356 BRITISH BIRDS. 



11 f to 13| inches. The tarsus never reaches 2 inches in lengthy but is 

 always longer then the middle toe and claw. The middle tail-feathers 

 of adult birds are three or four inches longer than the outer ones. A 

 further distinction between the two species is to be found in the colour of 

 the tarsus, that of Richardson's Skua being black and that of Buffon's 

 Skua slate-grey. 



It is not known that there is any difference in the colour of the sexes of 

 Richardson's Skua, but the female has the two elongated tail-feathers 

 somewhat shorter than the male. As in the Poraarine Skua, tliis species 

 has a light and a dark form, which are, to a considerable extent, connected 

 Avith geographical distribution, the light form being the predominant one 

 north of the Arctic circle, and the dark one being most abundant in birds 

 breeding soutli of the Arctic circle. Under these circumstances it would 

 be more consistent to regard the two forms as subspecifically distinct ; but 

 the uncertainty attaching to any explanation of the plumages of this Skua 

 which has hitherto been attempted makes it, perhaps, premature to recog- 

 nize the distinctness of the two forms in their nomenclature. So far as I 

 am able to judge, the facts appear to be as follows : — In the adult of the 

 dark form the whole of the plumage is an almost uniform dark sooty 

 brown, slightly suffused with slate-grey on the upper parts, and with a 

 bronzy yellow on the ear-coverts and sides of the neck. After the autumn 

 moult the colour of the plumage is rich and dark, but in the course of the 

 summer it fades into a paler and redder brown. In the adult of the light 

 form the slate-grey of the upper parts is a little more pronounced than in 

 the dark form; the general colour of the underparts is wliite, shaded with 

 brown on the sides of the breast, the vent, and under tail-coverts ; the 

 white on the throat extends round the sides of the neck and across the 

 lower ear-coverts almost to the nape, and is suffused with yellow. Bill 

 slate-grey, black at the tip ; legs and feet black ; irides hazel. Young in 

 first plumage are uniform sooty brown, every feather having a buff margin, 

 very cous])icuous on the wing-coverts and primaries, almost obsolete on 

 the tail-feathers. It is not known whether there is any difference between 

 the two forms in first plumage, but in the intermediate stages the greater 

 amount of white on the underparts of the light form is very conspicuous. 

 The adult plumage is probably not completely assumed until after the fifth 

 autumn moult. Young in down are uniform sooty brow^n, slightly paler 

 on the underparts. 



The light and the dark form habitually pair together; and Booth, 

 who observed both forms during the breeding-season in the north of 

 Scotland, states that in the majority of instances a light bird was paired 

 with a dark one. Saunders and Hancock are of opinion that the result 

 of this cross-breeding is a form intermediate between the two; but 

 the evidence appears to me to point exactly in the contrary^direction. 



