Towards the close of February, dependent on the moon, a marked climatic change takes place. A period of sudden heat usually sets in--a sequence of warm sunny days, breathless, and at noontide almost suffocating. But each afternoon with flowing tide there arises from the sea a S. W. breeze, gentle at first and uncertain but gaining strength with the rising flood. Already, shortly before this change, the duck-tribes had partially relaxed their full mid-winter activities--owing to abundant spring growths of food-plants, had become more sedentary; if not sluggish, at least reluctant to move. After the brief morning-flight not a wing stirred. But now, scan the mirror-like surface of some great _lucio_ and you will recognise a new movement distinct and dissimilar from regular hibernal habit. There float within sight (and the same is happening at a score of places beyond sight) not only the usual loose flotillas, but three, four, or five concrete assemblages of densely massed fowl whose appearance the slightest scrutiny will differentiate from the others. These are not sitting quiescent. The binoculars disclose a scene of perpetual motion, well-nigh of riot--one might be regarding a feathered faction-fight. Hundreds of units fight, splash, and chase, or throw up water with beating wings till surf and spray half conceals the seething crowd. That flicker of pinions and flying foam are, moreover, accompanied by a chorus of myriad notes--a babel of twirling sound blended in rising and falling cadences, comparable only to the distant roar of some mighty city. A more singular spectacle we have not encountered. Inquiry from one's companion elicits the reply that these assemblages are _hechando corros para irse_ (literally, "forming choruses preparatory to departure")--an expression which conveyed no more significance to us than it can to the reader.[64] We decided to return at daybreak to see this thing through, and after watching the phenomenon a score of times can now explain it. During the morning hours there are established focal points whereat assemble those units already affected by the emigrant furor. These (at first, perhaps, but a score or two) rapidly increase in numbers till each focus becomes the nucleus of a corro. The seasonal infection spreads, and as its influence impregnates the surrounding masses, these, singly or in scores or hundreds as the passion seizes them, hasten to join one or other of the mobilising army-corps. Within an hour or two the insignificant original nucleus has developed into a vast host all in a ferment of agitation, and being constantly reinforced by buzzing swarms of recruits from without.