The gradual increase or decrease continually going on in many species about us is little remarked; but the sudden infrequent appearance in vast numbers of large and comparatively rare species is regarded by most people as a very wonderful phenomenon, not easily explained. On the pampas, whenever grasshoppers, mice, frogs or crickets become excessively abundant we confidently look for the appearance of multitudes of the birds that prey on them. However obvious may be the cause of the first phenomenon--the sudden inordinate increase during a favourable year of a species always prolific--the attendant one always creates astonishment: For how, it is asked, do these largo birds, seldom seen at other times, receive information in the distant regions they inhabit of an abundance of food in any particular locality? Years have perhaps passed during which, scarcely an individual of these kinds has been seen: all at once armies of the majestic white storks are seen conspicuously marching about the plain in all directions; while the night air resounds with the solemn hootings of innumerable owls. It is plain that these birds have been drawn from over an immense area to one spot; and the question is how have they been drawn? Many large birds possessing great powers of flight are, when not occupied with the business of propagation, incessantly wandering from place to place in search of food. They are not, as a rule, regular migrants, for their wanderings begin and end irrespective of seasons, and where they find abundance they remain the whole year. They fly at a very great height, and traverse immense distances. When the favourite food of any one of these species is plentiful in any particular region all the individuals that discover it remain, and attract to them all of their kind passing overhead. This happens on the pampas with the stork, the short-eared owl, the hooded gull and the dominican or black-backed gull--the leading species among the feathered nomads: a few first appear like harbingers; these are presently joined by new comers in considerable numbers, and before long they are in myriads. Inconceivable numbers of birds are, doubtless, in these regions, continually passing over us unseen. It was once a subject of very great wonder to me that flocks of black-necked swans should almost always appear flying by immediately after a shower of rain, even when none had been visible for a long time before, and when they must have come from a very great distance. When the reason at length occurred to me, I felt very much disgusted with myself for being puzzled over so very simple a matter. After rain a flying swan may be visible to the eye at a vastly greater distance than during fair weather; the sun shining on its intense white plumage against the dark background of a rain-cloud making it exceedingly conspicuous. The fact that swans are almost always seen after rain shows only that they are almost always passing.