FAMILY STERCORARIIDAE 433 



Old Worlds and spends the winter season at sea, ranging far south 

 in southern oceans. 



Eugene Eisenmann informs me that on May 1, 1961, while on a 

 trip in the lower Bay of Panama he observed two flocks of at least 

 300 birds in the area between Isla Chame and Isla Otoque. As the 

 launch cruised among them for some time he noted that most were in 

 breeding dress. 



In Spain this bird is known as falaropo picofino. 



Family STERCORARIIDAE : Skuas and Jaegers ; 

 Gaviotas Salteadoras 



These are the predatory robber gulls that on their nesting grounds 

 kill and eat smaller avian companions, as well as small mammals. 

 When at sea, off their nesting grounds, they are scavengers on dead 

 fish and other animals and also harass gulls and other birds to force 

 them to drop or regurgitate food that they have taken. As this falls 

 the robber may seize it expertly in the air. The smaller jaegers nest 

 in the north, while the skuas that reach Panama breed in the 

 Antarctic. They are seen most often in the Gulf of Panama. 



Members of this family differ from gulls and terns in the possession 

 of a horny cere on the base of the bill. 



KEY TO THE SPECIES OF STERCORARIIDAE 



1. Larger, wing 380 mm. or more ; bill strong and heavy, more than 45 mm. long. 



Skua, Catharacta skua, p. 433 



Smaller, wing not more than 370 mm. ; bill smaller, more slender, less than 



40 mm. long 2 



2. Wing 350 mm. or more ; depth of bill at base slightly more than width ; adult 



with elongated central tail feathers twisted toward tip. 



Pomarine jaeger, Stercorarius pomarimis, p. 435 



Wing less than 345 mm.; depth of bill at base about equal to width; adult 



with elongated central tail feathers flat and straight 3 



3. Horny cere (supranasal saddle) decidedly longer than distal section of bill 



(dertrum) ; adult with middle rectrices less than 235 mm. 



Parasitic jaeger, Stercorarius parasiticus, p. 436 



Horny cere not longer than dertrum ; adult with central rectrices more than 



240 mm. long Long-tailed jaeger, Stercorarius longicaudus, p. 437 



CATHARACTA SKUA Briinnich: Skua; Salteador Grande 

 Figure 69 

 Catharacta skua Briinnich, Orn. Borealis, 1764, p. 33. (Iceland.) 



Gull-like in form, of large size, with a prominent white area in 

 each wing. 



