WOOD WARBLER
were dependent upon the periodic recurrence of a certain organic condition in both sexes, it is probable that seldom only would it reach its consummation; if, on the other hand, the necessary condition were always present in both sexes, no check would be imposed upon a too liberal yielding to the the sexual impulse. In order to overcome this difficulty the sexual instinct is, I believe—though it is a mere supposition on my part—allowed free play in the male, but is subject to some periodicity in the female. Thus she may be said, broadly speaking, to exercise a control, though such control is really biologically determined. During the period of sexual activity the male can usually be found in close proximity to the female. She nutters from branch to branch, he following in her wake, and as they roam thus about the territory in search of food, she calls often and he utters the last sentence of his song occasionally. Outbursts of excitement are of frequent occurrence, and the male generally takes the initiative, but not always, for sometimes the responsibility rests with her, that is to say it is she who suddenly and with but little warning darts off in pursuit. The routine of sexual behaviour does not appear to be similar on every occasion; I am doubtful whether it is always initiated by a pursuit, but it is difficult to speak with certainty upon this point. A mutual pursuit amongst the trees seems, however, to be a necessary part of the routine, and may even occur at the conclusion of the whole process, subsequent that is to say to the completion of the sexual act. The normal procedure is as follows. The male approaches the female, and settling beside her with drooping wings commences to nutter or quiver them rapidly. She may do likewise, whereupon both continue this wing napping or wing quivering for a period which, though of short duration, varies in length on different occasions. When the proceedings are initiated by a pursuit, the male, previous to darting at her, watches her intently for a few moments, and whilst doing so betrays suppressed excitement, his feathers being tightly compressed and his wings loosely
13
