Mr Edmondston on the Colymbus Grylle^ 101 
It is termed in Zetland Craa maa^ or the Crow gull, though 
I think they apply there the same name to the young kittiwakes 
after they have begun to ily, much in the same way as they de- 
nominate the young of two or three other species of gull Scoree. 
If it then be an undescribed species, we might have a Larus 
corpus added to the list. 
As, however, I have only had my attention directed to it 
since last autumn, what I have here stated regarding it must 
be considered as coming from limited experience, and rather as 
hints for the remarks of others, than as ascertained definitively. 
I shall, however, continue my inquiries regarding this doubtful 
bird, and communicate their result to the Society, 
3. Colymhus Grylle, or Blade Guillemot. 
Some obscurity still exists regarding the history of this spe- 
cies, which it is my present object to endeavour to remove. 
One opinion maintains, that this species changes its black 
summer plumage to a grey mottled appearance in winter. Ano- 
ther considers this difference of appearance to depend on diffe- 
rence of species, — while a third refers it merely to the distinc- 
tion of age ; the black being the adult, the grey the young 
bird. 
This latter opinion is that which I entertain, and I conceive 
it to be the only one capable of avoiding the inconsistencies 
that attach to the others. 
The black guillemot has been stated to produce only one 
young during the year. This, however, is incorrect. It lays 
two eggs ; and very seldom indeed is its nest found without 
two young ones. If this species do not migrate from Zetland 
durirrg the winter, it ought then to occur there at that season 
in considerably greater numbers than in summer. This is, how- 
ever, decidedly opposed to all my observations, The number, 
on the contrary, is certainly less. 
Partial migration, then, does take place ; and nothing ap- 
pears more easy to suppose, than that the old migrate, while 
the greater part of the young remain behind. 
The departure of the black individuals occurs soon after the 
young have quitted the cliffs ; and when migration might be ex-= 
