THE SKUA GULLS AND THEIR RECENT MIGRATION. 93 
plumages. I went up to Shetland last July, with the object of 
studying the young in down and in their first plumage, and 
arrived at some tolerably clear conclusions; but there are still 
several points which could not be cleared up by one investigator, 
nor in a single season. It may confidently be stated that there 
are two extreme varieties in the adults—the one having the 
breast and under parts of the same dusky hue as the back; and 
the other in which the under parts are white. These two varieties 
breed together, and pairs are found consisting of two white- 
breasted birds, or of one white and one dark bird, or of two dark 
birds. Where light forms are mated, the young are perfectly 
recognisable, and the same is the case with the offspring of two 
wholly dark birds; the young of a light and a dark form are also 
distinguishable ; but the question to be solved is, What are the 
offspring of these “mixed marriages” on attaining maturity ? 
Each pair has two young, but it is not altogether easy to find out 
whether one of these resembles one parent-and one resembles the 
other, because these young, from the downy stage upwards, leave 
the shallow “nest,” if it may be called so, and are to be found 
squatting about on the moors so far apart as to preclude any 
positive identification. The question can only be solved by 
selection of the young of such a pair, and by keeping them in 
captivity for several years; and it is to be feared that this 
experiment will not be carried out for some time to come. I do 
not think that the dark plumage is a sign of immaturity, or that 
birds which breed in this plumage will ever become light-breasted, 
for the dark breeding birds have the acuminate feathers on the 
nape of quite as burnished a yellow colour as the light forms, and 
that is a sure sign of age in all the Skuas; but whether the half- 
bred birds which are found breeding with partially white breasts 
ever lose their gorget and become entirely white, is still unknown 
to me. 
There are few more interesting sights than a breeding-place 
of Richardson’s Skua. As the visitor traverses the wild ionnanae 
furrowed by the rain with innumerable channels, sharp-winged 
angular forms rise against the sky, and are seen skimming the 
heather with a flight which has been described as “ hawk- like,” 
but which differs in its peculiar “balancing,” as distinct from 
“hovering,” from that of any other bird. As the intruder 
advances, a loud “ mee-ah” is heard, and this may be followed by 
