ADVANTAGES OF MIGRATION 



Why should a bird subject itself to the rigors of a long migratory 

 journey twice a year if it can find all the requirements suitable for 

 existence in one locality? It seems well to consider briefly the ends 

 that are served by this annual round trip between breeding grounds 

 and winter quarters. Obviously, the migratory habit enables a 

 species to enjoy the summer of northern latitudes and to avoid the 

 severity of winter. In other words, migration makes it possible for 

 some species to inhabit two different areas during the seasons when 

 each presents favorable conditions. If it was not advantageous to 

 make the trip twice a year, natural selection would have eliminated 

 the tendency, but bird migration has become the rule over much of 

 the world rather than the exception. 



By withdrawing in the spring to regions uninhabitable earlier in 

 the year, migrant species are generally assured of adequate space 

 and ample food upon their arrival in the winter-freed North, and 

 those nonmigratory kinds, which stay behind to nest, are also assured 

 of ample space for these activities. 



Every pair of birds requires a certain amount of territory for the 

 performance of its reproductive duties, the extent of which varies 

 greatly between different species. This territory must be large 

 enough to provide adequate food, not only for the parent birds but 

 also for the lusty appetites of their young. In the Arctic summer, 24 

 hours of daylight allow the young to feed or be fed almost con- 

 tinuously and rapid growth is apparent. The short breeding season in 

 northern latitudes exposes the vulnerable young to predation for a 

 brief period and prevents a build up of predator populations. 



It cannot be said that the winter or summer area of every species is 

 entirely unsuited to the requirements of all of its members at other 

 seasons, because some individuals pass the winter season in areas 

 that are frequented only in summer by other individuals of their 

 species. Such species may have extensive breeding ranges with wide 

 climatic variations so that some individuals may actually be 

 permanently resident in a region where others of their kind are 

 present only in winter. Also, some individual song sparrows and blue 

 jays, for example, have been known to change their migratory status 

 (e.g., a particular bird may migrate one year and not the next or vice 

 versa). Thus, different individuals or populations within these 

 species appear to have different tolerances for climatic conditions. 



The tendency of some birds to move southward at the approach of 

 winter is not always due to seasonal low temperatures. Experiments 

 have demonstrated many of our summer insect feeders, when 

 confined in outdoor aviaries, comfortably withstand temperatures 



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