Il6 SEA-BIRDS 



regions. Thirdly, they may occupy different habitats in the same 

 region. And, fourthly, they may live in the same habitat and region 

 but eat mainly different foods. Lack points out that the last position 

 is often reached when the two species differ markedly in size. His 

 analysis was confined to passerine birds, for which habitat-distinc- 

 tions are much more clearly marked than they are for sea-birds; 

 and we know of no closely-related species of sea-birds which do, in fact, 

 clearly occupy different habitats in the same region in this way. 

 The occupation of different kinds of nest-site on the breeding-cliffs 

 is very closely marked among sea-birds. Every species appears to 

 have its special niche. Although there remains apparent competition 

 for nest-sites at least between certain species at most sea-bird stations, 

 it is a rule that the denser the population of the station, and the greater 

 the number of species occupying it, the keener and more definite 

 does this nest-site selection become. Among the petrels this may result 

 in a division of the year into separate breeding seasons between species 

 using the same nesting burrow (Lockley, 1952). It would be interesting 

 to collect their parasites. 



A similar state of affairs appears to exist with the food of sea-birds. 

 Indeed, the situation gives the impression that there is a kind of non- 

 intervention agreement among the sea-birds by which each has chosen 

 its own particular kind of nest-place and its own particular kind of 

 food so as to avoid competition with the other species. The question is 

 how this state of affairs has come about in the course of evolution. 



A full answer seems impossible to give over the question of the 

 differential nest-site selection, at least in the present state of theory, 

 though a very great many facts have been collected and are still being 

 collected about the nest-site shortage on the breeding-grounds of 

 sea-birds. 



To the question of the differential feeding habits, a clear answer 

 is suggested by the work of Mayr and Lack. In general terms it is that, 

 unless two forms derived from the same parent-species which meet 

 together on the same area are already at least partly differentiated 

 as regards their feeding-habits, one will ehminate the other from the 

 area. If this is true — and there is every reason from the evidence to 

 believe that it is — then the origin of a species will depend on its pos- 

 sesion of a special pattern of food-preferences. We will find that the 

 population of the species will depend also, in normal circumstances, 

 on the natural supply of the food of that pattern. Indeed, we will dis- 

 cover that only under the interference of quite exceptional agents, 



