social gatherings, where the birds perform a dance and make the desolate marshes resound with their insane humanlike voices. A smaller kind, Porphyriops melanops, has a night-cry like a burst of shrill hysterical laughter, which has won for it the name of "witch;" while another, Rallus rythyrhynchus, is called "little donkey" from its braying cries. Strange eerie voices have all these birds. Of the remaining aquatic species, the most important is the spur-winged crested screamer; a noble bird as large as a swan, yet its favourite pastime is to soar upwards until it loses itself to sight in the blue ether, whenca it pours forth its resounding choral notes, which reach the distant earth clarified, and with a rhythmic swell and fall as of chiming bells. It also sings by night, "counting the hours," the gauchos say, and where they have congregated together in tens of thousands the mighty roar of their combined voices produces an astonishingly grand effect.