BRITISH WAEBLERS 



but had, for reasons unknown to us, recently adapted itself 

 to its present environment? The answer to this must be 

 that we should anticipate those conditions of instability 

 which are so clearly revealed to us ; instability, that is 

 to say, in just those particular features which were of 

 adaptive value under the former conditions of existence. 

 Since no useful purpose is any longer served in the exces- 

 sive depth of the nest and in the method of weaving it to the 

 upright supports the selective agency has been withdrawn, 

 and we consequently find nests varying considerably in depth, 

 and varying, too, in a remarkable degree in the manner 

 whereby they are secured. We find them, on the one hand, 

 placed in a fork and tightly woven to the upright supports, 

 and, on the other, flimsily slung by slender basket handles. 

 And just as the former type is unnecessarily secure, so the 

 latter — one might almost say — is foolishly insecure. In one 

 instance I watched with growing anxiety a nest that daily 

 became more tilted owing to the grass that formed the 

 handles becoming stretched. Disaster however in this case 

 overtook it from another source. The method of attachment 

 of which we are now speaking is in reality similar to that 

 adopted by the Eeed Warbler and some individuals of this 

 species, though when first seen it appears to be very different. 

 The situation in a great measure determines the type. If the 

 bird builds in a dense thicket of dogwood, where there is a 

 difficulty in finding three or more shoots close together, it has 

 to make the best of the position. The weaving instinct being 

 strong within it, it threads the grass round the nearest sup- 

 ports, which instead of being upright are often bent, and the 

 result is the flimsy handles referred to. When a nest rests 

 on no solid foundation and is dependent solely on the sup- 

 ports from which it is slung, it is possible, especially in wet 

 weather, that the weight of the bird brooding her well-grown 

 young will aggravate the weakness and thus bring about a 

 catastrophe. This appeared to be taking place in the case 

 referred to. It seems then as if a stronger structure than 



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