THE EUROPEAN CUCKOO 263 



passerine birds which are the chosen hosts recognise the cuckoo's egg 

 as something undesirable. Subsequently they frequently destroy it or 

 desert the nest in which it has been laid. It is not, of course, suggested 

 that the birds know the Ggg is a cuckoo's egg — what disturbs them is an 

 egg in some way different from their own and this sense of disharmony 

 prompts them to eject it or to begin to build a new nest altogether. In 

 these circumstances those eggs which resemble the fosterer's or vary in 

 the same direction, and consequently do not arouse anxiety or antag- 

 onism in the host, have the best chance of survival and development. 

 In this way selection gradually produces eggs more and more like those 

 of the foster parents. In the same manner elimination by natural 

 enemies produces plovers' eggs and terns' eggs which almost exactly 

 resemble the ground they nest on. In the latter case they have to be 

 concealed from egg thieves, while in the case of the cuckoo the host 

 constitutes the principal enemy. This theory, of course, assumes that 

 the various deviations from the original egg-type are fixed by heredity. 

 The genetics of the cuckoo's egg have not been investigated, but at any 

 rate it is known that egg-shell colour in the domestic fowl is transmitted 

 independently and equally by either sex. Some species of cuckoo like 

 the Indian hawk cuckoo {Hierococcyx varius) and all the species of the 

 genus Clamator, such as the red- winged crested cuckoo [Clamator 

 coromandus) have developed eggs which mimic the host's in every detail, 

 so that even an experienced ornithologist can be deceived. Sometimes the 

 texture of the shell or a small difference in weight reveals the truth, but 

 at times it is virtually impossible to tell which is the brood parasite's egg. 



Such a high degree of specialisation naturally restricts the cuckoo in 

 question to one or two closely related species of host, which is always a 

 dangerous position for a parasite to adopt. The advantage of a wide 

 circle of fosterers probably explains why the cuckoos, once embarked 

 upon parasitism, diverged to hosts with eggs unlike their own, which 

 were primitively white or pale bluish green. 



There are one or two questions which immediately spring to mind 

 after reading the foregoing account. It has been mentioned that some 

 birds are much more willing to accept the cuckoo's egg than others. The 

 hedge-sparrow, which we know was a favoured host in Shakespeare's 

 day, will brood almost anything foisted upon it from cuckoos' eggs to 

 pebbles. Nor does it attack the adult cuckoo. It is not surprising, 

 therefore, to find that as no selection takes place, no blue-type egg has 

 been developed by the cuckoo parasitising this species. Why then has 



