disappeared, leaving the Green
Woodpeckers in possession.

In this varied field of hostile behaviour which we have explored, one
feature stands out prominently, namely, that the interest which a bird
displays in other species varies not only at different seasons but even
from hour to hour. I have used the word "nature" as equivalent to
"interest," and I have spoken of the bird's nature changing or altering
according to the circumstances in which it was placed. But its nature is
its inborn constitution, and its constitution cannot change from day to
day, still less from hour to hour. So that, in a sense, and having
regard to strict scientific accuracy, it is misleading in this
particular connotation to say that the bird's nature changes.

[Illustration: H. Gronvold dcl. Emery Walker ph.sc.

A battle between a pair of Green Woodpeckers and a pair of Great
Spotted Woodpeckers for the possession of a hole in an oak tree.]

What then does happen? The instinct of pugnacity must form just as much
a part of the hereditary make-up of the migrant, when on migration, as
when finally it reaches its destination; still more must it form part of
the constitution of the Bunting when it leaves its headquarters
temporarily and joins the flock. And, if it is there, the question
arises as to why it does not respond. Now every instinct requires for
its response a stimulus of an appropriate kind, and, therefore, a
reasonable view to take would be that the necessary stimulus was
lacking. But this is a view which we cannot uphold, because on all these
occasions an opposing male was present--and, so far as it is possible to
judge by observation, that is the stimulus which in the main evokes a
hostile response. We must therefore look elsewhere than in its
direction for a reason which will adequately explain the behaviour.

Though it be true that every instinct requires for its functioning a
stimulus of an appropriate kind, yet it is also true that the condition
which will render it responsive must be present. What we have then to
consider is whether the phenomena which we have explored give us any
clue as to the particular nature of that condition. In the first place,
we have the general fact that the hostility is not confined to a few
species belonging to a few families, but that it is of wide
application--birds of prey, Warblers, Woodpeckers, all supply us with
evidence which serves to show, in greater or less degree, its nature and
exten