Page:The British Warblers A History with Problems of Their Lives - 4 of 9.djvu/21

 whereas the male in the one case may have arrived only a few days before the male in the other, an interval of as much as fourteen days may separate the arrival of the respective females. Now there is little doubt that it is the presence of a female in a certain territory that is one, though not the only, cause which induces the unpaired male in an adjoining territory to cross the boundary. The impulse to approach the female is at this period probably irresistible, and the result is that the owner frequently attacks the intruder by flying at and pecking him vigorously until he leaves, although this does not always happen, for he sometimes takes little or no notice of him. On these occasions the female shows that she, too, is under the influence of considerable excitement, spreading and flirting her tail, and at the same time incessantly uttering her quiet call note. When the one male pursues the other she also accompanies them, flying rapidly in and out of the bushes. I never remember seeing her actually attack an intruding male, but when a second female intrudes she does not hesitate to do so; the attack being very vigorous, and at such times the erection of the feathers and the spreading and the flirting of the tails are in every way similar to what one observes in the case of the males.

It is more difficult to understand what causes an unpaired male to cross its boundary and enter the territory of another single male, yet it is of frequent occurrence. In assuming that all these activities have some direct bearing upon the history of the individual and of the species, we may possibly be in error; but, if this is so, we must still regard them as an expression of some agency at work in the bird's metabolism, and thus they come to have a meaning equally important if we could but fathom it. When an unpaired male thus enters the adjoining territory of another single male a battle— or what frequently has every appearance of a battle—ensues; at one moment so strenuous that there need be no hesitation in pronouncing it as such, but at another less active and more