STERNA. 253 



b. Under wing-coverts white or pale grey. 



a 1 . Wing about 9 inches ; tarsus '9 inch S. HYBRIDA. 



b l . 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. FITLIGUNOSA. 



d. Upper parts mostly grey. 



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

 a 2 . Wing more than 14 inches; tarsus more than If 



inches S. CASPIA. 



b~. Wing less than 13 inches ; tarsus less than 1^ inches. 



a 3 . Culmen more than 2 inches S. CANTIACA. 



b 3 . Culmen less than 2 inches S. ANGLICA. 



rf 1 . Wing under 11 inches; tarsus less than 1 inch. 

 c 2 . Wing 9 to 11 inches ; tail 5| to 9 inches. 



c 3 . Wing and tail about equal length S. DOUGALLI. 



d 3 . Tail one third shorter than the wing. 



a 4 . First primary with the grey stripe along 

 the shaft of the inner web no broader 



than the outer web S. AECTICA. 



6*. First primary with the grey stripe along 

 the shaft of the inner web twice as 



broad as the outer web S. HIRUNDO. 



d 2 . 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 insignificant. The White- 

 winged Black Tern varies in length of wing from 7| to 8J inches, and its tarsus measures 

 | inch. The Whiskered Tern has generally a longer wing and a longer tarsus ; wbilstthe 

 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 under wing-coverts. 



