Why birds and beasts flock, no doubt, is for mutual protection from
natural foes. One has heard of swallows nesting on a cliff beneath an
eagle's eyrie, yet having nothing to fear from the eagle's attack
because of their combination; and every one knows how a party of small
birds will defy a hawk, or will mob and rout a cuckoo or a day-flying
owl. Possibly the reason for the great congregation of sparrows in
one chosen tree in a London square is mutual protection from cats.
Food is a most important factor in flocking; the keeper knows that the
scarcer the food of partridges the greater is their tendency to pack.
Birds may pack at night for mutual warmth--as when titmice snuggle on
branches, and wrens, to the number of ten or twenty, crowd a hole in
the thatch. Partridges gain something in warmth in snowy weather by
their habit of jugging at night--a good covey on a yard of ground.
But examination of the spot where they have passed the night shows
that the main pack has been divided into comparatively small parties,
in the same way as there were small parties among the great herds of
buffalo that travelled as one column across the plains of America.
Sheltered hollows are naturally chosen for jugging, where the keen edge
of the wind passes over the birds' heads. There is not always safety
or benefit in numbers; a flock may attract foes where individuals
would pass unnoticed, or may make short work of food which would keep
an individual for many days. With insects, great congregations may be
harmful, if an advantage to their bird enemies. Presumably, flocking is
a matter of general convenience.
During the first fortnight of October little parties of fieldfares
from Scandinavia drift over the fields, chuckling in their throaty
way, redwings are seen, our wood-pigeons are reinforced by countless
thousands from overseas, snipe come in, woodcock will soon be
here, parties of goldcrests, newly arrived, cry their sharp notes
among the larches, and the winter flocks of tits, with goldcrests,
tree-creepers, and nuthatches busily move in the woods. Everywhere
birds are in flocks. Chaffinches, greenfinches, and sparrows move in
vast congregations, plovers circle in clouds above the fallows, flocks
of rooks unite in the evening and thousands upon thousands of starlings
rise, fall, and circle in perfect unison, filling the air with the
rushing noise of wings.
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