180 
ICELAND GULL. 
of colour amongst several individuals which I had an op* 
portunity of examining, — some being darker, and others 
lighter, than the specimen I have described, — and the 
colour of the iris presenting a corresponding variation, from 
dark-brown to silver-grey. These changes are perfectly 
analogous to what occurs in the young of the greater part 
of the genus, and hence I was led to the conclusion of the 
present specimen being a young bird. But as, in those 
species to which this resemblance applies, the iris attains 
its permanent colour some time before the perfect plumage 
is assumed, I could only form a vague conjecture of its 
adult appearance. Fortunately, however, in the same flock, 
from which I killed the specimen above described, was a 
considerable number of individuals of this species, in what 
I decidedly consider maturity of plumage. In general 
appearance and habits, no difference could be detected. 
The back and upper part of the wings were light-blue, 
passing into white. All the rest of the body, and the prim- 
ary and secondary quills, dull- white. 
In the month of November last year, I observed a flock 
of upwards of a hundred of this species in the Bay of Balta 
Sound, in Shetland. They remained there for two or three 
weeks, going out to sea, in search of food, regularly at a 
particular period of the tide, and returning to rest for some 
time in the Bay. During this time I had ample opportu- 
nities of observing their appearances and habits, and of 
completely confirming all the views I had previously enter- 
tained concerning them. Unfortunately I could not at that 
time procure an adult specimen, from the want of sufficient- 
ly heavy shot, the peculiarly thick-set plumage resisting, 
at any considerable distance, the effect of the smaller sizes, 
which could then alone be procured in that distant quarter 
of the country. 
It is in Unst, the most northerly island of the group; 
