their coming
and going, but knowledge on many of the points involved is incomplete.
It is only of recent years that the nest of the Solitary Sandpiper has
been found, and yet this is a very common bird in the eastern United
States in certain seasons. Where is the scientist who can yet tell us
in what country the common Chimney Swift {63} passes the winter, or
over what stretches of sea and land the Arctic Tern passes when
journeying between its summer home in the Arctic seas and its winter
abode in the Antarctic wastes? The main fact, however, that the great
majority of birds of the Northern Hemisphere go south in autumn and
return in spring, is well known.

_Moulting._--By the time the young are able to care for themselves the
plumage of the hard-working parents is worn and frayed and a new suit
of feathers becomes necessary. They do not acquire this all at once.
The feathers drop out gradually from the various feather tracts over
the body, and their places are at once taken by a new growth. While
this is going on the birds are less in evidence than at other times.
They keep out of sight and few song notes are heard. Perhaps there is
some irritation and unpleasantness connected with moulting which causes
a dejection of spirit.

With swimming water birds the wing quills disappear nearly all at once
and the birds are unable {64} for a short time to fly; but being at
home in the water, where they secure their food, they are not left in
the helpless, even desperate, condition in which a land bird would find
itself if unable to fly. In a few cases birds begin to migrate before
this moulting takes place, but with the great majority the moult is
complete before they leave their summer homes.

_Why Birds Migrate._--Why birds migrate we can only conjecture.
Without doubt the growing scarcity of food in autumn is the controlling
factor with many of them; and this would seem to be an excellent reason
for leaving the region of their summer sojourn. Cold weather alone
would not drive all of them southward, else why do many small birds
pass the winter in northern latitudes where severe climatic conditions
prevail? Should we assume the failing food supply to be the sole cause
of migration, we would find ourselves at fault when we came to consider
that birds leave the tropic regions in spring, when food is still
exceedingly abundant, and journey northward thousands of miles to their
former summer haunts.

[Illustration: Y