38 KEY TO THE BIEDS OF EASTERN NORTH AMERICA. 



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



Larus atricilla. Laughing QuII. 

 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. lUisa tridactijle. Kittiwal<e Gull. 



See No. 23. 



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



Underparts, white; first primary entirely black, or dark l)rowu. 



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 M ; bill, yellow. 



Pagophila alba. Ivory QuII. 

 See No. 22. 



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

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



See No. 23. 



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



Lams dclawarcnsis. Ring°billed Gull. 



See No. 31. 



Primaries, pale pearl gray, becoming white at tip; bill, over 1.50j 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 ou primaries (adult"). Larus ylaucus. Glaucous Gull. 



See No. 24. 



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



Larus marin.us. Great Black=backed QuII. 



See No. 27. 



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

 or leas marked with white. 



Lams aryi'Matiis smithsonianns. American Herring QuII. 



See No. 30. 



* For directions for measurement, see page i). 



