It may be well to illustrate the foregoing remarks. Suppose that there is a small piece of woodland barely sufficient to hold three pairs of Willow-Warblers, and suppose that the male and female in the middle territory did not respond to one another's influence quite as readily as the adjoining males and females, what would be the result? The emotional tone of the central pair would stand at a lower level of intensity; and, since their congenital dispositions would lack the necessary reinforcement, the birds would tend to become less and less punctilious in keeping their boundaries intact, whereas the adjoining pairs, always on the alert and meeting with little opposition, would encroach more and more and gradually extend their dominion. And so, by the time the young were hatched, the parents would be in occupation of an area too limited in extent to insure the necessarily rapid supply of food, and would be compelled to intrude upon the adjoining ground. But knowing how routine becomes ingrained in the life of the individual, knowing that for weeks this pair had submitted to their neighbours, can we believe that they would be capable of asserting their authority and that the young would be properly cared for? Or suppose that different pairs of Kittiwake Gulls on the crowded ledges, or different pairs of Puffins in the crowded burrows, varied in like manner, would they all have equal chances of rearing their offspring? The struggle for reproduction is nowhere more severe than amongst the cliff-breeding sea birds; it is not for nothing that one sees Kittiwake Gulls, locked together, fall into the water hundreds of feet below and struggle to the point of exhaustion, or, as has been reported, to the point of death; it is not for nothing that Puffins fight with such desperation. And surely success will be attained by that pair whose emotional tone stands high and whose impulse to fight is therefore strong, rather than to the ill-assorted couple.
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.