THE GULLS 



233 



Fig. 40 



Breeding distribution of the ivory-gull, Pagophila 



eburnea 



{Xema sabini), nesting 

 in swampy tundra along 

 the arctic shore, is 

 often found close to, 

 or among breeding 

 arctic terns. However, 

 there are so many other 

 factors to be considered, 

 such as the relative 

 abundance of each 

 species from year to 

 year (this appears to be 

 unstable; in some years 

 the terns do not re- 

 appear in the old haunt), 

 and the dominance of 

 the species most suited 

 to the environment. 

 Palmer ( 1 94 1 ) even sug- 

 gests that the presence 

 of gulls is detrimental 



and eventually drives the terns away. The herring-gulls at Hirs- 

 holmene were not in a typical environment. A numerous herring- 

 gull colony, in a typical sloping cliff site or Baltic sand-dune, may 

 dominate the breeding area to the virtual exclusion of all other gulls, 

 as at Lambay Island cliffs, Ireland, where in 1939 we estimated 

 10,000 pairs present, with only 40 pairs of lesser black-backs and 20 

 of great black-backs. Such dominance can of course only occur in 

 colonies at or near the centre of the geographical distribution in 

 summer; thus on the edge of its distribution, in the Faeroe Islands, 

 the herring-gull is a rare, shy and unaggressive species (Lockley, 1938). 

 In the arctic the ivory-gull, Pagophila eburnea, is known as the ice- 

 partridge or snow-bird, living as it does constantly in the vicinity of snow 

 or ice, where its pure cream-white plumage matches its surroundings. 

 It constructs a well-insulated nest of moss about 18 to 24 inches across 

 and raised 6 to 9 inches above the ground ; but on cliffs a less elaborate 

 structure may be built of more variable materials. The nests may be 

 begun on snow- or ice-covered ledges. Two eggs are usually laid. 

 The new-born chick is covered with a white down, which later becomes 

 pearl grey; both colours are adaptively protective, the first matching 



