154 THE BIOLOGY OF BIRDS 



another variant which suited the moorland conditions and 

 has become a species. Ahhough it has been introduced 

 elsewhere, as on upland heaths in the north of England, on 

 many Welsh moors, and on many Irish hills, it flourishes 

 best in Scotland, where, we believe, it was first evolved. 



The Red Grouse is a resident bird, moving very little 

 unless the winter is unusually severe. The Spartan diet, 

 which its extraordinary digestive powers enable it to endure, 

 accounts for its survival in an environment which would be 

 fatal to most creatures of high degree. For it feeds on the 

 tips of the twigs of ling and heath, crowberry and blaeberry, 

 besides, in autumn, the seeds of sedges and grasses — very 

 frugal fare. It seems that the young grouse require more 

 digestible food than their parents demand, and there is 

 said to be a definitely insectivorous period, during which 

 the food-canal becomes educated for more difiicult 

 tasks. 



The Red Grouse is full of biological interest. Thus the 

 red warty ridge above the eye has the same pigment {zoofi- 

 erythrin) as in lobster and shrimp ; the feet are stockinged 

 in feathers that go as far as the ends of the toes ; the w'orn 

 claws are moulted off in late autumn, liberating a fresh set 

 well-suited for the winter's scratching for food. There 

 seems to be no specific " grouse-disease," but when the birds 

 live a too sheltered life and constitutions below par are 

 tolerated, then the parasites of the grouse — normally kept 

 within bounds — get the upper hand and are fatal to their 

 host. 



§ 7. Birds of the Sea-Cliffs 



There is a very characteristic bird-fauna on sea-cliflFs 

 like those of Flamborough Head, the Bass Rock, Ailsa Craig, 

 Handa Island, and Foula. There are some tenants which 

 remain throughout the year, as in the case of cormorants and 

 kittiwake gulls ; in other cases the birds are there at the 

 breeding season only, as in the case of guillemots and pufiins. 

 The permanent tenants of the cliff and the summer lodgers 

 are there and not elsewhere because of the presence of endless 



