160 NORTH AMERICAN FAUNA 61, FISH AND WILDLIFE SERVICE 



seen on Rat Island; July 4, 3 were seen on Semisopochnoi ; July 

 5 and 11, about 7 were sighted on Amchitka; July 29, at least 

 4 were seen on Ogliuga ; July 30, three or four were observed on 

 Kavalga; July 31, 2 were seen on West Unalga; and on August 2, 

 2 were seen on Uak. 



On the Alaska Peninsula, the black-color phase of the parasitic 

 jaeger is particularly common, though the light phase probably 

 predominates. In the Shumagins, all five birds seen were light 

 colored. 



Among the Aleutian Islands, however, the light-color phase 

 is a rarity. More than 100 parasitic jaegers were recorded, and, 

 of this number, only 4 were specifically mentioned in our field 

 notes as being light colored; nearly all the rest were mentioned 

 as being definitely dark. Possibly in no other area is the parasitic 

 jaeger population so uniformly dark. 



Stejneger (1885) says of the color phase that "On the Com- 

 mander Islands the dark form is the most common. A few only 

 with white lower surface were seen and one secured." 



Hartert (1920) obtained 4 adults in the Commander Islands 

 with white underparts, and he obtained 3 of the dark phase. 



Dall (1874) noted the same tendency, believing, however, that 

 the dark color was in the immature plumage ; this becomes obvious 

 when he says "nor have we ever obtained any in completely 

 adult plumage. All our specimens are of a nearly uniform dark 

 slate color." 



Bent (1921) has suggested that the dark color phase may be 

 a distinct species. That appears doubtful, however. Bent quotes 

 Grinnell as saying that he found a light and a dark bird mated. 

 On two occasions in the Aleutians we observed trios, one of 

 which was white. Unless we can show that normally the two 

 color types keep segregated in breeding, with only an occasional 

 crossbreeding that may be construed as hybridization, it will be 

 better to consider that they are color phases. 



A dark-color phase, becoming dominant or very prominent 

 locally, is known among other animal species — for example, in 

 the case of the marmot in parts of the Rocky Mountains, the 

 ground squirrel in eastern Alaska, and the Arctic fox in the 

 Aleutians. 



Food Habits 



The name of this bird suggests its food habits. It is known 

 to rob gulls and terns of their food. The Arctic tern and European 

 turnstone were seen pursuing parasitic jaegers, evidently rec- 

 ognizing them as foes. On Alaska Peninsula, there was evidence 



