VARIATIONS IN COLOURING OF STERCORARIUS
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 less 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.