BRITISH WARBLERS
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the excessive depth of the nest and in the method of weaving it to the upright supports the selective agency has been withdrawn, and we consequently find nests varying considerably in depth, and varying, too, in a remarkable degree in the manner whereby they are secured.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 Reed 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 supports, 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 supports 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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