e rise of the organic condition which leads to the establishment of
territories; and the hostility continues, though in diminishing degree,
throughout the breeding season, and dies away the following autumn.

For example, different Warblers resort to the elders (_Sambucus nigra_)
in September, and there pass much time feeding on the fruit which is
then ripe and often abundant. In the same bush there may be Blackcaps,
Garden-Warblers, Whitethroats, and Lesser Whitethroats, some preening
their feathers, others searching for the berries, others again, with
feathers relaxed, making feeble attempts to sing. Occasionally there may
be a scuffle, perhaps between a Blackcap and a Lesser Whitethroat, or
between a Garden-Warbler and a Blackcap, but it is of short duration and
lacks vigour. Apart, however, from such temporary disturbances, there is
no real rupture in their relations, and certainly nothing to lead one to
suppose that the bickerings are determined by the functioning of any
specific instinct. Yet only a few months previously some of them were
constantly at war, and their quarrels betrayed symptoms of great
persistence; and if we remember how the observed behaviour of the birds
suggests the fact that they were striving to attain something definite,
we shall understand the nature and extent of the change, and shall, I
fancy, be in a better position to estimate its biological worth at its
true value.

We can find many similar examples--flocks are to be found on arable
ground, on the water meadows, and on the mud-flats; here different kinds
of Thrushes feed on the berries of the yew, there different kinds of
Tits travel together in parties; hosts of Finches collect in the hollies
to pass the night and Buntings roost together in the gorse; and, in
fact, in whatever direction we choose to look in the autumn and winter,
we find various birds assembled together and living on amicable terms.
All of this changes in the spring, and the relationship undergoes a
gradual but noticeable alteration; so much so that whereas the
outstanding feature of bird life in the winter is sociability, that of
the spring is hostility.

So much, then, for the seasonal change of relationship; let us now turn
to particular cases and attempt to trace the condition which accompanies
such change.

Many migrants in the spring seem to follow the course of the Severn
during their journey northwards through Worcestershire; and where the
river bends to t