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 appro- 

 priated 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 approxi- 

 mately 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 neigh- 

 bouring 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 



28 



