PELAGIC BIRD FAUNAS OF THE INDO-PACIFIC OCEANS 477 



place is unknown. A bird containing an unlaid egg was shot of£ central 

 Peru and if the breeding station is adjacent this w^ould be a parallel 

 case in the Pacific to O. castro in the Atlantic of a transgression of the 

 breeding range through the equatorial belt. This cluster of forms is 

 suggestive of a multiple invasion, via the Bering Gap at each inter- 

 glacial period, with a southward extension of speciation in the following 

 glacial, the post-glacial form leiicorJwa not being differentiated. 



TERNS 



Sterna albijrons and S. nereis 

 Sterjia albijrons appears to be plainly a northern element, with a 

 breeding distribution from the British Isles and the Baltic coasts, south- 

 wards, its habitat including rivers as well as marine shorelines. It has 

 disseminated into the Indian Ocean through the Mediterranean-Indian 

 Ocean water ways and the river systems of Asia, reaching the periphery 

 of the western Pacific partly through the Malayan water gap. It now 

 breeds in New Guinea, the Philippines, and neighbouring islands and 

 coasts. In the Australian region it has produced a derivative, S. Jiereis, 

 which is still strictly allopatric with it, and which has spread some 

 distance further into the Pacific— to New Zealand and New Caledonia. 



Sterna dougallii 

 This species is probably a member of the northern element, with 

 its origin perhaps in the North Atlantic. It has failed to pass into the 

 Pacific by the Panama water gap but has penetrated into the Indian 

 Ocean by the Mediterranean-Indian Ocean Gap, and into fringes of 

 the western central Pacific through the Indo-Malayan gap. 



Sterna hirundo and S. macrura 



Sterna hirundo migrates a considerable distance south along the 

 western Pacific shoreline from its northern breeding grounds, regularly 

 visiting eastern Australian waters, 5. macrura has a much more extensive 

 migration in the Atlantic sector; in the southern hemisphere it deploys 

 into the southern Indian Ocean. 



Other northern terns have limited movements along the Pacific 

 coasts, and cannot strictly be considered pelagic birds. 



GULLS 



Many northern gulls occuj^y the coasts of the north Pacific, but 

 none of them has established breeding stations on the numerous islands 

 in mid-ocean or included them in their migrations. Lams glaucescens 

 is an occasional wanderer to the Hawaiian Islands. 



The relationship of the northern gulls to the southern hemisphere 

 forms was briefly mentioned in the earlier section on Southern Faunas. 



