My experience is that mammals and birds, with few exceptions--probably there are really no exceptions--possess the habit of indulging frequently in more or less regular or set performances, with or without sound, or composed of sound exclusively; and that these performances, which in many animals are only discordant cries and choruses, and uncouth, irregular motions, in the more aerial, graceful, and melodious kinds take immeasurably higher, more complex, and more beautiful forms. Among the mammalians the instinct appears almost universal; but their displays are, as a rule, less admirable than those seen in birds. There are some kinds, it is true, like the squirrels and monkeys, of arboreal habits, almost birdlike in their restless energy, and in the swiftness and certitude of their motions, in which the slightest impulse can be instantly expressed in graceful or fantastic action; others, like the Chinchillidae family, have greatly developed vocal organs, and resemble birds in loquacity; but mammals generally, compared with birds, are slow and heavy, and not so readily moved to exhibitions of the kind I am discussing. The terrestrial dances, often very elaborate, of heavy birds, like those of the gallinaceous kind, are represented in the more volatile species by performances in the air, and these are very much more beautiful; while a very large number of birds--hawks, vultures, swifts, swallows, nightjars, storks, ibises, spoonbills, and gulls--circle about in the air, singly or in flocks. Sometimes, in serene weather, they rise to a vast altitude, and float about in one spot for an hour or longer at a stretch, showing a faint bird-cloud in the blue, that does not change its form, nor grow lighter and denser like a flock of starlings; but in the seeming confusion there is perfect order, and amidst many hundreds each swift- or slow-gliding figure keeps its proper distance with such exactitude that no two ever touch, even with the extremity of the long-wings, flapping or motionless:--such a multitude, and such miraculous precision in the endless curving motions of all the members of it, that the spectator can lie for an hour on his back without weariness watching this mystic cloud-dance in the empyrean. The black-faced ibis of Patagonia, a bird nearly as large as a turkey, indulges in a curious mad performance, usually in the evening when feeding-time is over. The birds of a flock, while winging their way to the roosting-place, all at once seem possessed with frenzy, simultaneously dashing downwards with amazing violence, doubling about in the most eccentric manner; and when close to the surface rising again to repeat the action, all the while making the air palpitate for miles around with their hard, metallic cries. Other ibises, also birds of other genera, have similar aerial performances.