10 Introduction 



what was already pretty well understood that is, 

 the great distances traversed by certain birds, and 

 the general direction of their movements. On the 

 other hand a mass of the most valuable information 

 has been accumulated with regard to the methods of 

 migration. Species which ordinarily travel in flocks 

 can be separated from those which are apt to do so singly 

 or in pairs ; the numbers in the flocks have been proved 

 often to be incalculable ; the altitude has been reckoned 

 in certain cases and found to be so great that it is clear 

 that only the lower flocks are really brought within our 

 ken ; bad weather has not proved to be in all cases an 

 obstacle to migration, though the direction of the wind 

 has always to be considered. 



Apart from the distances traversed, the direction of 

 migration, that is, the broad lines in which different 

 species travel, is a question of great moment. Birds 

 which breed to the south of the equator certainly tend 

 to migrate northwards ; but so little is known of the 

 habits of these southern forms that we must follow 

 the course, usual at present, of confining our remarks 

 to those that breed in the northern hemisphere, while 

 noting that the movements are of much less extent in 

 the southern half of the globe. 



On migration the young usually start before the 

 parents, though in exceptional cases, such as that of the 

 Cuckoo, which is reared more often than not by one of 

 our resident species, they linger till a later date. Once 

 started, the direction is distinctly influenced by the 

 conformation of the land ; coasts, river- valleys, and so 

 forth making for ease of travel, high mountain ranges 

 for difficulty ; but even the last-named are not un- 

 commonly surmounted, and the old idea that straight 



