MIGRATION OF BIRDS 



207 



far the direction of migration is dependent on external factors. Light 

 and topographical conditions have apparently no influence at all on 

 the direction taken by the migrant swarms. As against this, it ap- 

 pears possible that the wind plays a certain role; in the majority 

 of cases investigated the hoppers were driven by the wind." 



Fio. 48. — Migration routes of North American birds. Though this map was pre- 

 pared chiefly to show the flyways used by waterfowl, most of these routes also 

 are utilized by innumerable land birds. For example, the important Mackenzie 

 Valley-Great Lakes-Mississippi Valley route is shown (with its tributaries) from 

 the Arctic coast to the delta of the Mississippi River. (After Lincoln, 1935.) 



MIGRATION OF BIRDS 



The migration of birds is presented rather fully because it is a 

 universal phenomenon of great interest and significance. IMuch atten- 

 tion has been given to it, and the diversity of opinion engendered 

 has led to endless and often fruitless discussion (cf. Walter, 1908). 



