FAMILY LARIDAE 439 



KEY TO THE SPECIES OF LARIDAE 



1. Culmen with tip distinctly decurved; the curving end of the maxilla over- 



hanging the end of the mandible; tarsus relatively longer (Gulls). 2 



Culmen with tip not decurved, more nearly straight throughout its length; 



end of maxilla not overhanging that of mandible; tarsus relatively 



shorter (Terns) 8 



2. Tail truncate or slightly rounded 3 



Tail definitely forked 7 



3. Under surface white, or washed and mottled with grayish brown; back 



plain gray, or grayish brown ; bill not completely black 4 



Entire body plain mouse gray ; bill completely black. 



Gray gull, Larus modestus, p. 440 



4. Size large, wing more than 410 mm. 



Herring gull, Larus argentattts smithsonianus, p. 441 

 Size smaller ; wing less than 390 mm 5 



5. Bill small, exposed culmen less than 35 mm.; tarsus less than 45 mm. 



Franklin's gull, Larus pipixcan, p. 445 

 Bill larger; exposed culmen 40 mm. or more; tarsus more than 45 mm. 6 



6. Tarsus and feet light-colored; bill light-colored, at least at base; if 



immature, upper tail coverts spotted with grayish brown. 



Ring-billed gull, Larus delawarensis, p. 441 



Tarsus, feet, and bill dark; if immature, upper tail coverts plain white or 



grayish white, without spots Laughing gull, Larus atricilla, p. 442 



7. Large; wing more than 390 mm.; bill slender, exposed culmen more than 



45 mm Swallow-tailed gull, Creagrus furcatus, p. 446 



Small ; wing less than 300 mm. ; exposed culmen less than 30 mm. 



Sabine's gull, Xetna sabini, p. 447 



8. Head smooth, not crested 9 



Back of head with feathers elongated in a definite crest 15 



9. Toes partly webbed, the webs extending only to the bases of the outer 



toe joints ; outer tail feathers broad and rounded at tip. 



Black tern, Chlidonias niger surinamensis, p. 448 



Toes fully webbed, the webs extended on the outer toe joints ; outer tail 



feathers narrowed and pointed at tip 10 



10. Size small ; wing not more than 180 mm. .Least tern, Sterna albifrotis, p. 457 

 Larger ; wing more than 200 mm 11 



11. Bill relatively short and heavy, with greatest depth equal to about one 



third length of exposed culmen ; tarsus longer than middle toe with claw. 



Gull-billed tern, Gelochelidon nilotica aranea, p. 449 



Bill relatively longer and more slender, with greatest depth definitely less 



than one third length of exposed culmen; tarsus shorter than middle toe 



with claw 12 



12. Back and wings light gray Common tern, Sterna hirundo hirundo, p. 452 



Back and wings dark sooty brown, sooty gray, or black 13 



13. Tail rounded; body and wings dark sooty or grayish brown. 



Brown noddy tern, Anous stoUdus, p. 461 

 Tail deeply forked 14 



