of the excitement. And if we are
satisfied that the fighting in the one case is purposive, so, too, must
we regard it as having some biological purpose to serve in the other.
But the Garden-Warbler is not the only bird that acts as a stimulus to
the instinct of the Blackcap; Whitethroats are often attacked, and the
Chiffchaff is a source of irritation. Even when a male Blackcap is
engaged in incubation, it will leave its nest on the approach of a
Chiffchaff, and, having driven away the intruder, proceed to sing
excitedly. At other times both male and female will combine to attack
this small intruder.

But this does not mean that the Chiffchaff suffers persecution; it is
itself most aggressive, as is shown by the fact that it will join in the
Blackcap quarrels and attack the combatants indiscriminately. Its
behaviour, however, requires further consideration, especially as
regards its relations with its nearest of kin--the Willow-Warbler; for
here we have a mutual intolerance which is somewhat remarkable, and
evidence of it can be found wherever the birds occupy the same ground.
Now it can be observed that the hostility is not limited merely to
occasional acts of intolerance, but that there is organised warfare
lasting, it may be, for many days in succession, and that the actions of
the birds bear the stamp of a persistent striving towards some end. On
one occasion the Willow-Warbler may be the aggressor, on another the
Chiffchaff, and at times it is difficult to say which of the two is
responsible for the quarrel. In size and in strength they are equal, and
the "will to fight" is as strong in the one as in the other, so that it
is seldom, if ever, possible to point to this one as the victor and that
one as the vanquished. Success or failure probably depends more upon the
cumulative effect of many combats entailing physical exhaustion, than
upon the issue of any one particular battle; and whilst observation
might quite well fail to distinguish any resultant change in the
relative positions of the birds, or any harmful effect upon their
constitutions, yet the area occupied by this one might be sufficiently
curtailed to prejudice the welfare of the young, or the vitality of that
one might be seriously impaired--and we should be none the wiser.

Neither the Marsh-Warbler nor the Reed-Warbler will tolerate strangers
within the small space of ground over which they exercise dominion. Of
the two, the Marsh-Warbler is perhaps