WILLOW WARBLER
adults, even appropriate the old territory, but what is of more concern to us is the extent to which they do so. Is it a rule for the same male to return to the same territory? And does the female seek her former mate? These are the questions which we should like to be in a position to answer, and they are questions to which I have never been able through actual observation to obtain a definite reply. The marking, of young birds may tell us whither they go, and whether they return to the locality of their birthplace, but upon the point in question it can clearly throw no light unless some system can be devised whereby the owners of certain territories are annually caught and released. In the case of some of the migrants males are the first to reach their destination, afterwards the sexes arrive intermingled, but not paired. A male takes up its territory and there awaits a female, upon whose arrival pairing takes place. The young are then hatched, and as soon as they are capable of taking care of themselves the parents desert their territory and appear to separate. Will they again meet the following breeding season? Is it likely, in the first place, that both will survive? We have no direct evidence to guide us in estimating the average duration of life of an individual under natural conditions, but we can be certain that it varies enormously in different species. However, we are here considering the migrants only; and bearing in mind the mortality which must and does occur amongst them during migration, from scarcity of food, from vagaries of climate, from disease and from natural enemies, and fixing our attention on the fact that there is no perceptible increase in their numbers year by year, how can we regard such an existence as otherwise than a precarious one? And are not the chances, therefore, of both surviving somewhat remote? Assuming, however, that both escape the numerous dangers that threaten them, and are consequently capable of breeding the following spring, will both male and female steadfastly make their way back to the same few acres of ground which they inhabited the
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