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DIMORPHISM IN THE EGGS OF THE HOUSE 

 SPARROW {PASSER DOMESTICUS). 



C. J. PATTEN, M.A., M.D., Sc.D. 

 Shejfield. 



(plate XIV.). 



The considerable range of variation to which the egg of the 

 House-Sparrow is subject has no doubt struck many ornitholo- 

 gists. But until I came to collect large numbers of the eggs of 

 this species, while prosecuting some investigations into avian 

 embryology, I was not of the opinion that such marked dimor- 

 phism existed. Unlike the case which I published in this 

 journal last year, of a clutch of Song-Thrush's eggs in which one 

 was devoid of spots, and was non-fertile with an aborted con- 

 dition of the yelk, all the shells in the eggs which I am now 

 about to describe have pigmental markings deposited on the 

 ground-colour. In some the markings are strikingly different 

 even in eggs from the same nest ; conversely in another case 

 whilethe pattern is fairly uniform, there is decided dimorphism 

 in size. 



Before proceeding further with details, I may here point out 

 that I have thought it worth while also figuring a very common 

 form of dimorphism in which three or four of the clutch have a 

 clear ground-colour with large and more or less discreet blotches 

 and spots [e.f.g. Plate XIV., fig. i) ; the remaining one or two 

 eggs having a muddy whitish brown ground-colour in which 

 there is a considerable diffusion between the latter and the pig- 

 ment-markings {h. Plate XIV., fig. i). From a large Sparrow- 

 colony situated in an ivy-covered house-front, from which all 

 the eggs in question have been taken, I estimated that some 

 twenty-live per cent, of clutches displayed this peculiarit}-. 

 and as I have found the same in Sparrows' eggs obtained from 

 nests more or less isolated from their fellows, I am not inclined 

 to think that here there is any mixing of the clutches of different 

 birds. Moreover, I have found the same variation taking place 

 in egg-clutches of the Redbreast, Willow-Warbler. Meadow- 

 Pipit, and other birds in which the markings are numerous and 

 well distributed over the shell. This form of variation I look 

 upon as purely physiological, depending upon an alteration in 

 the activity of the secretion of the pigment-glands, and the 

 rate of rotation of the shell down the oviduct ; and I believe 



Naturalist, 



