THE CUCKOO 



85 



darker than usual ; and this does conform in a very imperfect 

 way to the pale green eggs of the reed-warbler, with their 

 dense, dark green spots. It is most probable, though it is 

 not yet proved, that the hen cuckoo puts her eggs in nests of 

 the same kind in which she herself was brought up. The 

 same principles of natural selection which control the colours 

 of the lawful eggs in the nest therefore influence to some 

 extent those of the intruding cuckoo, 

 never become very like those of the host, 

 because when two cuckoos mate there is 

 often an admixture of blood from a different 

 class of cuckoos, contributed by the cock. 

 Though the hen cuckoos of the reed-warbler 

 cuckoo clan are faithful to the nests of the 

 reed-warbler, the approximation of their 

 eggs to the reed-warbler's eggs is perpetu- 

 ally hindered by their mating with the 

 hedge-sparrow or meadow-pipit or pied 

 wagtail clan, which perpetually introduce 

 the blood of a different stock. It is worth 

 noting that the indecisive colour and mark- 

 ings of cuckoos' eggs are just the result which 

 we should expect from a confusion of many types. Cuckoos' 

 eggs are found so rarely in certain nests — as, for example, the 

 swallow's and chaffinch's, and even the dabchick's — that it 

 seems clear that the right instinct of the mother sometimes 

 breaks down, and that she puts her egg in the first nest she 

 can find. If the attachment to the nest of a certain species 

 were absolute, it would also be difficult to explain the occur- 

 rence of cuckoos' eggs in nests of the marsh-warbler, which 

 is so scarce and local in this country as to provide a very 

 uncertain foster family for cuckoos depending on it. Yet 

 cuckoos' eggs do turn up in marsh-warblers' nests, and it 



