Now from a tree near by comes a low penetrating moan or muffled boom. The bird in front of me at once changes his whole demeanor. He continues his walking but it assumes more of a mincing character, uttering all the while several notes, like low but shrill squeaks or gurgles, mingled with snorts and snores, all rather subdued. These seem rather hit or miss, there being no regular sequence or similarity of the utterances. Several times these sounds are interrupted by the bird stopping, appearing to pick up something, and then to dash its head violently against its back, producing a low champing sound which seems to excite the females, who otherwise are wholly indifferent. Try as I may I can make nothing of this action, and later it is an indiscreet, impatient movement of mine at such a juncture that ultimately frightens the birds and ends my observations. I was delighted therefore when observing the Curassow in the north to see the bird repeatedly pick up pebbles or a feather or twig and champ them in its bill just as the wild bird did. The clicking sound resulted only when a hard object was picked up, but the dull thuds were made by the skull of the bird striking violently against its dorsal vertebræ, the object it had picked up being held meanwhile in its bill. (Fig. 137.) The wild Curassow soon drops whatever it has picked up and claps its wings together seven or eight times over its back, making a loud slapping sound. It then turns its back on its rival in the tree, plucks nervously at the wings, right and left, for a full minute or longer, and then reaches convulsively forward several times, with its head and neck, the bill being wide open, gulping in a great quantity of air. Its abdominal air-sacs swell, its wings are lowered and rounded out until the bird appears half as large again as usual. Thus it stands, half squatting with lowered head and tail, and within a period of five to ten seconds utters usually four notes of the deepest and most penetrating character. Now that I am within a few yards, they sound no louder than when several hundred yards away. The exertion put forth shows this vocal effort to be a strenuous one, and at the second performance the tones are rather low and confused. But the normal utterance, this climax of the whole challenge, is as follows: [Music] This may be imitated by anyone with a deep bass voice by humming the syllables _Um, um, um-um-um_, to the notes as I have written them. During this period the actor, as observed in the captive specimen, seems almost in a trance, standing with half closed eyes, oblivious even of a hand resting on the feathers of his back, and the recovery is slow, the bird seeming dazed for a short time.