120 SEA-BIRDS 



actually in the zones where the two species overlap. Thus no serious 

 work has been done on the food of the arctic and common terns in 

 the fairly wide zones of overlap in both New and Old Worlds, nor 

 is there any detailed investigation of the different foods of the different 

 divers or Leach's and storm-petrels, or of the different tropic-birds and 

 frigate-birds, or, rather surprisingly, of the herring- and lesser black- 

 backed gulls as a straight comparison, or of BrtJnnich's guillemot 

 and the common guillemot. 



In the tropics the three boobies — the blue-faced booby, the brown 

 booby and the red-footed booby — overlap considerably, sharing many 

 of the same breeding-places and feeding-grounds, e.g. round Ascension 

 Island. They differ in ecology quite markedly. Thus, the blue-faced 

 booby nests on fiat ground, making only a scrape, the brown booby 

 on the ground but usually on eminences and then with considerable 

 nest-material (it may also perch but not nest on trees), while the red- 

 footed booby nests exclusively on trees and bushes and is limited 

 to breeding on islands which can provide them. While their food 

 habits have not been closely compared it seems clear, from the account 

 of Murphy (1936) that they are different; the blue-faced booby appears 

 to be primarily an eater of small squids, the brown booby an eater of 

 flying-fish and many other species of surface-fish, the red-footed booby 

 primarily a diurnal eater of flying-fish but also a nocturnal feeder on 

 squids. 



If we study the skuas, gulls and terns we find perhaps the most 

 complicated food-systems and habitat-selection of any of the sea-birds, 

 and it will be a long time before research has sorted out the real 

 differences which undoubtedly exist between the different species and 

 forms. Two similar species overlap geographically very often, and there 

 are several cases where three similar species overlap, and one case 

 where four do so. In the southern United States, for instance, the com- 

 mon tern overlaps with the closely allied Forster's tern, and in northern 

 North America with the closely allied arctic tern. In many parts of 

 the arctic all three species of the smaller skuas breed in the same general 

 geographical area. In Greenland the glaucous gull overlaps with 

 the very similar Iceland gull or Greenland herring-gull, and with 

 the more closely related, but not so similar, greater blackback. In 

 Iceland the great blackback and glaucous gull overlap, and recently 

 they have been joined in the same general geographical area by the 

 herring-gull and lesser blackback, though so far there is no record of 

 all four species nesting together on the same cliff. 



