462 STERCORARIIDA 
sains the ascendancy (Saunders). In winter the Arctic 
Skua wanders southward along the Kuropean coasts; some 
birds sojourn at the Mediterranean basin, others proceed 
along the west coast of Africa, down to the Cape of Good 
Hope, and beyond to Tasmania and New Zealand. West- 
ward the migration-route extends to California on the 
Pacific side, and the Barbadoes on the Atlantic side of 
the American Continent. Eastward this bird reaches the 
Persian Gulf. It is at once manifest that the geographical 
distribution is very extensive. 
DESCRIPTIVE CHARACTERS 
PLUMAGE.—It Is generally admitted that this Skua shows 
distinct dimorphism of plumage; in other words, perfectly 
mature individuals are not all alike. Two varieties are recog- 
nised, the uniformly dark brown bird, which, strictly speak- 
ing, is Richardson’s Skua, and the white-breasted form, more 
truly polar in its breeding-range, and hence called the Arctic 
Skua (Plate XLIX., figs. 1 and 2). Between these, every 
syadation of intermediate form exists, and is found breeding 
in many of the Northern Islands of Europe. That there is 
but one species is clearly borne out by the fact that, on 
meeting, the birds pair readily, and without distinction of 
choice of colour-markings ; indeed, with regard to the Ice- 
landic birds, Mr. Coburn, who recently obtained an abundant 
series of dark, light, and intermediate forms, from North 
Iceland, is of the opinion that two types do not exist, but 
that the white-breasted birds are simply adults, and take 
several years to reach that stage (‘ Zoologist,’ 1901, p. 416). 
Dark race. Adult male nuptial.—Back, wings, tail, and 
its coverts, umber-brown ; head, neck, breast, and abdomen, 
sooty-brown. 
Light race. Adult male nuptial.—Head, back, wings, 
and tail, same as in the dark race; hind-neck, dull white, 
streaked with dark straw-colour ; abdomen, ash-brown ; 
throat and breast, white. The lanceolate cheek-feathers 
in both races are of a yellowish tinge. 
Adult female nuptial—Similar in the respective race to 
the male nuptial plumage. 
Adult winter, male and female.—Similar in the respec- 
tive race to the nuptial plumages, but with a tendency to 
striation on the flanks, and on the upper and under tail- 
coverts, in birds not quite matured. 
