WHAT CONTROLS THE NUMBERS OF SEA-BIRDS? I25 



cropping of its colonies. But the balance of populations is a very 

 subtle one, and a lengthy discussion is not called for. We merely wish 

 to draw special attention to the importance of food as a controlling 

 factor, and to suggest that more critical work ought to be done on this 

 neglected subject, and simplest of hypotheses. 



