264 FLEAS, FLUKES AND CUCKOOS 



the cuckoo not become entirely fixed to complacent fosterers? A 

 possible answer to this question may be that there are very few uncritical 

 fosterers; such birds are quickly over-parasitised and greatly reduced 

 in numbers and then, owing to their scarcity, the cuckoo is forced to 

 lay in other available nests. Therefore selection does not, in the long 

 run, favour the choice of uncritical hosts. It would appear that the 

 type of mimicry found in the European cuckoo is only developed if 

 fosterers exist which are neither too discriminating nor too com- 

 placent. 



As we have seen, there are strains or "gentes" of the European 

 cuckoo which favour different hosts in different districts. It is assumed, 

 and in some cases proved, that these birds return to breed in the areas 

 where they were originally hatched. The strains are to some extent 

 isolated both geographically and ecologically. Why then has the Euro- 

 pean cuckoo not broken up into a number of distinct subspecies or 

 species ? Stresemann believes that promiscuous sexual behaviour results 

 in a considerable amount of crossing between the gentes, which con- 

 sequently works against speciation. 



Colour and markings are not the only adaptations displayed by the 

 cuckoo's eggs. In the case of the European cuckoo, which parasitises 

 small birds, the eggs are relatively tiny, weighing one thirty-third only 

 of the parent bird. On the other hand, the great spotted cuckoo 

 {Clamator glandarius), a rare vagrant in Britain, which parasitises crows 

 and magpies, lays eggs which are larger than normal, namely one 

 eleventh of her own weight. 



The egg shells are also heavier and tougher than those of the host's 

 eggs. The hasty manner in which laying takes place and the projection 

 into nests with side entrances, not to mention the occasional trans- 

 portation in the bird's beak, sets a premium on shell-toughness. It may 

 also prove useful in cases where the fosterers make abortive efforts to eject 

 the egg — for they generally begin this operation by trying to peck a 

 hole in it. The eggs, as in all known brood parasites, develop more 

 rapidly than those of their hosts. 



Despite the amazing number of adaptations displayed by the 

 cuckoo the mortahty rate of its young is very high. Capek records that 

 out of 237 cuckoos' eggs laid, only 62 per cent, were hatched — but if he 

 could have taken into consideration those eggs which had been im- 

 mediately destroyed or built over by the fosterers the actual proportion 

 of faihires would undoubtedly have proved considerably higher. 



