WHAT CONTROLS THE NUMBERS OF SEA-BIRDS? Iig 



shallow-water crustacean Adysis oculata; the glaucous and ivory-gulls 

 of offal ; the eider of a crustacean Gammarus locusta and lamellibranch 

 molluscs; BriJnnich's guillemot of a prawn Spirontocharis gaimardii; 

 the black guillemot oi^ Mysis and gasteropod molluscs; besides various 

 other shallow- water creatures including butter-fish; and the puffin 

 of fish. Unfortunately we did not sample enough little auks to be 

 quite clear as to their preference, though it appeared to be entirely 

 crustaceans. Of the offal-eaters the fulmar ate a wide range of other 

 organisms besides Thysanoessa; the glaucous gull ate a few Euthemisto; 

 the ivory-gull ate nothing but Thysanoessa and offal; but every one of 

 the species, as far as could be detected, had a different food-spectrum 

 in spite of the abundance of Thysanoessa. 



It is clear that under conditions of super-abundance, species 

 that would otherwise compete can, at least temporarily, share the same 

 main food. To give another example: in north-central Iceland is 

 a large shallow lake, Myvatn, of quite exceptional fertility, in which 

 the hatch of insects in June is one of the marvels of nature. Myvatn 

 is probably the finest duck lake in the world: it supports a population 

 of several tens of thousands of ducks belonging to about ten established 

 species (fourteen have bred by or near the lake in the present century). 

 Almost every indication of differences in nest-site choice and food- 

 spectra between the ducks of Myvatn has disappeared, and in places 

 their nests are placed almost in rows, apparently indiscriminately, 

 in the grass and willow cover at the edge of the lake. All feed apparently 

 on the same supply of chironomid insects. Often they appear to make 

 mistakes and lay eggs in each other's nests. Again: it is a common 

 sight in a large colony of auks to find puffins, guillemots, razorbills 

 feeding together on shoals of sand-eels and small fry (perhaps joined 

 by other sea-birds such as gulls and shearwaters) which swarm inshore 

 during June, July and August in British waters. But it remains prob- 

 able that even under the special conditions in both examples quoted, 

 some differences in food-spectra persist, as well as the normal differ- 

 ences in feeding actions. 



There is, in the wide an scattered a iterature, already a good 

 deal of information of a qualitative, if nOt of a quantitative kind, 

 which tells us about differences in the food-patterns of closely related 

 sea-birds; and certainly serves to indicate that in all cases those patterns 

 are different except perhaps under the rare and exceptional conditions 

 of food super-abundance. It is true, however, that no deliberate and 

 quantitative research has so far been done on certain obvious pairs 



