368 TEE ZOOLOGIST. 



VARIATIONS IN COLOURING OF STERCORARIUS 

 CREPIDATUS. 



By Edmund Selous. 



The Arctic Skua (Stercorarius crepidatus) is usually described 

 as being dimorphous — that is to say, of two forms in regard to 

 its plumage, one dark and the other light. I have lately, whilst 

 in the Shetlands, written down the description of various birds 

 as they stood on the heath, after carefully watching them through 

 the glasses, at a very moderate distance. I might have added 

 greatly to the list, hardly any two individuals being alike in the 

 same degree that the individuals of most other birds are- — at 

 least this appeared to me to be the case. But my time for this 

 was limited, and my list consists of fifteen birds only. It is as 

 follows : — 



(1) The neck from just below the head, with the throat, 

 breast, and ventral surface as far as the legs, a beautiful creamy 

 white. The rest dark, as in the ordinary cases ; but I was not 

 careful to note the precise shade. The crown of the head — and 

 this, I think, is universal — sufficiently dark to appear black. 

 This bird represents, I think, the extreme of the light form in 

 which dark and light are almost equally divided. 



(2) The light colouring extends, speaking roughly, over the 

 same parts, but it is very much less bright and pure. It might 

 be described as a dun-cream or cream-dun, the two shades 

 seeming to struggle for supremacy. The cream prevails on the 

 neck, the dun on the other parts ; but even the neck is of a much 

 duller shade than in the bird just described (No. 1). There are 

 parts of the breast where the original sombre hue, a little 

 softened, encroaches cloudily upon the lighter surface. These 

 two birds cannot — I say this after due comparison — be described 

 as more or le'ss handsome in the same colouring. The lighter 

 surface, at any rate, is plainly different in shade, as also its 

 amount and distribution, though in a less degree. 



