A Few Interesting Facts About Bird 



Migration 



By Gayne T. K. Norton 



"Why do the birds fly soutli in the fall?" This is a question asked many 

 times each fall that is prompted by the great flocks of birds that leave and return 

 to North America periodically, and a question to which no conclusive answer 

 has yet been found. 



The Biological Survey of the United States Department of Agriculture has 

 collected the largest amount of data on bird migration ever assembled in the 

 United States ; there are over 500,000 facts on record, the collection of which 

 has taken twenty-five years and with which it is possible to make broader and 

 safer generalizations on this subject, which has always been a mystery to the 

 average person, than have ever before been made. 



Yet, despite the large amount of information on iile. and the fact that bird 

 migration has been noted for 2,000 years, the question : "\\'hy do North Ameri- 

 can birds migrate?" is still unanswered. Two theories are advanced, and of 

 these the Bulletin of the Biological Survey says in part : 



■"According to the more commonly accepted theory, ages ago the l"nited 

 Slates and Canada swarmed with non-migratory bird life, long before the .\rctic 

 ice-fields, advancing south during the glacial era, rendered uninhabitable the 

 northern half of the continent. The birds' love of home influenced them to 

 remain near the nesting site until the a])]jroaching ice began, for the first time, to 

 produce a winter — that is, a period of bad weather which so reduced the food 

 supply as to compel the birds to move or starve. 



"As the ice approached very gradually, now and then receding, these first 

 migrations were of short duration and distance; in .shoft. the habit of migration 

 possessed the birds at the same time that the changing seasons in the year replaced 

 the continuous semi-tropical conditions of the pre-glacial eras. 



"As the ice advanced southward the swing to the north in spring niigratidU 

 was shortened and the fall retreat to a suitable winter home, lengthened : until 

 during the height of the glacial period birds were practically confined to Central 

 and South America. The habit of migration- had been formed, however, and 

 when the ice receded toward its present position, the birds followed it northward, 

 and in time established their present long and diversified migration routes." 



That is the first theory: according to the second, the Ijirds" real home is in 

 the south : all bird life tends by overproduction to overcrowding, and at the end 

 of the glacial era, the h'lrch. seeking in all directions for suitable breeding grounds, 

 graduallv worked their way north. But the winter abiding place was still the 

 home and to this they returned as soon as the breeding season was over. 



Whichever theory is accepted, the fact remains that southward migration is 

 due to the changing food supply; North America possesses enormous summer 

 food supplies for the birds, but they must return south for the winter. The 



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