MIMICRY IN EGG-COLOUR 191 



Skylark than to any other bird's, but no Robin or 

 Hedge-Sparrow, even if colour-blind, could fail to 

 notice the difference of the alien egg. 

 (It is said that eggs resembling those of the foster- 

 parents, such as the blue form found in the Red,- 

 start's nest, tend to be confined to fosterers which 

 are rarely patronized, leading to the inference that 

 such are not easily duped, so that a perfect imitation 

 has been evolved ; but such an explanation involves 

 the assumptions that certain strains of Cuckoos 

 always lay in particular birds' nests, and always 

 produce eggs of the same colour, of which there is 

 no evidence at present. Certainly, as in the case 

 of Tree-Sparrows and Eagles, the same individual 

 birds do not always lay similar eggs ; a very 

 remarkable instance of this has occurred in the 

 case of the Nightingale, in whose nest have been 

 found an egg of the normal olive-brown, a blue 

 one, and two of intermediate shades. 



In some Oriental Cuckoos there is a very perfect 

 assimilation between the egg of the parasite and 

 the fosterer ; the " Brain-fever-bird " (Hierococcyx 

 varius) and the Pied Crested Cuckoo (Coccystes 

 jacobinus) lay in the nests of Babbling-Thrushes 

 (Crateropus and Argya) and their eggs are plain 

 blue like those of the Babblers ; and the Koel 

 (Eudynamis honor ata), the commonest and best- 

 known Cuckoo in India, is parasitic on House and 

 Jungle Crows (Corvus splendens and C. culminatus), 

 and lays an egg very like a small Crow's egg. It is 

 doubtful, however, whether the imitation is here 



