There are human dances, in which only one person performs at a time, the rest of the company looking on; and some birds, in widely separated genera, have dances of this kind. A striking example is the Rupicola, or cock of-the-rock, of tropical South America. A mossy level spot of earth surrounded by bushes is selected for a dancing-place, and kept well cleared of sticks and stones; round this area the birds assemble, when a cock-bird, with vivid orange-scarlet crest and plumage, steps into it, and, with spreading wings and tail,
262 The Naturalist in La Plata.
begins a series of movements as if dancing a minuet; finally, carried away with excitement, he leaps and gyrates in the most astonishing manner, until, becoming exhausted, he retires, and another bird takes his place.
In other species all the birds in a company unite in the set performances, and seem to obey an impulse which affects them simultaneously and in the same degree; but sometimes one bird prompts the others and takes a principal part. One of the most curious instances I have come across in reading is contained in Mr. Bigg-Wither's Pioneering in South Brazil. He relates that one morning in the dense forest his attention was roused by the unwonted sound of a bird singing--songsters being rare in that district. His men, immediately they caught the sound, invited him to follow them, hinting that he would probably witness a very curious sight. Cautiously making their way through the dense undergrowth, they finally came in sight of a small stony spot of ground, at the end of a tiny glade; and on this spot, some on the stone and some on the shrubs, were assembled a number of little birds, about the size of tom-tits, with lovely blue plumage and red top-knots. One was perched quite still on a twig, singing merrily, while the others were keeping time with wings and feet in a kind of dance, and all twittering an accompaniment. He watched them for some time, and was satisfied that they were having a ball and concert, and thoroughly enjoying themselves; they then became alarmed, and the performance abruptly terminated, the birds all going off in different directions. The natives told
Music and Dancing in Nature. 26^
him that these little creatures were known as the "dancing birds."
This species was probably solitary, except when assembling for the purpose of display; but in a majority of cases, especially in the Passerine order, the solitary species performs its antics alone, or with no witness but its mate. Azara, describing a small finch, which he aptly named Oscilador, says that early and late in the day it mounts up vertically to a moderate height; then, flies off to a distance of twenty yards, describing a perfect curve in its passage; turning, it flies back over the imaginary line it has traced, and so on repeatedly, appearing like a pendulum swung in space by an invisible thread.
Those who seek to know the cause and origin of this kind of display and of song in animals are referred to Darwin's Descent of Man for an explanation. The greater part of that work is occupied with a laborious argument intended to prove that the love-feeling inspires the animals engaged in these exhibitions, and that sexual selection, or the voluntary selection of mates by the females, is the final cause of all set musical and dancing performances, as well as of bright and harmonious colouring, and of ornaments.