AND BLACK-BILLED AUK. 
21 
to quitting the cliffs, about the middle of August, they 
have attained about one-half the size of the old ones : at 
this period the plumage is changed, the under part of the 
neck and sides of the head having become white. This is 
the state of plumage in which they have been described as 
distinct species, and they preserve it during the winter. In 
December they have attained the size of the adult birds. 
To assure myself of their changes, I kept several indivi- 
duals tame till they were able to fly, and thus distinctly 
traced them. On the approach of spring, they may be seen 
passing into the summer-plumage of the mature birds. 
These also change in winter, turning white on the lower 
part of the neck and sides of the head ; but, in the earlier 
part of this season, they may be distinguished from the 
young, by their greater size, and the less vivid black of the 
plumage. In the old Razor-Bill, also, the bill is larger, 
and more furrowed. In the adult Guillemot, the bill is 
considerably longer. As the season advances these distinc- 
tions become less apparent; but the old birds acquire their 
summer-plumage earlier in spring than the young. 
There appears, therefore, no reason for separating the 
Colymbus Minor and Alca Pica from the Foolish Guille- 
mot and Razor-Bill ; and hence two species are expunged 
from the list. In the discussion which has led me to this 
result, I have been necessarily minute ; but I have hardly 
been more so than the very ingenious and respectable or- 
nithologist from whom principally I have differed. The 
opinion I have supported is not a new one, but has been 
held by many naturalists, who have, however, chiefly con- 
tented themselves with its mere statement, without satisfy- 
ing their readers with an exposition of their reasons for it. 
This I conceived it to be useful to supply, especially as 
the advocate for the opposite opinion had displayed so 
much elaborate ingenuity in maintaining it. The discus^ 
