habitat only to be driven south again at the approach of winter. As the 

 size of the ice-covered area diminished, the journeys to the summer 

 breeding areas became even longer until eventually the climatic 

 conditions of the present age became established, and with them, 

 present patterns of the annual advance and retreat we call migration. 



The opposing theory is simpler in some respects and supposes the 

 ancenstral home of Northern Hemisphere migratory birds was in the 

 Tropics. As all bird life tends to overpopulation, there was a constant 

 effort of young individuals to pioneer and seek breeding grounds 

 where competition was less severe. Species better adapted to more 

 northern latitudes moved in that direction for nesting but were kept 

 in check by the glacial ice and forced to return southward with the 

 recurrence of winter conditions. Gradually, as the ice retreated, vast 

 areas of virgin country became successively suitable for summer 

 occupancy, but the winter habitat in the South remained the home to 

 which the birds returned after the nesting season. 



The above two theories presume that the Quaternary glaciations, 

 which occurred 10,000 to 1 million years ago, have been the 

 predominate influence on bird migration in North America and 

 Europe as we observe it today. There is no doubt these extreme 

 climatic and ecologic barriers played a part in shaping or modifying 

 some patterns, but as Moreau (1951) has pointed out, well-developed 

 migrations occur in parts of the world, including the Southern 

 Hemisphere or even within the tropics, where continental glaciation 

 was not a factor. Furthermore, migrations to fit various needs have 

 probably been going on ever since birds could fly. The tremendous 

 Pleistocene glaciations actually occupied less than a hundreth of the 

 time birds have existed on the earth and probably only determined 

 the details of migrations as we see them today (Moreau 1951). 



The northern and southern ancestral home theories appear 

 diametrically opposed to each other but Dorst (1963) concludes they 

 are perfectly compatible. Since the phenomena probably occurred 

 simultaneously, northern migrants then originated from two stocks: 

 the North Temperate Zone birds sought refuge to the south during 

 the glacial periods and the tropical avifauna expanded their range 

 during the interglacial periods. Dorst also stated this double origin is 

 more prevalent in North America where the tropical element is most 

 abundant. Birds representing this element include hummingbirds, 

 tyrant flycatchers, orioles, tanagers, and blackbirds. At some 

 latitudes, they nest in the same area as the shorebirds which are of 

 arctic parental stock. 



These theories assume migration is a genetic, inherited character, 

 but we now know in some species it can be modified in the lifetime of 

 one individual and the place some individuals return to nest or winter 

 is not the ancestral home but a place to which they had been 

 transported at an early stage in their development. Traditions that 

 have lived for countless generations may die overnight if experienced 

 individuals are lost or no longer active (Hochbaum 1955); migration 

 patterns remain constant only as long as the factors influencing these 



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