RARER BRITISH BIRDS. 
69 
The resemblance between this species and the common Tern 
is very close ; and indeed we may say, that without a very 
minute examination it is next thing to impossible to distinguish 
them. We did not recognise the specimens obtained at the 
Skerries until after our return home. 
In plumage, the Arctic Tern agrees very nearly with the 
common Tern in all its stages, but the comparative measure- 
ments of the bill and tarsi furnish good and invariable cha- 
racters. The length of the gape in the Arctic Tern is one inch 
and three-quarters, that of the tarsi barely five-eighths ; while, 
in the common Tern, the depth of the gape is two inches, and 
the length of the tarsi seven-eighths. The lower plumage of the 
Arctic Tern, during the breeding season, is of a pearly grey, 
nearly as dark as that of the back. In the common species, 
the lower plumage is white, or has merely a very slight shade 
of grey on the wings. 
