CHAPTER 4 



WHAT CONTROLS 

 THE NUMBERS OF SEA-BIRDS? 



ALTHOUGH MAN is SO much the most important predator of sea- 

 birds, except in very inaccessible places such as the Antarctic 

 Continent and some of the Subantarctic islands, the existence of other 

 animal predators should not be overlooked. The number and import- 

 ance of these is not great, and the effect of these predators upon the 

 population of their prey is small; but, on the other hand, the effect 

 of the numbers of the prey upon the numbers of the predators may 

 be marked. It is abnormal for a predator to take a crop from its 

 prey which is more than that prey can easily spare. At every important 

 sea-bird station there is normally a small, but often varied community 

 of animals which are predators on the individual members of the 

 main community or their eggs and young. At a typical British island 

 bird rock the predators are all birds. Ravens snap up young that fall 

 from the cliffs or adults which meet with flying accidents (below the 

 cHffs of Ailsa Craig we have seen a raven eviscerating a still living 

 gannet that had been starving for some days with a broken wing). 

 Hooded and carrion-crows nest, like the ravens, on the cliffs of many 

 sea-bird stations, and poach eggs and pick up what they can get. 

 Peregrine falcons also nest on the cliffs and prey upon the inmates. 

 At a peregrine eyrie on Skomer, in 1946, we found two eyasses sitting 

 among the remains of eleven Manx shearwaters and two puffins. 

 Even the short-eared owl may occasionally nest on British islands 

 and prey upon the sea-birds. One of the pair nesting on Skomer 

 used to operate to the neighbouring bird island of Skokholm, where a 

 pellet was found containing the remains of a storm-petrel; and on 

 Skomer the newly-arrived little owls also have persecuted the storm- 

 petrels. But the most important animal predators of sea-birds are 

 other sea-birds. In Shetland the arctic and great skuas of Noss chase 



