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XV. — Egg Coloration in the Cuckoo, Cuculus canons, and its bearing 

 upon the theory of Cuckoo Sub-species. By John Rennie, 



D.Sc, F.R.S.E., University o£ Aberdeen.^ 



(Read 27th October 1913. MS. received 12th November 1913.) 



Through the generosity of Mr E. Hay Fenton, Aberdeen University 

 possesses one of the finest collections of clutches of birds' eggs in the 

 country. This collection includes 300 Cuckoos' eggs together with the 

 foster clutches with which they were found. The value of Mr Fenton's 

 gift is further enhanced by the fact that he has along with it supplied much 

 important data regarding the eggs, and, in particular, has given a full list of 

 localities and dates of their taking. 



The material and information thus available have made possible the use 

 of the collection for scientific investigation, and I wish here to express my 

 indebtedness to Mr Fenton for calling my attention to the particular facts 

 submitted and discussed in this paper dealing with egg coloration in Ctundtis 

 canorus and its bearing upon Newton's theory of Cuckoo "gentes." This 

 theory has recently obtained some support from biometric evidence supplied 

 by Latter (3). 



It is well known that Cuculus canorus practises the parasitic habit 

 of foisting the duty of incubation and rearing of its offspring upon a large 

 number of different birds. Sharpe puts the number of species at 119. In 

 the Fenton collection we have evidence of about 60 species in Britain. 



It is generally held that individual Cuckoos in the main parasitise upon 

 particular species of fosters during the whole of their lives, transmitting this 

 bias to their offspring, and Newton has suggested that by this means several 

 " gentes " of Cuckoos have thereby become established. The main evidence 

 for this suggestion is found in the coloration and size of the egg. Latter 

 investigated the problem biometrically, taking length and breadth dimen- 

 sions in 1572 eggs, and claims that the balance of evidence is " decidedly in 

 favour of there being distinct sets of Cuckoos." He finds evidence, in 

 particular, of races of Kobin-cuckoos, Wren-cuckoos, Whitethroat-cuckoos, 

 and Hedge-sparrow-cuckoos, and says, "It now seems fairly certain that 

 Prof A. Newton's suggestion is correct, and that there are certain ' gentes ' 

 of Cuckoos whose members being closely related lay eggs of somewhat 

 similar dimensions, and in the main confine their attentions, generation after 

 generation, each to its own particular variety of foster parent. In other 

 words, the evidence here adduced points to the conclusion that the species 



1 Reprinted from Proceedings Royal Physical Society of Edinburgh, Vol. six., No. 5, 

 pp. 97-107, Feb. 1914. 



