After cold weather set in the storks went away, probably on account of the scarcity of water, for the owls remained. So numerous were they during the winter, that any evening after sunset I could count forty or fifty individuals hovering over the trees about my house. Unfortunately they did not confine their attentions to the mice, but became destructive to the birds as well. I frequently watched them at dusk, beating about the trees and bushes in a systematic manner, often a dozen or more of them wheeling together about one tree, like so many moths about a candle, and one occasionally dashing through the branches until a pigeon--usually the Zenaida maculata--or other bird was scared from its perch. The instant the bird left the tree they would all give chase, disappearing in the darkness. I could not endure to see the havoc they were making amongst the ovenbirds (Furnarius rufus--a species for which I have a regard and affection almost superstitious), so I began to shoot the marauders. Very soon, however, I found it was impossible to protect my little favourites. Night after night the owls mustered in their usual numbers, so rapidly were the gaps I made in their ranks refilled. I grew sick of the cruel war in which I had so hopelessly joined, and resolved, not without pain, to let things take their course. A singular circumstance was that the owls began to breed in the middle of winter. The field-labourers and boys found many nests with eggs and young birds in the neighbourhood. I saw one nest in July, our coldest month, with three half-grown young birds in it. They were excessively fat, and, though it
64 The Naturalist in La Plata*
was noon-day, had their crops full. There were three mice and two young cavíes (Cavia australis) lying untouched in the nest.
The short-eared owl is of a wandering disposition, ard performs long journeys at all seasons of the year in search of districts where food is abundant; and perhaps these winter-breeders came from a region where scarcity of prey, or some such cause, had prevented them from nesting at their usual time in summer.
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
A Wave of Life. 65
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