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.
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