44 



SEA-BIRDS 



Fig. 5 

 Breeding distribution of Larus canus,. the common gull, and the closely 

 related L. delawarensis, the ring-billed gull 



interbreed, nor apparently compete. Perhaps it is recapitulating some 

 of the early stages in the origin of Briinnich's guillemot. To some 

 extent Briinnich's guillemot, with its razorbill-like beak, appears to 

 replace the razorbill in the arctic, where it may occupy the same 

 ecological (feeding and breeding) niche in relation to the common 

 guillemot as the razorbill does in relation to that bird in the south 

 part of the common guillemot's range. 



The student of variation will find much material for his researches 

 among the North Atlantic sea-birds. Several species of North Atlantic 



