1917-] Breeding-habits of the Cuckoo . 195 



It is, of course, evident that if tliere were only two 

 Cuckoos in a certain district, one of which laid eggs of a 

 rufous type and tlie other eggs of a grey type, it would be 

 easy to assign them to a specific female, and this would also 

 ai)ply to eggs having an unusual colouring, or abnormal 

 sl'.ape due to some irregularity in the oviduct ; but this is 

 quite a different matter to Dr. Key's claim that he could 

 assign every e^g found in a district near Leipzig to one 

 of thirty-four different females, many of whom, from his 

 description, laid eggs of very similar types. 



I think, for instance, that most oologists would recognise 

 the impossibility of rearranging into correct clutches a 

 mixed assortment of eggs of the Gulls, Terns, or other 

 species in which the colouring varies considerably, and I 

 liave often seen clutches of these eggs, clearly laid by the 

 same bird, in wiiich individual eggs differed strikingly. 

 And the dithculty would, of course, be even greater in 

 dealing with eggs of such species as the Skylark, Meadow. 

 Pipit, and others. Nor would it be possible to visit even a 

 small colony of Guillemots in two successive years and pick 

 out eggs laid by the same female. 



AVhat grounds have we, then, for considering the Cuckoo 

 an exception ? 



Further, it is an acknowledged fact that one or two eggs 

 in each clutch laid by the Tree-Sparrow are distinctly paler 

 than the others, while uniform blue eggs have been found 

 in otherwise normal clutches of the Linnet and Song- 

 Thrush; and, indeed, examples of variations in colour and 

 shape amongst eggs of the same clutch are by no means 

 uncommon, and even Dr. Key himself points out that 

 Walter, after seven years' experience in Pomerania, found 

 all Cuckoos' eggs the same colour and indistinguishable 

 from each other. 



I may add that I have in my collection two Cuckoos' 

 eggs, both taken in nests of the Hedge-Sparrow, and in a 

 locality where I feel sure there was only one female Cuckoo, 

 and, although these are similar in type, I do not think any 



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