单选题　　By the time we had completed our preparations, the sky was beginning to grow rapidly lighter, although the sun was not yet above the horizon. We waited unpatiently, hardly daring to move or to whisper to one another. Suddenly, we heard the quick beating of wings and one of the birds we were looking for flew up from the valley beyond. He flied straight to the tree and settled on a thin branch, stripped bare of leaves, which grew diagonally upwards from the main trunk. Immediately he began to clean himself, combing with his beak the long soft feathers which grew from beneath his wings and extended past his tail in a glorious red cloud. Charles''s camera, on a trial run, sounded unexcepted loud, but the bird took no notice and continued with his cleaning until at last he drew himself up and shook himself. Then, head held high, he called a single, loud, sharp note which resounded over the valley. He seemed in no hurry to begin his dance, as he continued to call for a quarter of an hour. Two other birds flew from the valley and settled in other parts of the tree. They were hens, uninteresting brown creatures, which, attracted by his call, had come to watch his dance. He paid no attention to them, and continued his unmusical cries, occasionally cleaning some more feathers. The hens kept silent, moving from branch to branch. Once one of them drew too near his dancing ground, but he beat his wings and she drew back. 　　His calls began to increase frequently, and I carefully fixed the recording machine so that they produced the highest reading on the scale. With electrifying suddenness, the bird lowered his head and throwing his fine tail feather over his back, he ran down his branch, a shaking flame of color, making his loud love cries. Up and down the branch he danced in a state of wild excitement. After half a minute he seemed to get out of breath, for his cries stopped and he danced silently.