38 KEY TO THE BIRDS OF EASTERN NORTH AMERICA. 



Underparts, white; first primary, entirely black or dark brown (adult). 



Larus atricilla. Laughing Gull. 



See No. 33. 



General plumage, white; back, white; first primary entirely white, or with a blackish 

 spot at tip; hind toe, with nail; bill, yellow. Pagophila alba. Ivory Gull. 



See No. 22. 



Back, pearl gray ; inner web of primary, white, broadly tipped with black ; a small rudi- 

 mentary hind toe without nail. Rissa tridactyle. Kittiwake Gull. 



See No. 23. 



* Group 3. Wing, 13 to 15 Inches long. 



Uuderparts, white; first primary entirely black, or dark brown. 



Larus atricilla. Laughing Gull. 



See No. 33. 



General plumage, white ; first primary, entirely white, or with a dusky spot near tip ; hind 

 toe, with nail; bill, less than 1.50; wing, less than 14 ; bill, yellow. 



Pagophila alba. Ivory Gull. 

 See No. 22. 



Underparts, white ; inner web of primary, white, broadly tipped with black ; a small 

 rudimentary hind toe, without nail. Larus tridactyla. Kittiwake Gull. 



See No. 23. 



Underparts, white ; terminal portion of first primary, black, with white spot near tip. 



Larus delawarensis. Ring-billed Gull. 



See No. 31. 



Primaries, pale pearl gray, becoming white at tip; bill, over 1.50; wing, over 14. 



Larus leucopterus. Iceland Gull. 



See No. 25. 



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

 Section 1. Length, over 21 ; bill, over 2 ; underparts, white. 



No black on primaries (adult") . Larus glaucus. Glaucous Gull. 



See No. 24. 



Back, slaty black; primaries, black with more or less white (adult). 



Larus marinus. Great Black=backed Gull. 

 See No. 27. 



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

 or less marked with white. 



Larus argentatus smithsonianus. American Herring Gull. 



See No. 30. 



* For directions for measurement, see page 9. 



