WHAT CONTROLS THE NUMBERS OF SEA-BIRDS? II5 



(1952), and all that it is necessary to say here is that there is a real 

 possibility that the fulmars may have contracted the psittacosis from 

 a dead parrot or parrots. However, while this disease has killed quite 

 a number of human beings who have split or eaten infected young 

 fulmars in September, there is no evidence that it has made any serious 

 difference to the actual fulmar population. It is not even proved that 

 there is a large mortality among the young fulmars, some at least 

 of which appeared to recover. A strain of ornithosis of another kind 

 has been found in herring-gull, lesser blackback, laughing gull, 

 royal tern and little tern. 



From this short survey of the possibilities we find that man most 

 importantly controls the number of sea-birds; but that the only 

 other organisms in a long list of predators and parasites that may also 

 do so in any serious way or to any significant extent are (under special 

 circumstances) rats and possibly a virus or viruses. But it is quite 

 certain that none of the other animals which live on or in sea-birds 

 can possibly be responsible for the changes in sea-bird populations 

 which are clearly going on all the time. 



There remain three other factors to investigate; for it is possible 

 that the numbers of sea-birds may be limited by the availability of 

 nest sites, by climate and weather, and by the availability of food. 



Before we proceed to a further discussion of these points we must 

 remind the reader of an important principle, and for the first time, 

 state another important principle. The principle already stated (p. 37) 

 is that so carefully established by E. Mayr (1942) and D. Lack (1944) — 

 that in the course of evolution new species of birds originate when 

 forms of the parent species differentiate in geographical isolation and 

 subsequently meet in the same area. The second principle, which we 

 have not so far encountered, was stated by the Russian zoologist 

 G. F. Gause in 1934. It is that two closely related species with identical 

 ecology cannot live together in the same place. Generally speaking 

 throughout the world of animal ecology these two principles have 

 been accepted, though not without some reservations by critical 

 workers such as W. H. Thorpe (1945). 



As Lack states, when two such differentiating forms of the same 

 parent-species meet, they will tend to compete ecologically but will 

 eventually reach one of four more stable positions. First, one species 

 may eliminate the other completely (this is likely to happen if they 

 still have the same fundamental ecology). Secondly, the two may 

 withdraw once more to occupy separate but contiguous geographical 



