NATURAL HISTORY OF SOUTH AFRICA 



THE FOOD SUPPLY 



Many species of birds are compelled to migrate 

 to secure a sufficiency of food. If swallows, for 

 instance, were not migratory, only a comparative few 

 would find sufficient food to carry them through the 

 winter, even in temperate climates ; while in those 

 countries like the British Isles where the winters are 

 severe, the swallows would probably 

 become extinct. By migrating from 

 the northern hemisphere of the 

 earth to the south in the autumn, 

 migratory birds are perpetually in 

 spring and summer conditions ; and 

 at these times insect food is at its 

 maximum of abundance. This is a 

 wise provision of Nature. Even with 

 the present large number of migra- 

 tory birds, the insect hosts are able 

 to more or less hold their own. 

 a severe blizzard or other sudden weather 

 changes occur, great numbers of birds perish, 

 and there is a corresponding increase in the insect 

 world, and insect plagues are prevalent. Sometimes 

 a catastrophe overtakes the migrating hosts of birds. 

 They are occasionally caught in a wind storm of great 

 violence and driven out to sea to perish miserably; 

 or they may be struck down by hail and heavy rain. 

 Should this occur, insect pests increase alarmingly. 

 Insectivorous birds cannot be too abundant. If there 



236 



Suet hung by string 

 from branch. Suet 

 is an excellent 

 winter food. 



Wh 



en 



