1917-] Breeding-habits of the Cuckoo. 207 



Wasman even admits that the eggs in the nest prohahly 

 afford the female a clue in identifying the species that 

 reared her ; and other authorities are of opinion that the 

 Cuckoo, when possible, deposits her eggs in the nests of 

 the same species that reared her. My own theory goes 

 further than this. I think it is only natural to assume thnt 

 each Cuckoo lays eggs of the same type all her life, and it 

 is probable that the egg-coloration is transmitted, more or 

 less exactly, from mother to daughter, and thus there are 

 families or *' strains " of Cuckoos laying eggs of a certain 

 type. It is possible that the male may influence the egg- 

 coloration, but this seems unlikely. 



Now w'c may assume that Natural Selection would favour 

 those Cuckoo families whose eggs most nearly resembled 

 those of the foster-parents, and hence, for example, Cuckoos 

 laying blue eggs would become nuraerovis in districts where 

 the Redstart was common, wdiile it is obvious that if such 

 coloured eggs had to be placed in the nests of other species 

 they might frequently be rejected, and, consequently, this 

 family would either die out by degrees or migrate to some 

 other district where they could find eggs resembling their 

 own. If this theory be correct it would, I think, produce 

 the following results : — 



(1) Cuckoo families laying eggs of a very distinctive 



coloration "would either become very localized in 

 districts where suitable foster-parents existed in 

 sufficient numbers or would gradually die out. 



(2) Cuckoos laying eggs of a more neutral colour and more 



nearly resembling the types of the species most 

 suitable as foster-parents would become far more 

 numerous, since they would have a wider choice of 

 nests in which to deposit their eggs, and the colour 

 would be a protection. 



(3) Cuckoos laying eggs of the Wren or Willow-Warbler 



type would become very rare, since when their eggs 

 were deposited in the nests of these species they 

 would not be hatched, and, if placed in other nests, 

 a large proportion would also not be accepted. 



