VOL. VII.] PLUMAGES OF THE ROOK. 133 



10-13). An examination with a glass of the chins 

 of a series of birds from August to January, shows 

 the gradual effect of this wearing process very 

 clearly. By January (and in some individuals in 

 December) the chin and upper-throat look quite bare 

 at a little distance, but a close examination shows 

 that there are many remains of degenerate feathers 

 and plumules and longish filoplumes, and these remains, 

 getting smaller and smaller as the summer goes on, 

 are still to be seen with a glass up to the autumn-moult. 

 An examination under a microscope of a series of these 

 feathers, taken from birds shot at intervals, shows that 

 they are worn down gradually aU round rather than 

 broken off short (Plate 10, Fig. 25). They are of such a 

 delicate nature that I am inclined to think this shows that 

 they cannot often be subjected to violent friction, and that 

 when the Eook digs it does not usually allow its chin and 

 throat to touch the ground. Indeed, it would be hardly 

 possible for it to thrust its head so far into the ground 

 as to touch the lower part of the bare patch on the 

 throat, while the portion of the chin which lies within the 

 angle of the mandibles would be protected to a certam 

 extent by the projecting ridges of the mandibles. When 

 we consider the abrasion which goes on in a normal 

 contour feather, the action of wind and weather alone, 

 it seems to me, is quite sufficient to account for 

 the wearing away of this delicate " down." A line of 

 " down " just alongside and parallel to each lower 

 mandible (Plate 4, Fig. 12), remains imworn longest, but 

 this is not because the projection of the mandible 

 protects it from friction by " digging," but because this 

 part of the skin folds inwards when the " pouch " is 

 empty, and the " down " along this " crease " is thus 

 not exposed. 



I much regret that owing to a want of knowledge of 

 the process of feather-development, I have been unable 

 to investigate the cause of the debility of the feather- 

 papillse. I hope that someone who has studied 



