sings. Again, nearly every male at one time or another
in the course of the season is aroused to action by the intrusion of a
rival. The emotional tone of the owner of the territory is then raised,
and the intruder is pursued and attacked; but this alone is not
sufficient, it seems as if the chain of instinctive activities, when
once aroused by appropriate stimulation, must pursue its course to the
end--and the end in such a case is only reached and complete
satisfaction only gained when the bird has not merely returned to his
"headquarters" but has given vocal expression to his emotion. Finally,
we must bear in mind these two facts, that the "headquarters" is
occupied solely by the male--it forms no part of the life of the
female--and that it is the male only that sings.

Many such subtle incidents of behaviour as the foregoing can be
perceived but not readily described, and trifling though they may seem
to be in themselves, yet in the aggregate they yield full assurance of a
close relationship.

The distant song of a male, or the presence of an intruding male, have
also stimulating effects, though in somewhat different ways. The former
evokes the normal reply, that is to say the bird, if silent, is liable
to utter a corresponding reply; the latter arouses hostility into which
is infused much feeling tone, the bird sings hurriedly while in pursuit
of its rival, and, which is more remarkable still, even in the midst of
an encounter. Both the normal reply and the emotional song must be
similar in origin--different aspects of the same situation--and both are
clearly related to the other male.

The arrival of a female may also be followed by an emotional outburst
which can be heard at intervals for some days; on the other hand, the
song may continue as before or, for a time, entirely cease.

To take the emotional outburst first. This would appear to be
susceptible of explanation on the hypothesis that the voice contributes
to a more effective pairing situation; an hypothesis which admittedly,
at first sight, gains some support from the fact that a second or a
third male is frequently present. But, in truth, the presence of a
second male makes the situation, so far as the relationship between the
song and the female is concerned, all the more perplexing; for, as we
have already seen, the instinct of pugnacity, when aroused by the
appearance of an intruder, is also liable to be accompanied by a
similarly extravagant song