WHAT CONTROLS THE NUMBERS OF SEA-BIRDS? Ill 



terns, kittiwakes and even gannets, to force them to vomit fish, which 

 is then seized by the pursuer. Seldom do these parasites (a truer 

 word for these particular birds than predators) actually kill their prey. 

 However the bonxies, or great skuas, frequently hunt and drown 

 the local kittiwakes, as R. Perry (1948) has observed, dropping on 

 top of them and forcing them down on to the water, often working 

 in couples. The great skuas of Noss and Foula indulge in predation of 

 kittiwakes, as well as guillemots, until the gannets are feeding young, 

 and do not take to parasitism of their main host until then. They 

 also chase great blackbacks and herring-gulls, and make them vomit. 

 The great skuas, like the great blackbacks, watch the cliffs for the 

 young guillemots on their first flights to the sea. Both also take fledge- 

 ling kittiwakes and fledgeling herring-gulls. The arctic skuas of Noss 

 get most of their prey by harrying kittiwakes, guillemots and terns 

 over the sea a couple of miles from the island. 



The herring-gull is an important predator of other sea-birds, 

 although it has not the aggressive qualities of the great blackback. 

 It is particularly good at robbing gannets of their egg, taking quick 

 advantage of any unusual disturbance in a gannetry, such as the 

 arrival of human beings. Fulmars are much more difficult to dislodge 

 from their eggs than are gannets, but once human investigators have 

 put the fulmars off, the ever- watchful herring-gulls snap the eggs up. 

 The herring-gull is probably the worst egg-robber of the bird-cliffs, 

 and it will take nestlings of any species of gull (including its own), 

 of terns and of the auks. 



The lesser blackback, though not quite so aggressive as the herring- 

 gull, has been known to kill adult puffins and Manx shearwaters, and 

 the young of many cliff-breeding birds. The great blackback eats 

 everything so far mentioned. It will smash eggs by taking them up 

 to a height and then dropping them. In general, however, most of 

 of these gulls, predatory though they are, seek their living away from 

 the colony. Often very large colonies of herring-gulls, lesser blackbacks 

 and great blackbacks — sometimes even all three — are situated at the 

 top of a teeming bird-cliff. If all the gulls were to rely only on what 

 they were to get from the bird-cliff the community might collapse. 

 They should be properly described as opportunist predators, who 

 earn their main living elsewhere, — nowadays, as we have already 

 pointed out (p. 104) — very largely as parasites upon the fishing industry 

 of the coast. 



In the Arctic the glaucous gull to a large extent takes the place 



