1917-] Breeding-hahits of the Ctickoo. 205 



argues, the food supplied to the young Cuckoo influences 

 the subsequent colour of the eggs the problem is solved, 

 and " mixed types '* of eggs and variations are easily ac- 

 counted for by the Cuckoo being unable to find sufficient 

 nests of the species that reared her, and being forced in 

 consequence to entrust her eggs to others. For instance, 

 if a female Cuckoo, reared by a Whitethroat and laying 

 eggs of that type, were forced to deposit her egg in the nest 

 of a Garden-Warbler, the young Cuckoo, if a female, would 

 lay eggs of a " mixed " type. 



But, as already pointed out, there are strong arguments 

 against this theory, and it is not easy to accept. 



Boraston refers to the theory that when the eggs of the 

 Cuckoo resemble those of tlie foster-parent they are hatched, 

 and that thus the strain of birds laying such eggs becomes 

 "naturally selected/' and that, when the resemblance does 

 not exist, the eggs are not hatched and the strain gradually 

 becomes eliminated. He argues, however, that, if this were 

 so, the foster-parents would themselves become the "natural 

 selectors, '' and that to make them responsible for "selecting" 

 a type of Cuckoo's egg resembling their own and thus 

 deceiving their posterity, is an untenable theory. Further, 

 after examining 7Q nests of 31 different species containing 

 Cuckoos' eggs, he comes to the conclusion that the alleged 

 specialization into types resembling those of the foster- 

 parents shows no constancy, and that the difference between 

 eggs deposited in nests of the same species is just as great 

 as between those laid in nests of different species. 



Professor Newton argues that assimilation in colour is un- 

 necessary in the case of the H edge-Sparrow, but that, since 

 other species are more particular in accepting differently 

 coloured eggs, assimilation to their types is more requisite, 

 and that the operation of natural selection in egg-coloration 

 would be most needed in those cases in which the foster- 

 parents are not easily duped — that is in the cases occurring 

 less frequently. And it is in such cases that it is found, 

 since eggs deposited in nests of the Red-backed Shrike, 

 Redstart, Buntings, and Icterine Warbler resemble those of 



