DISTRIBUTION OF THE GULLS ATSTD TEBKS. 391 



There are, however, several remarkable instances of this apparently 

 voluntary restriction which are not to be accounted for by defi- 

 ciency or variation in food, nor even by climatic changes. These 

 cases are more frequently to be found amongst the GruUs (Larince) , 

 which, being to a great extent omnivorous, are the scavengers of 

 the shores ; whilst the Terns (Sternince) obtain their sustenance 

 almost entirely from the sea or from inland waters, and are 

 also, by their slender shape and length of wing, obviously adapted 

 for long and sustained flights. Yet even amongst the Terns there 

 are several remarkable cases of isolation and restriction ; and it 

 is in this group that we are more especially enabled to trace seve- 

 ral interesting links in the chain of dispersion accompanied by 

 gradual variation. 



It is not necessary to occupy more space with preliminary 

 remarks; but I my observe that my investigations during the 

 past seven years lead me to accept about 53 recognizable spe- 

 cies of Terns and Skimmers, 50 of Gulls, and 6 of Skua Grulls, 

 a considerable reduction from the 160 species upheld by Bona- 

 parte in his latest revision of the Longipennes. Even of the 

 accepted species, there are, however, several which are little more 

 than climatic varieties, and they will merely be alluded to in the 

 course of my remarks upon the general distribution of the groups 

 of which they form part. Nor is it my intention to lay any stress 

 upon solitary and accidental stragglers to places far removed from 

 their normal habitat, especially where these apparitions are those of 

 immature birds, which generally wander far more than adults — my 

 object being to bring forward the broad features of the geogra- 

 phical distribution of the members of this family, without dwel- 

 ling upon trifling and irrelevant exceptions. 



For convenience of treatment it will be better to commence 

 with the Stercorariince, or Skua GruUs {Lestridincd, lUiger), which 

 inhabit the Arctic and Antarctic seas. Two out of the four 

 northern species are very closely allied. The most Arctic in 

 its habitat, Stercorarius parasiticus (L.), commonly known as 

 Buffon's Skua, being an elegant long-tailed form of the stouter 

 and shorter-tailed species 8. crepidatus (Gm.), which, to avoid 

 mistakes, I call Richardson's Skua, a vernacular name originally 

 applied to a melanic variety of the species, but adopted as the 

 least liable to cause confusion. The former breeds throughout 

 the regions to the north of the Arctic circle, straying southward 

 in winter, both in the Atlantic and Pacific. Its extreme range on 



LllSrjr. JOUEIf. — ZOOLOGY, TOL. XIY. 28 



