282 SEA-BIRDS 



(averages), given in the table below, show that previous estimates 

 [Handbook of British Birds, Vol. 4) were wide of the mark. 



The disparity between the periods for the puffin and those for the 

 common guillemot and razorbill (and probably great auk) can now 

 be related to the amount of exposure of the breeding site. In the dark- 

 ness and security of the burrow the puffin o^gg and chick is safe and 

 can develop slowly and fully. In the little auk and the black guillemot 

 colonies there is rather more danger from predatory creatures. (Winn 

 found that high tides, and young herring-gulls, caused mortality to 

 black guillemot eggs and chicks). The guillemot and razorbill are 

 constantly threatened by predatory birds. The most successful guille- 

 mots and razorbills therefore must be those which are hatched in 

 the shortest time and which spend the briefest period as chicks on the 

 ledges. When recording the fledging period of these two auks we noticed 

 a significant correlation ; some of the strongest chicks were those which 

 flew to the sea within two weeks, and some of the less vigorous chicks 

 were those which remained for nearly three weeks on the ledges. 

 Where there are large colonies of breeding gulls near those of cliff- 

 breeding auks the young auks experience a heavy mortality. In 

 British colonies the herring-gulls are the greatest egg-thieves and the 

 great black-backed gulls the greatest persecutors of the nestling 

 guillemots and razorbills. During our observations of the fledging 

 periods of these auks, which were made at Skomer and Skokholm, 

 we ringed as many as possible of the chicks. Those which disappeared 

 before the fledging day were taken by black-backed gulls, whose 

 fresh castings we discovered to be full of the remains of young auks, 

 from which we were able to recover some of our rings. 



Nestling auks are fed on fish or plankton, or both. The size of the 

 fish varies according to the age of the chick, as well as to its species. 

 As the chicks grow, which they do very rapidly in the case of the open- 

 site auks, larger fish are brought in. This is particularly true of the 

 guillemot and may be associated with the structure of the bill. When 

 the chick is about a week old, an adult guillemot will bring home a 



