1917-] Breeding-habits of the Cuckoo. 219 



and Meadow-Pipit. In all other species it is much more 

 occasional, and it never occurs in the case of the Wren, 

 Willow-Warbler, Wood- Warbler, ChiffchaflF, and Hedge- 

 Sparrow ; yet, of these latter, the first four invariably 

 refuse to hatch the egg, while the Hedge-Sparrow willingly 

 accepts it — a strange fact which is hard to fit in to any 

 theory. 



6. In some localities the percentage of eggs resembling 

 the type of the species in whose nests they are deposited is 

 very high, nearly all being similar ; in others it is very low. 

 The high percentage usually occurs where one species is 

 mainly used as foster-parent, and the low percentage where 

 several are used. 



7. *The explanation of the resemblance of Cuckoos' eggs 

 to the normal type of those of the foster-parents is probably 

 that the nest is selected after the egg is laid, and that the 

 female Cuckoo is influenced in her choice mainly by the 

 colour of the eggs in the nest and the resemblance they 

 have to her own. Quite possibly she may also be influenced 

 to some extent by the nest being similar to that in which 

 she herself was reared, especially when she cannot find eggs 

 which match her own. Also she would normally return in 

 spring to the locality in which she herself was reared. 

 Further, when she had once laid, she would know the colour 

 of her eggs and could select nests beforehand if she came 

 across them. 



8. ^The prevalence of certain distinctive types of eggs in 

 specific localities can be assigned to the law of adaptation. 

 The strain of Cuckoos laying distinctive eggs, e. g. blue or 

 very rufous types, would have a large number destroyed, 

 since many species would refuse to accept them and thus 

 this strain would gradually die out except in localities where 

 many nests with similar eggs wei'e to be found. On the 

 other hand, the strain of Cuckoos laying more neutral- 

 coloured eggs would have more young reared, since their 

 eggs would be more easily accepted and thus would flourish 

 in most districts. 



