76 THE TERNS 



or resemble the Arctic and common. As the latter afford the standard 

 of comparison, they are taken out of their order and placed first. 



Though easy to distinguish when in the hand, chiefly by means of 

 the marked difference in the breadth of the dark stripe on the inner 

 web of the long quills (see the figure on p. 61), the Arctic and 

 common-terns are so much alike in general appearance that they 

 are sometimes confused when seen free in their native haunts. As 

 incorrect identification has for result incorrect statements as to 

 geographical distribution, it is worth while to note briefly the char- 

 acters likely to aid the naturalist in distinguishing the species, when 

 he is unable to handle them. The most reliable is the coloration of 

 the bill. In the Arctic form this is blood-red, except for an occasional 

 small black line on the ridge of the upper mandible at its tip, which 

 may, for practical purposes, be ignored. The bill of the common-tern 

 is dusky coloured for about a third of its length from the tip, the basal 

 two-thirds being coral-red. The shorter tarsi of the Arctic constitute 

 another difference, but it is of little use unless both species are under 

 observation at the same time, except to those to whom they are 

 familiar. Differences in the shade of the beautiful silvery grey of the 

 plumage are of uncertain value for identification, because they vary 

 for the eye with each change of light. By some the relative length 

 of wing and tail is held to afford a reliable means of distinction. 

 This is based on the assumption that when the common-tern is 

 sitting or standing the tips of its wings reach either as far as, or 

 beyond, the extreme tips of the tail, whereas in the case of the 

 Arctic the wing-tips fall somewhat short of the tail-tips. There can 

 be no doubt that, on the average, the common has, for its size, longer 

 wings and shorter tail streamers, as the following figures show : 



1. (H. SAUNDERS l ). 2. (NAUMANN *). 



Common. Arctic. Common.* Arctic. 



Length (full) . . 14-25 14'50 inches 30'6 to 35'30 34 to 39 c.m. 



Tail .... 6-50 7'50 147 15'8 17 19 



Wing . . . 10-50 10-0 26-50 27-10 26 27 



1 Manual of British Birds, 2nd ed., pp. 648, 650. ' Vogel Mitteleuropas, xi. 128, 138. 



