STERNA. 253 



b. Under wing^-coverts white or pale grey. 



a\ Wing- about 9 inches ; tarsus '9 inch S. hybrida. 



6\ Wing about 8 inches ; tarsus '65 inch S. nigba. 



B. Fork of tail more than 1 inch deep ; length of wing from 6^ 

 to 16 inches. 



c. Upper parts nearly black S. puliginosa. 



d. Upper parts mostly grey. 



c^. Wing more than 11 inches; tarsus more than 1 inch. 

 a^. Wing more than 14 inches; tarsus more than If 



inches S. caspia. 



h'-. Wing less than 13 inches ; tarsus less than 1|- inches. 



a^. Culmen more than 2 inche,<! S. cantiaca. 



b^. Culmen less than 2 inches S. anglica. 



d\ \Ving under 11 inches; tarsus less than 1 inch, 

 c'-. Wing 9 to 11 inches ; tail 5^ to 9 inches. 



c^. Wing and tail about equal length S. dougalli. 



d^. Tail one third shorter than the wing. 



fl^ First primary with the grey stripe along 

 the shaft of the inner web no broader 



than the outer web S. arctica. 



6*. First primary with the grey stripe along 

 the shaft of the inner web twice as 



broad as the outer web S. hiritndo. 



d'^. Wing 7 inches or less ; tail 4 inches or less S. minuta. 



not appear to be any difference in colour between the three Marsh-Terns, and scarcely any 

 structural differences, except those of size, which are very insigniticaut. The White- 

 winged Black Tern varies in length of wing from 7| to SJ inches, and its tarsus measures 

 f inch. The Whiskered Tern has generally a longer wing and a longer tarsus ; whilst the 

 Black Tern has a wing of the same length as that of the White-winged Black Tern, but 

 a shorter tarsus, measuring only f inch long. These measurements generally suffice to 

 determine the species of European examples, but small examples of the Whiskered Tern 

 are indistinguishable from large examples of the White-winged Black Tern, unless traces 

 of summer plumage can be found on the imder wing-coverts. 



