WHEN ALL THE WORLD IS YOUNG 115 



of the insectivorous birds. So when Scotland was 

 visited some years ago by a plague of voles, the 

 voles found themselves visited by a plague of owls. 



When voles overstep the bounds of their usual 

 increase, they soon swarm over fields and parishes 

 and counties, as thickly as you can see them at 

 midsummer in the corner of the field whence the 

 barn owl fetches them for its brood. The task of 

 keeping them down becomes far beyond the resident 

 owl population, often reduced almost to zero by 

 the gamekeeper. Then, as happened in Scotland 

 at the time of the last vole plague, numbers of 

 owls arrive from the Continent, where somehow 

 they get news of the need for them. 



The owl organism so thrives on the unaccustomed 

 plenty of food that clutches of eight and nine 

 eggs are laid, and two or three broods are brought 

 off in the season if the vole manna last so long. 

 Then, their great work of destruction done, the 

 tyrants are liable to have a hard time for lack of 

 victims. Nature will no more suffer superfluous 

 owls than superfluous voles, and the feathered 

 army, expanded at the special call for its services, 

 must be contracted again into normal limits. 

 Starvation attacks not only its winter numbers 

 but their fecundity in spring. Two or three eggs 

 are once more enough for a clutch and one brood 

 enough for the season, until their numbers become 

 normal to a normal supply of voles. 



Most of the rodents are given to gigantic increase. 

 The hamster and the lemming are well-known 



