WHAT CONTROLS THE NUMBERS OF SEA-BIRDS? II7 



such as man, or exotic species of animals, introduced either consciously 

 or unconsciously by man, or sudden cataclysms of climate, will the 

 fundamental numbers of a species be determined by any other factor 

 than food-supply. 



Of course, the food-supply often depends directly on other factors 

 such as the weather; and upon the available food supplies the gradual 

 processes of history and evolutionary change are also working. But, 

 owing to the uniqueness of every species' food-pattern, direct competi- 

 tion with other related species may not be a normal factor in population 

 control! When such direct competition is found, as it is occasionally 

 under peculiar circumstances, it is perhaps the exception that proves 

 the rule. 



The first case that we can discover in the literature which precisely 

 states the existence of specific food-patterns among closely related 

 sea-birds is an analysis by the Russian zoologist A. N. Formosov, 

 quoted by the pioneer Gause in his Struggle for Existence. In 1923 

 Formosov examined a mixed colony of terns consisting of many 

 hundreds of each of four species on an island on the west side of the 

 Crimean peninsula. Slightly paraphrasing Formosov's notes, he writes 

 that, "the nests of the terns are situated close to one another and the 

 colony presents a whole system. The entire population of the colony 

 belongs to four species, the Sandwich tern, the common tern, the 

 gull-billed tern and the little tern, and together they chase away 

 predators, e.g. hen-harriers, from the colony. However, as regards the 

 procuring of food there is a sharp difference between them, for every 

 species pursues a definite kind of animal in perfectly definite conditions. 

 Thus the Sandwich terns flies out into the open sea to hunt certain 

 species of fish, the gull-billed tern feeds exclusively on land and can 

 be met in the steppes at a great distance from the sea shore, where it 

 destroys locusts and lizards. The common tern and the little tern catch 

 fish not far away from the shore, sighting them while flying and then 

 falling upon the water and plunging to a small depth. But the light 

 little tern seized the fish in shallow swampy places, whereas the common 

 tern hunts somewhat farther from the shore. In this manner these 

 four similar species of tern living side-by-side upon a single small island 

 differ sharply in all their methods of feeding and procuring food." 



However, it is uncommon for all these four species to nest together 

 in the same colony. Usually there is also a fairly marked difference 

 in their selection of nest-sites. For instance, the little tern is more 

 inclined to nest on shingle than the others and the gull-billed tern 



