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