55 



KEY TO GULLS (Cont.) 



22' Secondaries and tertials not gray with conspicuous white tips. 12 



NOTE: Albino gulls of many species may key out to this step. Iden- 

 tification of albino gulls usually requires comparison with known 

 material by someone very knowledgeable about gulls. Interesting 

 specimens should be saved. Pure white birds or birds with ex- 

 cessively worn and bleached feathers should be treated cautiously. 



23 Wing length <360 mm; plumage all white; legs black; bill black with 

 yellow tip. IVORY GULL, adult 



23' Wing >425 mm; plumage usually with some buffy or gray. 61 



24 Tail nearly all white (dark smudge may be present on a few tail 

 feathers). 25 



24' Tail not all white. 34 



25 Head entirely white OR head and neck with dusky streaks; wing length 

 usually >330 mm. 26 



25' Head black, brown or gray OR head white with a blackish smudging 



on top extending down back of the neck; wing usually <330 mm. 15 



26 Wing tips without black. GLAUCOUS-WINGED GULL, adult 



NOTE: Glaucous-winged Gulls interbreed with Western and Herring 

 Gulls. The progeny often have very dark wing tips and may 

 be confused with Western, Herring or Thayer's Gulls. Beach 

 walkers in northern California, Oregon, Washington and British Col- 

 umbia should be cautious of this problem. 



26' Wing-tips with black (sometimes dark gray). 



27 Outer three primaries not distinctly darker than inner primaries 

 and secondaries; mantle dark, slate gray. 



27' Outer three primaries distinctly darker than inner primaries and 



secondaries; mantle pearly to silver-gray. 30 



28 Wing length >420 mm; two white spots (mirrors; see Fig. 16) or 

 much terminal white in outer primaries; Bering Sea and Aleutians. 



SLATY-BACKED GULL, adult 



