22 THE GROUSE SUBFAMILY 



the eagle it is largely protected by its plumage, which at all times 

 of the year bears a marvellously close resemblance to its surroundings, 

 but the fox and the gull cannot so easily be avoided. During the 

 winter months, indeed, the eagle commonly seeks its quarry at lower 

 levels, finding it even more difficult to see its feathered victims amid 

 the snow than amid the barren rocks during the summer. That 

 they hold the eagle in great dread is shown by Mr. Millais, who tells 

 us that on hills where they are subject to constant alarms from 

 this bird they are always very wild. Thus it is not surprising to 

 find that natural selection has been vigorously at work eliminating 

 all but the largest winged birds. For its size, indeed, the ptarmigan 

 has the largest wings of any of our game-birds. Thus they can 

 ascend and descend perpendicular slopes of several hundred feet at 

 great speed, and without apparent effort. Even young birds, no 

 bigger than the thrushes, Mr. Millais tells us, can do this. To 

 escape the swoop of their powerful enemy they will sometimes 

 take a sudden dive and a swing towards the rocks, conforming 

 precisely to the contour of the mountain, and in this way com- 

 pletely evade capture. Another factor in determining the size of 

 the wings is the need for migration to escape the rigours of 

 exceptionally severe weather. But on this head enough has already 

 been said. 



We must turn now to the subject of coloration and plumage 

 changes, which, though it does not, strictly speaking, fall under the 

 head of habits, is closely connected therewith, and is, moreover, of 

 such exceptional interest that it demands exceptional treatment. 

 The males of the capercaillie and the blackcock are strikingly 

 dissimilar, even though they have much in common. The females, 

 on the other hand, differ conspicuously from their mates, and much 

 more nearly resemble one another than do the males. Thus we may 

 infer that originally both sexes, when adult, were coloured like their 

 respective females, as they are still when in the juvenile plumage. 

 That there is weighty evidence in support of this assumption is 



