298 NATATOSES. 



those shelves and ledges of the rock which overhang the sea. 

 A question has been raised as to whether these and other 

 birds, which place their eggs upon shelving rocks, do not 

 cement them by some means or other, so as to prevent them 

 from falling ; because, when those who visit such places lift 

 the eggs, they find some difficulty in again balancing them 

 steadily upon the rock. Now it may be stated as a universal 

 habit with all birds to turn their eggs during the process of 

 incubation, and among domestic poultry, the goodness of a 

 brood-hen is estimated by the frequency with which she rolls 

 about the eggs under her ; and those hens which do not per- 

 form that operation so diligently, are incapable of hatching a 

 numerous brood without addling some of the eggs. The sea 

 birds, no doubt, turn their single egg in a similar mariner : 

 which would of course be incompatible with the operation of 

 cementing, even though the impossibility of that operation 

 were not, as it is, apparent enough upon other grounds. 

 These single-egg birds sit constantly during their incubation, 

 the males feeding them the while ; and if they are not forced 

 up by some alarm, on which occasions they are exceedingly 

 clamorous and agitated, they probably do not once quit the 

 egg from the time that it is dropped till the young bird 

 breaks the shell. The general means, therefore, by which 

 the egg is made to remain on the shelving ledge, is the pres- 

 sure and adhesion of the parent bird ; and when the birds 

 are suddenly forced up, numbers of the eggs, if they are 

 abundant and the situation very unstable, always fall down ; 

 and the prevention of that fall, either by jostling the egg 

 while leaving it, or by the action of the wind in her absence, 

 may be one of the reasons why the female of these birds sits 

 so constantly. 



The razor-bills resort to their breeding situations in the 

 month of May. They nestle, or rather breed, higher on the 

 precipices than perhaps any other of the shelve birds, though 



