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