THE BIRDS' AIR LINE 



come from the West to the East in recent times. 

 In migrating, therefore, they go back over the 

 old Instinct Way, or over the trail of their an- 

 cestors. 



Long, roundabout journeys like this are the 

 exception. I have spoken of them because they 

 seem to explain better than more direct air lines 

 how these wonderful highways, thousands of 

 miles in length, may grow, little by little, from 

 small beginnings. 



MAIN TRAVELED ROADS 



Now let us trace some of the more popular 

 routes. If we were studying the travels of 

 European as well as of American birds we should 

 learn some of the most interesting facts. For 

 instance, we should find that in flying from 

 Europe to Africa birds cross the Mediterra- 

 nean Sea at a point where the water is so 

 shallow that it is believed the two continents 

 were formerly connected there. The land bridge 

 which, it is thought, formerly guided the 

 birds in their flight has disappeared, but the 

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