BRITISH WARBLERS
It was amongst some willows growing in the water with little or no undergrowth close at hand that could afford adequate protection for the nest. That a male should have appropriated such a territory seemed to me at the time unusual, and therefore I was scarcely surprised when, after singing regularly for about ten days, and frequently intruding upon a neighbouring territory, it wandered one morning up an adjoining hedgerow and disappeared in search of a new home, which I believe it eventually found in a plantation approximately a mile away. Each territory has its boundaries which are jealously guarded by the owner. But these boundaries must not be regarded as clearly defined. The conception, that is to say, of a boundary must not be that of a line which is at no time crossed by the owner, or which cannot be crossed by an intruder without a commotion resulting. Such a conception would imply powers altogether beyond the mental capacity of a bird. These boundaries must be thought of in a wider sense than this, although it is remarkable how well defined they sometimes are. I have already pointed out that the territories are the result of a natural law, and that it is unnecessary to regard the individual bird as being in any way cognisant of a territory as such. The securing and defence of a territory is not an individual acquirement, but is rendered possible by congenital nervous dispositions which have been gradually evolved by selection and determine their possessor to act in this particular way. Neither is it essential for the principle of territory that there should be absolute boundaries, or that an intruder should never be allowed to enter a neighbouring preserve without being attacked. All that is required is that on the average those individuals which are the more active in attacking and driving away such other individuals as approach them too closely should be left in possession of a territory, and thereby gain some advantage; and the advantage in the case of the migrants arises from the fact that an ample supply of food for the helpless offspring is secured. The dimensions of the territories of the Marsh
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