JAN., 1909. BIRDS OF ILLINOIS AND WISCONSIN CORY. 45 



Under parts, white; inner web of primary, white, broadly tipped 

 with black; a small rudimentary hind toe, without nail. 



Rissa tridactyla. 

 Kittiwake Gull. 

 See No. 12. 



Under parts, white; terminal portion of first primary, black, with 

 white spot near tip. Larus delawarensis. 



Ring-billed Gull. 

 See No. 17. 



Primaries, pale pearl gray, becoming white at tip; bill, over 1.50; 

 wing, over 14. Larus leucopterus. 



Iceland Gull. 

 See No. 14. 



*GROUP 4. Wing, 15 to 17 inches long. 

 SECTION 1. Length, over 21; bill, over 2; under parts, white. 



No black on primaries (adult). Larus hyperboreus. 



Glaucous Gull. 



See No. 13. 



Back, slaty black; primaries, black with more or less white, 

 (adult). Larus marinus 



Great Black-backed Gull. 



See No. 15. 



Back, pale bluish, gray, or pearl gray; primaries, dull black or 

 brownish black, more or less marked with white. 



Larus argentatus. 

 American Herring Gull. 

 See No. 16. 



SECTION 2. Length, over 21; bill, over 2; under parts, not white. 



Outer webs of primaries, ash color (immature ) . 



Larus hyperboreus. 

 Glaucous Gull. 

 See No. 13. 



*For directions for measurement, see page at. 



