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.