Egg Coloration in the Cuckoo, Cuculus canorus. 97 
XV.—Egg¢ Coloration in the Cuckoo, Cuculus canorus, and its bearing 
upon the theory of Cuckoo Sub-species. By John Rennie, 
D.Sc., F.R.S.E., University of Aberdeen. 
(Read 27th October 1913. MS. received 12th November 1913.) 
THROUGH the generosity of Mr R. Hay Fenton, Aberdeen University 
possesses one of the finest collections of clutches of birds’ eges 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 
eift 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 Cuculus 
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 
“oentes” 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 Robin-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 whese 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 
VOL. XIX. Aconian Insti? 
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