Probably the most remarkable theory that has been advanced to 

 account for migration is contained in a pamphlet mentioned by Clarke 

 (1912: V. I, 9-11) as published in 1703 under the title: "An Essay 

 Toward the Probable Solution of this Question: Whence come the 

 Stork and the Turtle, the Crane, and the Swallow, when they Know 

 and Observe the Appointed Time of their Coming." It was written 

 "By a Person of Learning and Piety," whose "probable solution" was 

 that migratory birds flew to the moon and there spent the winter. 



Some peoples, who easily accepted the migratory travels of the 

 larger birds, were unable to understand how the smaller species, some 

 of them notoriously poor fliers, could make similar journeys. They 

 accordingly conceived the idea that the larger species, as the storks and 

 cranes, carried their smaller companions as living freight. In some of 

 the Mediterranean countries, it is still believed that these broad-pin- 

 ioned birds serve as aerial transports for the hosts of small birds that 

 congregate upon the shores awaiting opportunity for this kind of pas- 

 sage to their winter homes in Africa. Similar beliefs have been found 

 among some tribes of North American Indians. 



Advantages of Migration 



Before presenting some of the present theories concerning the origin 

 of bird migration, it seems well to consider briefly the ends that are 

 served by this annual round trip between breeding grounds and winter 

 quarters. It is apparent that the migratory habit enables a species to 

 enjoy the summers of northern latitudes while avoiding the severity 

 of the winters. In other words, migration makes it possible for some 

 species to inhabit two different areas during the seasons when each 

 presents favorable conditions. In the performance of its reproductive 

 duties, every pair of birds requires a certain amount of territory, the 

 extent of which varies greatly in different species. Generally, how- 

 ever, this territory must be large enough to provide adequate food, not 

 only for the parent birds, but also for the lusty appetites that come 

 into being with the hatching of the eggs. Thus, if all birds were to 

 remain constantly either in tropical or in temperate regions, there 

 would be intolerable overcrowding during the breeding season. By 

 the spring withdrawal to regions uninhabitable earlier in the year, the 

 migrants are assured of adequate space and ample food upon their 



