62 THE MIGRATION OF BIRDS. 



they live come back to the exact locahty of their 

 previous nests. This long journey, gradually grow- 

 ing longer and longer during thousands of years, until 

 it is now at least a thousand miles in length, has 

 grown to be a deeply-rooted custom sanctioned by 

 the practice of ages of experience and need, and 

 looked upon now as part of the Flycatcher's very 

 existence ! 



The above interesting and thoroughly demon- 

 strable instance shows two very important ways in 

 which Migration has been initiated. First, by a 

 gradual movement south during a short winter, 

 growing more extended in space as the climate 

 became more severe, and more prolonged in time 

 as the winters became longer. Second, a gradual 

 movement north again with the return of milder 

 climatal conditions. In both movements, however, 

 the first journeys undertaken were probably of very 

 small extent, and slowly became longer as the 

 summer area of distribution became wider. During 

 this period of Glaciation, bird-life was utterly 

 banished from all northern and many temperate 

 lands. During the past 60,000 years a general 

 avian exodus has been in progress. All the hardier 

 birds have emigrated back as far north as they can 

 live, and many have become sedentary in the more 

 temperate areas, but the winters still continue too 

 long and too cold for the majority of insectivorous 

 birds breeding in those areas, with the inevitable 

 consequence that Migration among them is the 

 almost universal rule. 



