248 BIRDS OF MIDDLESEX. 



difficult to imitate),* it is impossible to distinguish 

 them on the wing. 



The Arctic Tern is so frequently confounded with 

 the Common Tern, which it certainly very much 

 resembles, that I may be excused for departing, in 

 this case, from my rule of abstaining from descrip- 

 tions, to point out here the leading features by which 

 the two species may be distinguished. Many authors 

 give measurements of the wing, tail, and total length ; 

 but, from an examination of a great number of spe- 

 cimens of each, it appears that individuals of the 

 same species vary so much, according to sex and 

 age, that these measurements are practically of but 

 little use. We are also told that the Arctic Tern 

 is always much darker beneath than its congener, 

 but I have seen some Common Terns, shot in June, 

 which were quite as dark as many Arctic Terns 

 which I have examined. Again, many naturalists 

 say that while the bill of the Common Tern is red, 

 with a black tip, that of the Arctic is invariably red 

 throughout its whole length ; but this is only the 

 case with fully adult birds. The young of both 



* Mr. W. Thompson remarks that the cry of the Common 

 Tern resembles the sound "pirre," and that of the Arctic, 

 " che-eep, cheep, or chip" when uttered quickly ; while that 

 of the Roseate Tern sounds like "crake,'" uttered in a hoarse 

 grating key. See * Nat. Hist, of Ireland,' vol. iii., pp. 273, 

 284. 



