MIGRATION 105 



to the rank of " British birds," a very large proportion are 

 migrants; but while some come to us in the spring, remain to 

 breed, and depart in the autumn, others come to us in the autumn 

 and depart in spring. 



These movements are due, not to matters of temperature, 

 but of food. A large proportion of our summer migrants are 

 insect eaters, and since they cannot procure enough food during 

 the winter months to sustain life, they are driven to such 

 regions where a supply of food can be depended upon. The 

 birds that come to us in the autumn do so to escape the rigours 

 of a winter more severe than ours, and the consequent failure 

 of food. And this is further demonstrated by the fact that hard 

 weather in the northern parts of our islands drives the birds 

 wintering there, residents and migrants, farther and farther south. 

 When, in an unusually severe winter, the whole country is more 

 or less laid under snow and ice, then these fugitives die in 

 thousands, not of cold, but of starvation, being unable to dig 

 through the frozen earth for food. 



Thus, then, migration is indirectly due to temperature, as may 

 be shown in the case of insectivorous birds, for the insects upon 

 which they feed die in the autumn, leaving eggs to hatch out on 

 the return of congenial weather. And so it comes about that 

 migration routes must run north and south, as indeed is actually 

 the case. 



The swallow may serve as a good example of this. This bird 

 during the summer months ranges over the whole of Europe, and 

 winters in Africa, some on the West Coast, others on the East, 

 these last following the Nile Valley and eventually reaching the 

 Cape. The evidence so far collected shows pretty conclusively 

 that the swallows which winter in West Africa are those which 

 passed the summer in Great Britain and the more western parts 

 of Europe, while those which travel down the east side of Africa 

 are birds which spent the summer months in Eastern Europe. 

 The swallows of Asia, as one would expect, winter in India 

 and Burma ; similarly, the swallows of North America pass 

 southwards into Brazil. 



Most of our summer migrants, it should be noted the cuckoo, 

 swift, warblers, chats, wagtails, and so on winter in Africa. 



