MIGRATION. 



49 



epoch ; there is, however, in many species a constant widening of the area of distribution, 

 in consequence of the gradual increase in the number of the individuals which compose the 

 species, which is in its results the equivalent of emigration, and might be called " constant " 

 as opposed to " spasmodic " emigration. The evidence of this past emigration must be 

 sought for in the route of the present migration. As a typical example of a species whose 

 present distribution shows indelible traces of a gradual, but ultimately a very great, extension 

 of its original range, we cannot select a better bird than the Arctic Tern {Sterna arctica). 



" The geographical distribution and the migrations of the Arctic Tern are perhaps 

 more curious and interesting than those of any other British bird ; but no ornithologist 

 appears to have understood their peculiarities or attempted to explain them. The Arctic 

 Tern appears to. have been originally an oceanic species, visiting in summer the North 

 Atlantic, and breeding in Spitsbergen, the coast of Norway, the basin of the Baltic, the 

 Faroes, Iceland, Greenland, the shores of Baffin's and Hudson Bays, and the east coasts 

 of Canada and the United States as far south as Massachusetts. In winter its range was 

 confined, as it now is, to the Atlantic. It visits the Azores, the Canaries, and the west 

 coast of Africa down to the Cape. It enters the Mediterranean as far east as the Adriatic, 

 and has been known to round the Cape and wander as far as Madagascar. On the 

 American side it ranges as far south as the coasts of Brazil, occasionally crossing the 

 isthmus of Panama to the coasts of Northern Peru. This winter range appears to have 

 always been extensive enough ; but as their numbers increased, the Arctic Terns appear 

 to have extended their breeding-range east and west. The eastern line of migration 

 apparently extends across country from the Gulf of Finland to the lower valleys of the 

 Petchora, the Obb, the Yenesay, and the Lena, down which some of the birds migrate to 

 the Arctic Ocean, following the break-up of the ice on these great rivers. The western line 

 of migration extends along the shores and lakes of Arctic America, the two streams of 

 migrants meeting at Behring's Straits, where the Arctic Tern breeds in great numbers, 

 although it apparently is unknown in the North Pacific. This geographical distribution, if 

 I have understood it rightly, is a most interesting case of the breeding-range having 

 been extended until it has become circumpolar ; but, in consequence of the old routes of 

 migration having been strictly observed, the winter-quarters remain unchanged. In point 

 of fact, the Arctic Tern has not yet discovered the existence of the Pacific Ocean, and 

 evidently regards Behring's Sea and the Bay of Panama as a couple of lakes ! " l . 



Other equally interesting examples of recent important extensions of range may be 

 found amongst European birds. The Petchora Pipit [Anthus gustavi) and the Arctic 

 Willow- Wren (Phi/ttoscopus borealis) both winter in the islands of the Malay Archipelago. 

 The natural breeding-grounds of Arctic birds wintering in this locality is North-east 

 Siberia, to which it is almost certain they were once restricted. Both species have now 

 extended their breeding-grounds until they include North-eastern Europe, where they 

 nest in company with other migratory birds belonging to the same genera ; but instead of 



1 Seebohm, * British Birds,' iii. p. 285. 



Gradual 

 extension 

 of range. 



Range of 

 Arctic Tern. 



Fly-lines of 

 Arctic birds. 



