64 



KEY TO TERNS AND SKIMMERS (Cont.) 



16 Tips of upper wing coverts buffy, each with a dark center within 



buffy area. LEAST TERN, immature 



16' Coverts pearly gray. LEAST TERN, adult 



17 Dark stripe along shaft on inner web (contrasting with white along 

 outer edge) distinct only in primaries 10 and 9; inner primaries 

 largely dusky throughout. 18 



17' Dark stripe referred to in 17 very distinct on at least the outer 



6 primaries. 19 



18 Upper wing coverts with buffy edges. FORSTER'S TERN, immature 

 18' Upper wing coverts entirely pearly gray. FORSTER'S TERN, adult 



19 Inner web of primary 10 colored like 9, having both a dark stripe 

 along shaft and a dark border along outer edge from tip inward 



about 1/3 the way toward wing bone (Fig. 24c). 20 



19' Inner web of primary 10 without dark border referred to in 19, 



OR border extending for only a few mm (Fig. 24d). 21 



20 Greater upper wing coverts broadly tipped with white. 



ALEUTIAN TERN, immature 



20' Greater wing coverts dark. ALEUTIAN TERN, adult 



21 Viewed from above, outer web of primary 10 usually about as dark 

 (almost black) as stripe along shaft on inner web (as in Fig. 24a); 

 primary 10 completely dark along outer 20-33 mm of tip; tarsus 



17-21 mm, longer than middle toe without claw. 22 



21' Outer web of primary 10 usually decidedly darker than stripe on 

 inner web (as in Fig. 24b); primary 10 dark along outer 15-25 mm; 

 tarsus 13-16 mm, shorter than middle toe without claw. 23 



22 Feathers of back and wing coverts with buffy tips. COMMON TERN, immature 

 22' No buffy edges on any feathers. COMMON TERN, adult 



