behaviour of the migrant, that lacks the "will to fight" when on migration but is pugnacious when it has secured a territory, shows it; the behaviour of the resident, which temporarily joins the flock and is there sociable, shows it; and it is shown also by the determination with which both opponents fight when the question of ownership of a station is in dispute. And of all the facts we have reviewed, this is perhaps the most important in relation to our present subject, for it demonstrates that the change from sociability to hostility is not merely an incident of the sexual season, not merely an indirect result of the functioning of the general disposition which leads to the establishment of a territory, but that it is intimately associated with the whole process, and that the particular part of the bird's nature which is concerned is so nicely balanced that it will respond under one condition and one only.
Thus we are led to the only conclusion which seems consistent with the facts, namely that there is a relationship between the "territory" and the hostility.
If we are satisfied that all this warfare is not merely an expression of an instinct which is serviceable in another direction, what part does it play in the whole scheme of reproduction? The young of many birds are delicate at birth and unable to withstand exposure to cold, and in the previous chapter we came to the conclusion that the territory was serviceable in that
