27 



KEY TO PETRELS 



1 Bill length <18 mm; wing length <180 mm; tarsus <30 mm OR if 33-37 



mm then feet with yellow on webs; tarsus decidedly longer than middle 

 toe with claw (STORM PETREL). 2 



T Bill >25 mm; wing >200 mm; tarsus >38 mm OR if 33-37 mm then feet 



are entirely black; tarsus not longer than middle toe with claw. 10 



2 Body, head and neck light to medium bluish-gray; wings with blackish 

 flight feathers but with gray coverts. FORK TAILED STORM-PETREL 



2' Body, head and neck blackish-gray to sooty brown, except for rump 

 which may or may not be white and secondary upper wing coverts which 

 may or may not be distinctly buffy. 3 



3 Wing length >165 mm; tarsus 29-33 mm but if close to 33 mm then feet 

 entirely black and no white on rump. BLACK STORM PETREL 



3' Wings <162 mm; tarsus 25-33 mm but if close to 33 mm then webs of 



feet with light (yellow) spots and with white feathers in rump, flanks 

 and under tail coverts. 4 



4 At least some white on some rump feathers (upper tail coverts). 5 

 4' No white on any feathers of rump or flanks. 8 



5 Tarsus > 30 mm; light (yellow) area on webs of feet; some and usually 

 many under tail coverts white. WILSON'S STORM PETREL 



5' Tarsus <26 mm; feet entirely black; under tail coverts not white. 6 



6 Upper tail coverts white to the tips including central two (feather 

 shafts dark); tail short, longest tail feather (from skin to tip) 

 56-57 mm; longest (central) white rump feathers extend about 2/3 the 

 length of the tail (Fig. 19 b); culmen often <12.9 mm (10.6-14.0 mm). 



GALAPAGOS STORM PETREL 



6' Rump extensively white with most of the longer upper tail coverts 

 in center of rump having dark tips except' perhaps the most central 

 ones which may sometimes be entirely white or entirely dark OR only 

 a few feathers on either side of rump have some white (sometimes 

 confined to one white spot on one feather on either side); longest 

 rump feathers extend only about 1/2 the tail length (Fig. 19 a). 7 



