Page:The British Warblers A History with Problems of Their Lives - 8 of 9.djvu/15

 cause of the abundance or paucity we are more or less ignorant. The only factor we can point to with certainty as affecting the question is the danger attendant on migration, for large numbers must succumb annually from one cause or another on the perilous journey to and from the winter home, but no suggestion has yet been put forward which would adequately explain so wide a fluctuation. Many subtle causes might contribute towards such a result; even a slight alteration in the proportion of the sexes would influence the numerical standing of a species, but I have no evidence of its occurrence in Nature.

At the commencement of the migratory movement males arrive before females. The first males generally precede the first females by a week or ten days, but the difference in the time of arrival of the sexes is, though considerable, not so great as that found amongst some migrants. Since the bird is one of the later migrants to reach its breeding grounds, a day gained or lost when the season is already advanced is of importance, so that we should expect to find the females following quickly in the wake of the males, even though this were not borne out by actual observation.

The behaviour of the species is very similar to that of the Blackcap, to which it is closely related. Upon arrival at its destination the male appropriates a territory and forthwith proceeds to proclaim the fact by incessantly pouring out its song. It would seem that some at least of the males arrive during the night, for fresh arrivals are generally first heard during the early hours of the morning. Of course it is open to question whether these males may not have actually arrived on the previous day; they may have been fatigued or hungry, and therefore not inclined to sing. Judging however by the behaviour of other species, taking into consideration how important it must be for every individual to proclaim its possession of a territory as early as possible, and bearing in mind that migration, generally speaking, does for the most part occur during the night, we shall probably be right in