ly^ SEA-BIRDS 



the centre as the old birds die and leave vacant places. In the centre 

 lies the greatest safety for the individual. 



The smaller the flock the greater the exposure of the individual 

 to peripheral dangers until, when a colony of only one or two pairs 

 is reached, the core is wide open to those dangers. Conversely there 

 is protection in numbers — and there is also stimulation. It is well 

 known that in certain species (the budgerigar is quoted as a classic 

 example) two or three pairs together breed and produce more young 

 per adult than one pair breeding alone. Darling has suggested that 

 there is a threshold below which a social species breeds irregularly 

 or fails to breed, although he admits that this concept must be applied 

 warily, since the numerical threshold varies with the environmental 

 complex. Stimulation and protection in breeding by isolated pairs of 

 a social sea-bird may, however, come from related or unrelated 

 species using the same breeding ground; thus two pairs of gannets 

 have bred successfully for many years, although without increasing 

 their breeding numbers there, in the centre of a large colony of guille- 

 mots (Saltee Islands) ; and the reverse is recorded — a few pairs of 

 guillemots breeding successfully within a large colony of gannets 

 (Grassholm, and also Alderney). It is generally accepted that small 

 colonies of guillemots and razorbills, gulls and other sea-birds produce 

 fewer young per head of breeding adult than large colonies. It has 

 been suggested that this may be because small colonies are composed 

 of younger birds ; if this were invariably so then it would argue that 

 individuals, as they grow older, move from the small colonies to the 

 large! This, however, is not the case; we have known small colonies of 

 shearwaters, and of guillemots and razorbills at Skokholm which have 

 existed for decades (within a few hundred yards or less of large colonies) 

 with their hard core of old ringed breeders. Their breeding success has 

 been variable, fair in the shearwaters, poor in the guillemots; but 

 habit compels them to stick to the old site, whatever the success. 

 Single pairs of guillemots and razorbills have generally had no breed- 

 ing success. It is true however that small new colonies of sea-birds 

 are invariably composed of young birds, which are always the colonists, 

 and they take several years (and sometimes fail in the end) to build 

 up a permanent breeding core. The fulmars which are spreading to 

 new sites in western Europe are probably all young birds which have 

 not bred before; also, for years Great Saltee Island was visited by 

 immature Manx shearwaters before breeding took place there recently. 

 How does the advantageous stimulation of the large flock work? 



