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THE CUCKOO’S SECRET 
frequently the case, although they may be as aware 
as I am of the exceptions, often found, which prove 
the rule. Side by side with this fact we have to 
place the obvious truth, that the less the contrast 
in appearance between the egg of a Cuckoo and 
the egg or eggs of the fosterer beside which it is 
laid, the better chance the Cuckoo’s egg has of being 
accepted, brooded, and hatched, by the fosterer. 
Both facts clearly point to the same conclusion, 
i.e. that successful mimicry in her eggs is a con- 
dition of survival for the Cuckoo. For her it is 
the counsel of perfection to lay, rapidly and with- 
out detection, in the nest of one of her regular 
fosterers, an egg of such appearance as does not 
cause the foster-parents to desert. In observed 
cases where she has failed to achieve this ideal, a 
simple explanation is to hand. For example, one 
may come across an obviously reddish Cuckoo’s 
egg, eminently suitable, in the eye of an expert, for 
a Robin’s nest, but laid alongside the blue eggs 
of a Hedge-sparrow. This condition of affairs, 
however, is in my opinion the result of some 
irregularity in the even tenor of the Cuckoo’s 
normal routine. It cannot be doubted that this 
reddish egg is no accidental variation laid by a 
Cuckoo which usually lays eggs of a green or grey 
