STRUCTURE AND SEA-BIRDS OF THE N. ATLANTIC 25 



hundred and forty-one. Of these one primary sea-bird, Alca impennis 

 the great auk, and one secondary sea-bird, Camptorhynchus labradorius 

 the Labrador duck, are now extinct. Of the survivors, eighty-two 

 primary and thirty secondary sea-birds actually nest, or have nested, 

 on or near a North Atlantic or Mediterranean shore or a shore of that 

 part of the Arctic (north of the Circle) that communicates directly 

 with the North Atlantic (this brings in six arctic species: ivory-gull, 

 Ross's gull, little auk, white-billed northern diver, brent-goose and 

 Steller's eider). Two further species {Larus pipixcan and L. Philadelphia, 

 see above) are purely inland breeders. 



Most remarkably, the number breeding on the Old World and 

 New World sides is almost exactly the same. We can derive the following 

 summary of breeding-species from Appendix A ; the totals include 

 the two North American purely inland species, and the two extinct 

 species. Doubtful ("?" in the Appendix) and casual cases are deliber- 

 ately included — most of them are from tropical West Africa north of 

 the equator where the breeding of the species in question seems likely 

 but, owing to the scanty exploration of the coast, is not formally 

 proved. 



We can see that if we add the six purely arctic breeders to those 

 species which are common to both east and west sides of the North 

 Atlantic, we have fifty-five, out of a total of ii6, or about half. Of 

 the remaining sixty-one species, 24 breed only on the west side of 

 the North Atlantic, four on the west side and in the Arctic; and six 

 purely on the east side, and seven on the east side and in the Arctic. 

 Those on the east side include four *sea-birds' which breed in the 

 Mediterranean area but not in the North Atlantic (the crested pelican, 

 pigmy cormorant, Mediterranean black-headed gull and the lesser 

 crested tern) . 



The general conclusion is of considerable ecological interest, 

 showing how exactly the sea-bird communities of both sides reflect 

 one another. Although only about two-thirds of the members of the 

 community on one side of the Atlantic are found in that of the other, 

 the species comprising the remaining third 'balance each other' and 

 occupy very much the same 'niches' or places in nature. Opposite 

 species which pair off by occupying similar niches are grouped together 

 in the list in the Appendix, p. 292. 



