ii8 A HISTORY OF BIRDS 



to content themselves with preying upon the young only, which 

 they seized for the sustenance of their own offspring, feeding 

 them upon the entrails of their victims. 



Wherever large breeding colonies of marine species are to be 

 found the larger Gulls and Skuas are sure to be met with, these 

 living almost entirely on the eggs and young of their more 

 helpless neighbours. Of the depredations caused by Skuas 

 among the Penguin colonies of the Antarctic, Dr. E. A. Wilson, 

 one of the naturalists of the Discovery expedition, tells some 

 lurid stories. Writing of McCormick's Skua and the colonies 

 of the Adelie Penguin he says : " Hanging round the rookery, 

 with the unmistakable look of a thief, the Skua will run up to 

 a chicken almost as big as himself, drag it by degrees away 

 from the more crowded part of the rookery, and then gradu- 

 ally worry it to death. . . . The Penguin chick pipes his loudest, 

 but the old birds standing round take very little notice. Oc- 

 casionally one in passing will make a run at the Skua, drive 

 him off for a moment, but the chick is separated from the I'est, 

 and the old Penguin has no mind to stop and shelter him, so 

 back the Skua comes to complete his work. Literally, in a 

 rookery such as that of Cape Crozier, one cannot walk ten 

 yards without coming on a dead Penguin chick. Many of 

 these . . . are dried and flattened mummies, trodden down 

 and flattened into the stones and guano that cover the ground. 

 But an enormous proportion are seen to be fresh victims, if one 

 visits a rookery in January, when the Skuas have not only 

 themselves but their young to feed." " But," he also remarks, 

 " the Skua even robs its own kind, and in a nesting colony of 

 some twenty or thirty birds, the numbers that have apparently 

 lost their eggs, or one at least, by robbery is always fairly large." 

 This fact, coupled with the pugnacity of the young (p. 320), 

 tends to keep down the numbers of these predaceous birds, which 

 otherwise would long since have exterminated the Penguins. 



The foregoing illustrations of the severity of the struggle 

 for existence deals only with a struggle sustained by species 

 which yet contrive to maintain a hold on life. But we must 

 pass now to another aspect of this theme — to an aspect which 

 shows how the extermination and extinction of species is 

 brought about as soon as such species become too highly 

 specialised to respond to changes in their environment. 



