oung robins quarreling at their bath. Photographed in
the yard of Mrs. Granville Pike, North Yakima, Washington]

{65}

There is a theory held by many naturalists that the migrating instinct
dates back to the glacial period. According to this theory North
America was inhabited originally by non-migrating birds. Then the
great Arctic ice-cap began to move southward and the birds were forced
to flee before it or starve. Now and then during the subsequent period
the ice receded and the birds returned, only to be driven again before
the next onrush of the Ice King. Thus during these centuries of
alternate advance and retreat of the continental glacier, the birds
acquired a habit, which later became an instinct, of retreating
southward upon the approach of cold weather and coming back again when
the ice and snow showed indications of passing away.

_The Gathering Flocks._--To the bird student there is keen delight in
watching for the first spring arrivals and noting their departure with
the dying year. It is usually in August that we first observe an
unwonted restlessness on the part of our birds which tells us that they
have begun to hear the call of the {66} South. The Blackbirds assemble
in flocks and drift aimlessly about the fields. Every evening for
weeks they will collect a chattering multitude in the trees of some
lawn, or in those skirting a village street, and there at times cause
great annoyance to their human neighbours.

Across the Hudson River from New York, in the Hackensack marshes,
behind the Palisades, clouds of Swallows collect in the late summer
evenings, and for many days one may see them from the car windows as
they glide through the upper air or swarm to roost among the rushes.
These Swallows and the Blackbirds are getting together before starting
on their fall migration.

In Greensboro, North Carolina, there is a small grove of trees
clustered about the courthouse which is a very busy place during the
nights of summer. Here, before the first of July, Purple Martins begin
to collect of an evening. In companies of hundreds and thousands, they
whirl about over the tops of the houses, alight in the trees, and then
almost {67} immediately dash upward and away again. Not till dark do
they finally settle to roost. Until late at night a great chorus of
voices may be heard among the branches. The multitude increases daily
for six or eight weeks, additions, in the form of new family groups,
c