trill with unwearying zeal, yet no sooner does a mate appear
than its emotion is manifested in other directions. The Reed-Bunting is
vociferous during February and March; but when a female arrives, periods
of silence are frequent and the instinct of the bird becomes
progressively less susceptible to stimulation. After the manner of the
race the male makes temporary excursions from its territory accompanied
by his mate, and it is noteworthy that when he returns and she is absent
he sings, but that the moment she joins him, or even comes into sight,
he is silent. In fact, in greater or less degree, a change is noticeable
in the song of many resident and migratory species under similar
circumstances, a deterioration so marked that we learn by experience to
regard it as a certain indication of the arrival of a mate.

Thus it becomes clear that there are certain specific factors in the
external environment with which the instinct can be definitely related,
and in the order of their importance they are (1) the territory as a
whole; (2) the headquarters; (3) an intruding male; (4) the female.

To what extent are these relationships interrelated? Are they all
mutually dependent upon one another, or is there one which conditions
the remainder?

In the first place it is evident that if a male were not to establish a
territory, no opportunity would be afforded for making use of any
special post or for acquiring a habit in relation to it, and so without
further consideration we may say that the connection between the song
and the headquarters, whatever it may be, is primarily dependent upon
the establishment of a territory.

Next, we have the fact that the distant voice, or still more so the
presence, of another male has an exciting influence and evokes a
corresponding reply. Here we have a direct relationship, and one which
at first sight appears to be exclusive of cross-correlation. But is it
really so; does no circumstance arise under which even the proximity of
a rival fails to evoke response? The reply is not doubtful. Such a
circumstance _does_ arise--when a male for one reason or another passes
outside the limits of its accustomed area. This aspect of behaviour has
already been fully discussed in connection with the question of
hostility, and everyone, I imagine, must by now be pretty well familiar
with the facts. However, it does not often happen that we are given
such an aid to interpretation as is vouchsafed to us in