ORIGIN AND EVOLUTION OF MIGRATION 



The origin and evolution of bird migration has been discussed in 

 ornithological literature for centuries. As we have seen from the 

 foregoing discussion, migration exists in many forms throughout the 

 world and probably arose to satisfy many different needs in different 

 orders of birds at the same time. New pattens, traditions, and 

 routes are arising today as well as disappearing. Currently, the 

 migration patterns we see are a composite result of historic 

 influences mixed with present day influences. Even though the 

 migration of several different species may be very similar, the 

 patterns exhibited today can be the result of quite different 

 evolutionary processes. Because it cannot be substantiated by 

 experimental facts, any explanation of how a particular pattern or 

 route originates is pure conjecture. 



The general anatomical and physiological attributes of birds 

 enable them to develop more diverse and spectacular migratory 

 behavior than any other group of animals. Their potential for long 

 sustained flights is of primary importance in pre-adapting birds to 

 successful migrations. Migration has long since become a definite 

 hereditary habit of many species of birds that recurs in annual cycles, 

 evidently because of physiological changes which prompt a search 

 for an environment suitable for reproduction and survival. Like the 

 bird's other habits its migratory behavior is just as characteristic as 

 the color of its plumage and, like it, evolved through natural selection 

 because it was advantageous for the survival of the population. Its 

 origin has been thought by some to be a mystery locked in past ages, 

 but by study of the history of how birds came to occupy their present 

 ranges, information becomes available which suggests theories that 

 may be developed and explored. Two that are commonly mentioned 

 are termed the "Northern Ancestral Home Theory" and the 

 "Southern Ancestral Home Theory." 



According to the former of these hypotheses, in earlier ages when 

 conditions of climate, food, and habitat were favorable for existence 

 of birds throughout the year much further north than is the case 

 today, many species remained in these nothern latitudes as 

 permanent residents. Today, such conditions are found only in more 

 southern regions where migrations are much shorter or nonexistent. 

 Gradually, however, in the Northern Hemisphere the glacial ice 

 fields advanced southward, causing a southward movement of 

 conditions favorable to northern birds, until finally all bird life was 

 confined to southern latitudes. As the ages passed, the ice cap 

 gradually retreated, and each spring the birds whose ancestral home 

 had been in the North moved in again to fill newly opened breeding 



95 



