PART OF THE HEAD OF THE ROOK. 163 
from the egg may be inferred from the fact that 
although numerous individuals, whose shrill voices 
evidently denote that they are young birds of the 
season, may be seen in the months of June and July 
with the base of the bill and anterior part of the 
head abundantly supplied with feathers, yet for seve- 
ral months prior to the breeding-season not one can 
be perceived, at least as far as my own observations 
extend, which has not those parts denuded. 
From what has been stated, it is evident that the 
phenomenon under consideration has a physiological, 
not a mechanical cause, though the removal of the 
plumage may be facilitated by the frequently re- 
peated act of thrusting the bill into the ground ; 
and the circumstances which seemed to support 
the ‘opposite conclusion admit, for the most part, 
of an easy explanation upon this view of the subject. 
The difference observable in the extent and complete- 
ness of the nudity at the base of the bill and the 
anterior part of the head of the Rook probably 
depends upon the progress which has been made in 
moulting, especially among the younger birds; and 
the earlier denudation of the more prominent parts’ 
may be occasioned by the friction consequent upon 
the manner in which the bill is employed in pro- 
curing food. The short filiform processes so common 
on the depressed and less exposed parts present a 
difficulty of which no satisfactory solution suggests 
itself ; but the state of the plumage on the head of 
M 2 
