COMMON AND ARCTIC-TERNS 79 



England coast. 1 At the Fame Islands both species breed on the 

 same stretch of sand and gravel. They also breed together at Walney 

 Island in North Lancashire. The common-tern may once have been 

 a purely fresh-water species, and the Arctic purely maritime, but to 

 assume this does not help us to explain their divergent evolution. 



When we turn to the difference in their geographical distribution 

 we come upon one of the most remarkable recent discoveries in this 

 branch of ornithology ; it is that the range (summer and winter) of the 

 Arctic-tern has a wider latitude than that of any other vertebrate 

 animal. It has been met with not only among the ice-blocks on the 

 road to the North Pole, at as high a latitude as 82, but as near to 

 the South Pole as 74 1' latitude, attracted there especially by the 

 surface-swimming crustaceans that abound in the Antarctic seas 

 during the southern summer. 2 It has been found in every ocean. 

 Its range includes that of its congener, which lies well within the 

 temperate and tropical zones. But, again, the fact that the Arctic, 

 alone of the two species, ranges into the frigid zones, north and 

 south, leaves us still in the dark as to the cause of its specific 

 peculiarities. Yet unless the explanation is to be found in this fact, 

 the search elsewhere seems hopeless, for between the present habits 

 of the two species, to which we now turn, there is no difference that 

 can be called important. 



The common-tern arrives earlier at its summer quarters, the time 

 varying with the season and the climate. It falls generally about 

 the end of April. 3 The Arctic appears early in May. Both species 

 migrate usually in bands, and, like the black-tern, in a leisurely 

 manner, descending to fish in any promising water that presents 

 itself. They journey both by day and night, and at a great height. 



1 Auk, xii. 33-48. 



8 Ibis, 1907, p. 345 (W. E. Clarke on the " Results of the Scottish National Antarctic 

 Expedition"). The Antarctic birds may prove to be a local race or subspecies. See " Classified 

 Notes," p. 63, footnote. 



8 For definite dates see the " Classified Notes." At Muskeget the earliest common-terns 

 arrived during the years 1892-95 as follows : 1892, May 10 ; 1893, May 8 ; 1894, May 8 ; 1895, May 1 

 (Auk, xii. 33 ; xiii. 47). The late dates are due, no doubt, to the fact that the New England coast 

 has a colder climate than the British Isles, though situated in a more southerly latitude. 



