18 THE MIGRATION OF BIRDS 



Granting this, however, as the first cause, we 

 are only on the threshold ; the question still re- 

 mains unanswered, what actually impels the birds 

 to seek fresh food supplies or to look for safe nesting 

 places ? .The natural answer, the cravings of 

 nature and sexual impulses fails to give satisfaction 

 in every case. Wanderings in search of food might 

 lead in any direction, and probably did in the first 

 place, but now birds in the main travel south in 

 search of food and north in search of home, and 

 many of them perform immense journeys, passing 

 over or through lands which are capable of sup- 

 porting a wealth of bird-life even in the winter 

 months. 



The majority of Arctic birds or those nesting in 

 high latitudes leave before the great harvest of 

 autumn fruits, and even our common swift begins 

 to depart for all do not go at once towards the 

 end of July, when insects are more abundant than 

 at any other time of the year. Food supply has 

 not failed when most birds start their journey in 

 search of food ! Again in spring, when it is claimed 

 that the powerful sexual impulses are sufficient 

 reason to account for the northward journey, hosts 

 of sexually immature birds and of others which are 

 apparently mature but do not breed that spring, 

 migrate northwards, some even arriving before the 

 mature birds of their own species. 



