GULLS AND DIVERS 67 



In the case of the petrels a limit is reached, 

 beyond which further diminution is impossible. 

 Indeed, the unit is so very frequent, that it may 

 almost be regarded as the typical number. The 

 guillemot, the razorbill, the little auk, the puffin, 

 the shearwater, the solan-goose, all deposit and sit 

 on one egg at a time ; and they discharge the duty 

 with more than the gravity of a sitting hen on her 

 sixteen. The unit of production must be taken as 

 representing the unit of danger, and means that 

 the sea birds have so little to fear, that, in a majority 

 of cases, one egg is found sufficient to maintain, or 

 probably slightly increase the species. 



Five is the usual number found in a land bird's 

 nest. Four is not uncommon. Three is very rare, 

 occurring with some owls, and hawks. Two is con- 

 fined to the swift, and the night-jar, whose nests are 

 very seldom found ; and the pigeon, which builds out 

 of reach of many enemies. And one is unknown. 



Starting from five with the land bird, the tendency 

 is upward, not downward ; starting from three with 

 the sea bird, the tendency is downward, not upward. 



A few heavy-looking gulls, of a dirty brown 

 plumage, evidently young, represent the vast 

 armies of pure whites, and slaty blues that have 

 departed. These are either last season's birds, or 



