MIGRATION AND ORIGINS 



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Figure 9. — Hypothetical life zones of the Wisconsin age: (A) arctic glaciation, (B) tundra, 

 (C) Hudsonian, (D) Canadian, (E) Transitional, (F) Upper Austral, (G) Lower Austral. 

 Heavy broken line marks the limit of 20-inch mean annual rainfall. (From L. S. Dillon, 

 "Wisconsin Climate and Life Zones in North America," Science, vol. 123, fig. 11, 1956.) 



that some are of eastern and Asiatic species show that birds from all 

 sources became differentiated by their relation to the Alaskan nesting 

 grounds. , 



Some hardy species of birds, like those which winter in the Arctic 

 may have survived in ice-free lowlands of northern Alaska and Yukon, 

 during maximum glaciation in the Mankato substage of Wisconsin 

 time but land birds could not migrate over continental ice caps. The 

 migratory populations now nesting in the northern part of America 

 must, therefore, have become established within the last 10,000 years. 

 During the earlier glacial maxima of Wisconsin time the existence 



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