In treating of the ninth stage I said that the black had increased so much that it led directly up to this tenth stage. In this the black covers the whole of the under parts and flanks from the lower neck to the abdomen, being interspersed here and there only by a single drab feather; the flanks show most drab, but it can easily be seen that the black colouring matter is being passed into these feathers. The other portions of the bird, from the head to the mantle, have also become much darker, but the outer and greater wing-coverts have become of a paler slate, while the median coverts are darker than in preceding specimens. Length 26½ in.; wing 15½ in.; weight 5 lb. 2 oz.; bill 1·95 in.; tarsus 2·62 in. In the eleventh stage it need only be said that the black has still further intensified, become more glossy, extends entirely over the flanks, the outer feathers of which have broad white margins, and that there is only just a trace of drab left. It is quite certain that if the bird had lived a few weeks longer there would not have been a solitary trace of drab left.
It is quite clear to me that these birds select for their breeding site a spot where there is black sand and scattered lumps of black lava with portions of the surface weathered drab! just such spots as I found in Iceland (ante, 1901, p. 409), and which are also to be found in any volcanic area in the far north. Here, by throwing themselves into that upright attitude which I saw the Greylags (Anser cinereus) do, they would make themselves look, even from a short distance, exactly like weather-beaten lumps of black lava. The white front to the head is of the highest value to the bird for protective purposes, as it shines conspicuously, but is comparatively such a small speck, and terminates so abruptly by reason of its black border, and then is so softened down by the red bill that you cannot associate it at first sight with a living creature; and so it completely distracts attention from the sombrely clad body of the bird! Thus we see that what appears to us—if we examine a cabinet specimen or an illustration—to be a glaringly conspicuous mark for betraying the bird, is utilised by nature as a potent factor for the bird's protection at the most important period of its life!
Adult female.—Length and weight not kept; wing 15¾ in.; bill 2·12 in.; tarsus 2·65 in.
