448 



COLLEGE ZOOLOGY 





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Figure 317. A spectacular migration of snow geese, flying over the waters ot back Bay, Vir- 

 ginia. Picture shows only a small part of this majestic migration. (Photographed from a helicopter 

 by Margaret Bourke White. Courtesy of Life magazine.) 



altitude of Vi mile or more, but there are 

 some daytime migrants. Each species has a 

 more or less definite time of migration, and 

 one can predict with some degree of ac- 

 curacy the date when it will arrive in a given 

 locality. The speed of migration is as a rule 

 rather slow, and a daily rate of 38 miles is 

 about the average for the robin. 



Many theories have been advanced to ac- 

 count for the migration of birds, such as the 

 temperature, amount of light, condition of 

 the food supplv, and physiologic state of 

 the gonads. Other theories attempt to ex- 

 plain how birds find their way during migra- 

 tion. A review of all available evidence 

 makes it fairly clear that many problems 

 raised by bird migration still remain un- 

 solved; however, there seems no reason to 

 believe that they are insoluble. 



Nests, eggs, and 

 young of birds 



Some birds, like geese and eagles, usually 

 mate for life, but the majority live together 

 for a single season only. The nesting period 

 varies according to the species. 



As a rule birds conceal their nests or 

 build them in places that are practically in- 

 accessible. Some species do not build a nest, 

 but lay their one or more eggs directly upon 

 the ground (Fig. 318). A nest-building bird 

 may construct a flimsy platform of twigs 

 (mourning dove) or an intricate hanging 

 basket (oriole), but most species build dis- 

 tinctive nests. 



A few birds not only build no nests, but 

 they neither incubate their eggs nor take 

 care of their oflPspring. This is true of the 



