ADAPTATION TO HAUNTS 155 



shelves and nooks on which the eggs can be laid. There 

 may be tier upon tier for hundreds of feet, like a giant's 

 bookcase. 



Conspicuous among the summer visitors are the guille- 

 mots, dark slate grey or dark brown above, and brilliant 

 white below. For a large part of the year they are rovers 

 on the open sea ; it is only in spring and summer that they 

 frequent the cliffs. They float buoyantly on the water or 

 swim rapidly on the surface with their long webbed feet ; 

 and under water they use their long narrow wings with 

 great effect. They are not fond of flying on the open sea, 

 but a sudden alarm will prompt them to take to wing, and 

 they hurry spluttering over the waves. On the cliffs they 

 shuffle about rather awkwardly, but this is mainly because 

 of the difficulties of the situation. The whole of the instep 

 (tarso-metatarsus) is horizontal, and that enables the guille- 

 mot to grip the rock when it alights and to scramble safely 

 on very steep gradients. It is interesting to notice that this 

 splay-footedness is secondary, for the young guillemot stands 

 with its instep almost vertical, as in ordinary birds. 



When spring comes the rovers return from the open sea 

 to the cliffs, where they have been unrepresented all through 

 the winter. Why should hundreds of thousands come 

 together ? The answers are : (i) that guillemots are scions 

 of a gregarious (plover) stock ; (2) that it is instinctive in 

 migratory birds to return to the place of their birth ; and 

 (3) that there is only a small number of suitably shelved sea- 

 cliffs with good fishing in the vicinity. 



As adaptive to the cliffs we have mentioned the horizontal 

 instep, and we may add the top-like shape of the egg, which 

 does not roll when jostled, the reduction of the clutch to 

 one, which is held lengthwise underneath the body and in 

 part lifted on to the long feet, and the sluggishness of the 

 young bird which eats heavily and sleeps much on its 

 dangerous cradle. Guillemots are monogamous, for the 

 season at least, and there is on the crowded cliff some 

 approach to conventions in human society. Thus there is, 

 to begin with, keen competition for a shelf or niche, but a 



