CHAPTER III

.

A WAVE OF LIFE,

For many years, while living in my own home on the pampas, I kept a journal, in which all my daily observations on the habits of animals and kindred matters were carefully noted. Turning back to 1872-3, I find my jottings for that season contain a history of one of those waves of life--for I can think of no better name for the phenomenon in question--that are of such frequent occurrence in thinly-settled regions, though in countries like England, seen very rarely, and on a very limited scale. An exceptionally bounteous season, the accidental mitigation of a check, or other favourable circumstance, often causes an increase so sudden and inordinate of small prolific species, that when we actually witness it we are no longer surprised at the notion prevalent amongst the common people that mice, frogs, crickets, &c., are occasionally rained down from the clouds.

In the summer of 1872-3 we had plenty of sunshine, with frequent showers; so that the hot months brought no dearth of wild flowers, as in most years. The abundance of flowers resulted in a wonderful increase of humble bees. I have never known them so plentiful before; in and about the plantation adjoining my house I found, during the season, no fewer than seventeen nests.

The season was also favourable for mice; that is, of course, favourable for the time being, unfavourable in the long run, since the short-lived, undue preponderance of a species is invariably followed by a long period of undue depression. These prolific little creatures were soon so abundant that the dogs subsisted almost exclusively on them; the fowls also, from incessantly pursuing and killing them, became quite rapacious in their manner; whilst the sulphur tyrant-birds (Pitangus) and the Guira cuckoos preyed on nothing but mice.

The domestic cats, as they invariably do in such plentiful seasons, absented themselves from the house, assuming all the habits of their wild congeners, and slinking from the sight of man--even of a former fireside companion--with a shy secrecy in their motions, an apparent affectation of fear, almost ludicrous to see. Foxes, weasels, and opossums fared sumptuously. Even for the common armadillo (Dasypus villosus) it was a season of affluence, for this creature is very adroit in capturing mice. This fact might seem surprising to anyone who marks the uncouth figure, toothless gums, and the motions--anything but light and graceful--of the armadillo and perhaps fancying that, to be a dexterous mouser, an animal should bear some resemblance in habits and structure to the felidas. But animals, like men, are compelled to adapt themselves to their surroundings; new habits are acquired, and the exact co-relation between habit and structure is seldom maintained.

I kept an armadillo at this time, and good cheer and the sedentary life he led in captivity made him excessively fat; but the mousing exploits of even this individual were most interesting. Occasionally I took him into the fields to give him a taste of liberty, though at such times I always took the precaution to keep hold of a cord fastened to one of his hind legs; for as often as he came to a kennel of one of his wild fellows, he would attempt to escape into it. He invariably travelled with an ungainly trotting gait, carrying his nose, beagle-like, close to the ground. His sense of smell was exceedingly acute, and when near his prey he became agitated, and quickened his motions, pausing frequently to sniff the earth, till, discovering the exact spot where the mouse lurked, he would stop and creep cautiously to it; then, after slowly raising himself to a sitting posture, spring suddenly forwards, throwing his body like a trap over the mouse, or nest of mice, concealed beneath the grass.

A curious instance of intelligence in a cat was brought to my notice at this time by one of my neighbours, a native. His children had made the discovery that some excitement and fun was to be had by placing a long hollow stalk of the giant thistle with a mouse in it--and every hollow stalk at this time had one for a tenant--before a cat, and then watching her movements. Smelling her prey, she would spring at one end of the stalk--the end towards which the mouse would be moving at the same time, but would catch nothing, for the mouse, instead of running out, would turn back to run to the other end; whereupon the cat, all excitement, would jump there to seize it; and so the contest would continue for a long time, an exhibition of the cleverness and the stupidity of instinct, both of the pursuer and the pursued. There were several cats at the house, and all acted in the same way except one. When a stalk was placed before this cat, instead of becoming excited like the others, it went quickly to one end and smelt' at the opening, then, satisfied that its prey was inside, it deliberately bit a long piece out of the stalk with its teeth, then another strip, and so on progressively, until the entire stick had been opened up to within six or eight inches of the further end, when the mouse came out and was caught. Every stalk placed before this cat was demolished in the same businesslike way; but the other cats, though they were made to look on while the stick was being broken up by their fellow, could never learn the trick.

In the autumn of the year countless numbers of storks (Ciconia maguari) and of short-eared owls (Otus brachyotus) made their appearance. They had also come to assist at the general feast.

Remembering the opinion of Mr. E. Newman, quoted by Darwin, that two-thirds of the humble bees in England are annually destroyed by mice, I determined to continue observing these insects, in order to ascertain whether the same thing occurred on the pampas. I carefully revisited all the nests I had found, and was amazed at the rapid disappearance of all the bees. I was quite convinced that the mice had devoured or driven them out, for the weather was still warm, and flowers and fruit on which humble bees feed were very abundant.

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 was noon-day, had their crops full. There were three mice and two young cavies (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 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.

Whenever we are visited by a dust-storm on the pampas myriads of hooded gulls--Larus macnlipen-nis--appear flying before the dark dust-cloud, even when not a gull has been seen for months. Dust-storms are of rare occurrence, and come only after a long drought, and, the water-courses being all dry, the gulls cannot have been living in the region over which the storm passes. Yet in seasons of drought gulls must be continually passing by at a great height, seeing but not seen, except when driven together and forced towards the earth by the fury of the storm.

By August (1873) the owls had vanished, and they had, indeed, good cause for leaving. The winter had been one of continued drought; the dry grass and herbage of the preceding year had been consumed by the cattle and wild animals, or had turned to dust, and with the disappearance of their food and cover the mice had ceased to be. The famine-stricken cats sneaked back to the house. It was pitiful to see the little burrowing owls; for these birds, not having the powerful wings and prescient instincts of the vagrant Otus brachyotus, are compelled to face the poverty from which the others escape. Just as abundance had before made the domestic cats wild, scarcity now made the burrowing owls tame and fearless of man. They were so reduced as scarcely to be able to fly, and hung about the houses all day long on the look-out for some stray morsel of food. I have frequently seen one alight and advance within two or three yards of the door-step, probably attracted by the smell of roasted meat. The weather continued dry until late in spring, so reducing the sheep and cattle that incredible numbers perished during a month of cold and rainy weather that followed the drought.

How clearly we can see in all this that the tendency to multiply rapidly, so advantageous in normal seasons, becomes almost fatal to a species in seasons of exceptional abundance. Cover and food without limit enabled the mice to increase at such an amazing rate that the lesser checks interposed by predatory species were for a while inappreciable. But as the mice increased, so did their enemies. Insectivorous and other species acquired the habits of owls and weasels, preying exclusively on them; while to this innumerable army of residents was shortly added multitudes of wandering birds coming from distant regions. No sooner had the herbage perished, depriving the little victims of cover and food, than the effects of the war became apparent. In autumn the earth so teemed with them that one could scarcely walk anywhere without treading on mice; while out of every hollow weed-stalk lying on the ground dozens could be shaken; but so rapidly had they devoured, by the trained army of persecutors, that in spring it was hard to find a survivor, even in the barns and houses. The fact that species tend to increase in a geometrical ratio makes these great and sudden changes frequent in many regions of the earth; but it is not often they present themselves so vividly as in the foregoing instance, for here, scene after scene in one of Nature's silent passionless tragedies opens before us, countless myriads of highly organized beings rising into existence only to perish almost immediately, scarcely a hard-pressed remnant remaining after the great reaction to continue the species.

##