94 ' Mr Edmondston 07i the KUtiwake^ ^c. 
The voice is very similar to that of the kittiwake, but stronger, 
and not so well defined. Both these species follow nearly the 
same periods of migration, and, I suppose, visit the same lati- 
tudes. It is curious, however, that while some of the young of 
the kittiwake remain throughout the winter in Zetland, I know 
of no instance where an individual of the Arctic gull has been 
met with there during that season. That singular instinct, which 
seems to condemn it to depend for its chief supply of food on 
the superior industry or dexterity of other birds, irresistibly im- 
pelling it to follow the main body of those caterers for its sub- 
sistence, in their great periodical migrations. 
2. Larus Rissa, or Kittiwake.- 
Of the multitudes of the Larus rissa^ or Kittiwake, that an- 
nually arrive in Zetland to breed, large flocks are observed to 
keep apart from those which repair to the usual haunts for in- 
cubation, resting on the water, or on low rocks ; and, from their 
not breeding, are termed, in the dialect of the country, Yeeld 
Kittiwakes. This singular fact in their history has been stated 
by Dr Edmondston, in his “ View of the Zetland Islands,” but 
I am not conscious that any explanation has yet been offered of it. 
I regret that indisposition prevented me last season from pro- 
curing any specimens for dissection, since I suspect those Yeeld 
Kittiwakes to be merely the young of the first year, which, al- 
though attained to perfect plumage, have not yet acquired the 
faculty of propagation. Of all the individuals of this species 
which are met with in Zetland during the summer, I have never 
observed one that is not in the adult plumage ; and it seems 
more reasonable to suppose, that so large a number have not 
attained the power of continuing their species, than that they 
have lost it ; or that they are all of one sex. 
If this opinion be correct, it will establish a fact, which it may 
be useful to keep in view in ornithological distinctions, but to 
which sufficient attention has not always been paid, that although 
a bird may be arrived at mature plumage, it does not necessari- 
ly follow that, in other respects, it is perfect. 
I suspect this general remark may apply to the three more fami- 
liarly known species of the Greater and the Lesser black-backed, 
and the Herring Gulls. During the summer, when the her- 
