l exhaustion as might affect his power
of reproduction, its direct effect upon the combatants cannot be
otherwise than harmful--in fact it is a necessary evil which for the
good of the species must be kept strictly within bounds. Bearing in
mind, then, these two facts, namely that the distribution of the males
is never stable and that overmuch fighting may defeat the end in view,
we can appreciate the importance of any factor which will lead to a more
uniform distribution and at the same time insure security by peaceable
means.

The proximate end of the male's behaviour is isolation--how is it to be
obtained? If, after having occupied a territory, the bird were to remain
silent, it would run the risk of being approached by rivals; if, on the
other hand, it were merely to utter the recognition call of the species,
it would but attract them. In neither case would the end in view be
furthered, and isolation would solely depend upon alertness and the
capacity to eject intruders. Supposing, however, that the song, just as
it serves to attract the females, serves to repel other males, a new
element is introduced deserving of recognition; for those males that had
established themselves would not only be spared the necessity of many a
conflict, but they would be spared also the necessity of constant
watchfulness, and so, being free to pursue their normal routine--to seek
food, to rest, and, if migrants, to recover from the fatigue of the
journey, they would be better fitted to withstand the strain of
reproduction; and those that were still seeking isolation in an
appropriate environment, instead of settling first here and then there
only to find themselves forestalled, would avoid and pass by positions
that were occupied, establishing themselves without loss of time in
those that were vacant. Without the aid of something beyond mere
physical encounter to regulate dispersal, it is difficult to imagine how
in the short time at disposal anything approaching uniformity of
distribution could be obtained. Hence, both in the direction of limiting
combat, of insuring accommodation for the maximum number of pairs in the
minimum area, and of conserving energy, the song, by conveying a
warning, plays an important part in the whole scheme.

And if this be so, if the song repels instead of attracting, it follows
that the more distinct the sounds, the less likelihood will there be of
confusion; for supposing that different species were to d