62 THE RIDDLE OF MIGRATION 



The reason for this digression is evident. No con- 

 sideration of conditions prevailing in the north as 

 they affect bird Hfe can be complete without taking 

 note of this striking phenomenon of cycles. Whether 

 there is a connection or not between these and 

 migration remains to be discovered, but one thing 

 is certain. Migratory birds have solved the prob- 

 lem of periodic decimation that overtakes many of 

 their non-migratory brethren. As far as we know, 

 no migratory species show the ten-year cycle.^ 

 They take an annual risk and undoubtedly suffer 

 some annual reduction in numbers, but they avoid 

 the danger of almost total extermination every ten 

 years. In going south they guarantee themselves 

 an all year contact with the health-giving ultra-vio- 

 let rays of the sun. This applies particularly to 

 seed-eaters to whose continued welfare radiation 



3 Some owls (e.g., snowy and short-eared) and probably certain 

 hawks, show a cycle in numbers. Their rate of reproduction 

 apparently becomes heightened with augmented food supply and 

 we hence get larger numbers during years in which mice are super- 

 abundant on their breeding grounds in the north. Mice show a 

 four-year cycle throughout most of the northern hemisphere and 

 their peak seems to be reflected in the abundance of certain 

 predatory birds. This is entirely distinct from the ten-year rabbit 

 and grouse cycle and appears to be a direct question of food 

 supply. 



