48 Field Museum of Natural History — Zoology, Vol. IX. 



*GROUP 2. Wing, 7.50 to 8.50 inches long. 



Head and under parts, black (adult). A patch of black behind 

 the eye ; back of head, dusky ; rest of head, white (immature ) . 



Hydrochelidon nigra surinamensis . 

 Black Tern. 

 See No. 29. 



*GROUP 3. Wing, 8.50 to 9.50 Inches long. 



Inner web of outer tail feather, gray; entire under parts, white. 



Sterna forsteri. 

 Forster's Tern. 

 See No. 25. 



*GROUP 4. Wing, 9.50 to 11 inches long. 



Outer web of outer tail feather, darker than inner web; back, 

 pearl gray; breast, washed with pearl gray; bill in adults, red, tipped 

 with black; tarsus, usually over .70. Sterna hirundo. 



Common Tern. Wilson's Tern. 

 See No. 26. 



Inner web of outer tail feather, darker than outer web ; back, pearl 

 gray; bill in adults, black, slightly yellowish at tip ; entire under 

 parts, white. Sterna forsteri . 



Forster's Tern. 

 See No. 25. 



Outer web of outer tail feather, darker than iimer web ; back, pearl 

 gray; breast and under parts, pearl gray; bill, in adults, entirely red; 

 tarsus, usually less than .70. Sterna paradiscea. 



Arctic Tern. 

 See No. 27. 



* GROUP 5. Wing, 11 to 13 inches long. 



Bill, black, not tipped with yellow; feet, blackish; back, pearl 

 gray; bill, comparatively short and stout. Gelochelidon nilotica. 



GuU-billed Tern. 

 See No. 22. 



* For directions for measuremeilt, see page 21. 



