The argument, then, is briefly this. In the spring, a marked change takes place in the character of the males of very many species; instead of being gregarious they either avoid one another and become hostile, or, if their conditions of existence require that they shall still live together, they become irritable and pugnacious. This change is made known to us by the battles of varying degrees of severity which are such a feature of bird life in the spring; and since a female can commonly be observed to accompany the combatants, the possession of a mate appears at first sight to be the proximate end for which the males are contending. But when the circumstances which lead up to the quarrels are investigated closely, the problem becomes more difficult; for it is not merely a question of males fighting in the presence of a female, as is generally supposed to be the case, but on the contrary there is a complexity of strife which is bewildering--males attack females or _vice versa_; female fights with female; or a pair combine to drive away another pair, or even a solitary individual no matter of which sex. This complexity of strife makes against the view that the possession of a mate is the reason of the fighting. But an even stronger objection is to be found in the fact that males are hostile when no female is present--and hence we must seek elsewhere for the true explanation. Now if the behaviour of a male be closely observed, it will be found that its pugnacious instinct gains or loses susceptibility according to the position which it happens to occupy--when its ground is trespassed upon, the impulse to fight is strong; but when it crosses the boundary it seems to lose all interest in the intruder. Moreover, in some species, the male rejoins the flock at intervals during the early part of the season and for a time leads a double existence, passing backwards and forwards between its territory and the neutral ground. Its behaviour under these circumstances affords some valuable evidence, for the bird displays little if any hostility when accompanying the flock, yet when it returns to the ground over which it exercises dominion, no male can approach without being attacked. The conclusion, therefore, seems to be inevitable, namely that the actual occupation of a territory is the condition under which the pugnacious nature of the male is rendered susceptible to appropriate stimulation. CHAPTER IV THE RELATION OF SONG TO THE TERRITORY If we listen to the voices of the Waders as, in search of food, they follow the slowly ebbing tide, we shall notice that each species has a number of different cries, some of which are uttered frequently and others only occasionally. Not only so, but if we study the circumstances under which they are uttered, we shall in time learn to associate certain specific notes with certain definite situations.