WILLOW WARBLER
there they will be found for the first two hours or so of daylight singing incessantly, but for the remainder of the day they behave as they do during the winter months, roaming about in flocks; it is not until some weeks later that the females become sexually developed and commence to seek for mates. Territory is an essential to reproduction. By some means a male must proclaim the fact that he is in possession of a territory, and by some means the female must be made aware of the presence of a male fit to reproduce. I see no difficulty therefore in believing that all the particular sounds emitted at this season are thus of some use. And since, on the average, the stronger males only will be capable of obtaining a territory and thereby attaining to reproduction, may it not in justice be argued that a gradual increase in their power of producing sounds will automatically result? It is difficult to obtain any concrete evidence in proof of this. I will mention a few facts which may be used in support of it, and there leave it. The males we hear in full song in the spring are those which own territories and ultimately reproduce; I can recall no instance of a wandering male, that is to say, a male without a territory, in full song in the height of the breeding season. Males in search of territories sometimes pass through those already occupied and, as far as I have been able to observe, they remain silent during their transit. In the case of many species the song is uttered less frequently upon the arrival of the females and in some instances practically ceases. When two males occupy adjoining territories, and one of them utters its song whilst on the boundary, the other not infrequently attacks it forthwith.
The migration of the females extends over some days. The first females arrive about a week or ten days after the first males, but this interval is subject to variation according to the seasons. Thus it happens that males with adjoining territories do not necessarily all obtain a female on the same day. The difference is sometimes considerable, and may even amount to as much as a week. This discrepancy in the time
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