78 THE TERNS 



kikikerr or kikiqwerr. The qwerrr (repeated) may be uttered alone, also 

 the kik, kik kik. . . . For the Arctic, also, kikikerr, or kerr, kerr, 

 kikikikikiki kerr, or titwerr, titiwer, titiwer. The variations were noted at 

 different times, and are probably to be accounted for by variation in 

 distance and direction. The birds used the note when fighting among 

 themselves, attacking other species or an intruder. 



It is when we compare two closely allied yet well-defined species, 

 resembling each other so closely in external appearance and habit as 

 do these terns, or, to take another example, the chiff-chaff and 

 willow-wren, that the problem of the origin of specific variations 

 assumes its most puzzling form. Seeing that the chicks of both 

 Arctic and common-terns have black-tipped bills, one may assume 

 that it is the Arctic which has diverged most from the common 

 ancestor. If so, by what process or for what reason did the Arctic 

 lose, except as a mere survival, the black tip to its bill, and why has 

 it shorter legs and wings, and a narrower stripe on the primary 

 quills ? Why, on the other hand, has it become the larger species 

 and, for its size, developed a longer tail ? 



The cause probably lies in the influence of differing environment, 

 but it would be difficult to trace it either in the present habitat or 

 present geographical distribution of the two species. The only 

 important difference in habitat is that the Arctic shows a decided 

 preference for salt as compared to fresh water, but it is only a prefer- 

 ence, for the species has been found breeding on inland lakes in 

 Ireland, Scandinavia, and Arctic America. 1 The common-tern is 

 frequently found on fresh water even as far away from the sea as the 

 Swiss lakes, and one of its specific names, fluviatilis, points to its liking 

 for rivers. But that it shows a decided preference for fresh water is 

 not clearly demonstrated. Some of its largest colonies are found by 

 the sea, for instance at Bomney Marsh in Kent, by the tidal estuary 

 at Ravenglass, Cumberland, and at Muskeget Island off the New 



1 Ussher and Warren, Birds of Ireland, p. 323; H. Saunders, Manual of British Birds, 2nd 

 ed., p. 650; A. L. V. Man niche, Terrestrial Mammals and Birds of N.E. Greenland. 



