Page:The British Warblers A History with Problems of Their Lives - 9 of 9.djvu/39

 the more reason is there for its survival. The reactions that we meet with in bird life may thus be said to have been developed firstly as a means for arousing the requisite amount of pairing hunger in the female, secondly as a warning to intruders, and thirdly as a protection for the helpless offspring. Clearly everything centres round the strength of the probability in each case.

Emotional behaviour as a whole is so intricate that it is difficult to lay hold on anything very definite, but observation discloses one fact at least which seems to be of some importance and which is referred to in the life of the Marsh Warbler, namely, that the visible manifestations are uniformly strong or uniformly weak in the same species at different emotional periods. An example will make my meaning clearer. The Blackcap exhibits many peculiar antics and assumes many peculiar attitudes during the period of sexual activity and again when it has young. The Willow Warbler assumes one peculiar attitude during sexual activity and again, though in a minor degree, when its nest or young are intruded upon. Now the sexual behaviour of these two species is widely divergent; the one bird is intensely demonstrative, no limit being placed apparently upon the antics it can and does perform, the other has only one definite performance which really calls for remark. Great however as the difference is between them, it need not trouble us here, since a certain amount of latitude is permissible in the application of the utility hypothesis to the behaviour at this special time; let us therefore grant that in each case it serves its purpose equally well in its own particular sphere—arouses, that is to say, the pairing hunger of the respective females. Our difficulty arises in regard to the behaviour when the parental instinct is uppermost; for it is then no longer a question of calling forth a response in this female or in that, but of influencing the behaviour of an intruder. A certain definite standard of response is consequently necessary in the case of both species, since a very definite part has to be played, and this standard must have been gradually evolved by