STERCORARIIDtE — THE SKUAS AND JAEGERS — STERCORARIUS. 331 



which they have swallowed. As soon as the fish has been disgorged, the Hkua swoops 

 down upon it with so rapid a movement and so sure an aim as frequently to seize 

 the prize before it reaches the water. This bird is on this account known to some as 

 the "Parasitic Gull." It is supjjosed to be a bird of great longevity. Yarrell states 

 that a specimen brought alive to Dr. Keill in the summer of 1820 — then a nestling 

 — was alive at the Cannon-mills in October, 1843. Its plumage in its twenty-fourth 

 year had become very pale, and its head was grayish white. Another bird was kept 

 alive by Mr. G. T. Fox for ten years, undergoing no change of color at any of its 

 moultings. 



This bird lays two or three eggs, olive-brown in color, blotched with darker brown, 

 2.75 inches in length and 2.00 in breadth. An egg in the Smithsonian Collection, 

 from Greenland (No. 2658), measures 2.90 by 1.95 inches, has a ground-color of a 

 dark grayish drab, with irregular spots of raw-umber and sepia. Another specimen, 

 measuring 2.55 by 1.95 inches, has markings much deeper in color and more distinct. 



Genus STERCORARIUS, Brisson. 



Stercorarius, Briss. Orn. V. 1760, 149 (type, Larus i^nt'O'Silkiis, Linn.). 

 Lestris, Illig. Prod. 1811, 272 (same type). 



The difference between this genus and Megalestris consists chiefly in the smaller size and more 

 slender, graceful form of Stercorarius, the increased slenderness extending to all parts of the organ- 

 ization. One of the three known species differs considerably in form from the other two, which 

 are so much alike that they are sometimes with dithculty distinguished from each other. 









S. parasiticus. 



Synopsis of Species. 



A. Middle rectrices broad and rounded at ends. 



1. S. pomarinus. Wing, about 13.50-14.00 inches; middle tail-feathers, 8.00-9.00 ; culmen, 



1.45-1.55; tarsus, 2.10; middle toe (without claw), 1.60-1.75. 



B. Middle rectrices attenuated and pointed at ends. 



2. S. parasiticus. Wing, 11.80-13 15 inches (average, 12.67) ; central rectrices, 7.70-10.25 



(8.66); culmen, 1.15-1.40(1.27); tarsus, 1.50-1.85 (1.70) ; middle toe, 1.20-1.45 (1.34). 

 Tarsi black in adult ; nasal shield longer than the distance from the anterior edge of the 

 nostril to the tip of the bill. 



