ROUTES OF MIGRATION 



General Considerations 



While it is beyond question that certain general directions of flight 

 are consistently followed by migratory birds, it is well to remember 

 the term "migration route" is to some extent a theoretical concept 

 referring to the lines of general advance or retreat of a species, rather 

 than the exact course followed by individual birds or a path followed 

 by a species with specific geographic or ecological boundaries. Even 

 the records of banded birds usually show no more than the place of 

 banding and recovery. One ought to have recourse to intermediate 

 records and reasoning based on probabilities to fill in details of the 

 route actually traversed between the two points. In determining 

 migration routes, one must constantly guard against the false 

 assumption that localities with many grounded migrants are on the 

 main path of migration and localities where no grounded migrants 

 are observed are off the main path. 



There is also infinite variety in the routes covered during 

 migration by different species. In fact, the choice of migration 

 highways is so wide that is seems as if the routes of no two species 

 coincide. Differences in distance traveled, time of starting, speed of 

 flight, geographical position, latitudes of breeding and wintering 

 grounds, and other factors contribute to this great variation of 

 migration routes. Nevertheless, there are certain factors that serve to 

 guide individuals or groups of individuals along more or less definite 

 lines, and it is possible to define such lines of migration for many 

 species. 



Except in a few species, individuals probably do not follow 

 precisely the same route twice. This is especially true in the group of 

 soaring birds that utilize thermals. Mueller and Berger (1967b) 

 recaptured only three migrants in subsequent years at Cedar Grove, 

 Wisconsin, after banding over 50,000 birds there. In general, those 

 populations of species with very discernible breeding or wintering 

 grounds have readily discernible migration routes. However, even 

 the whole migration process of certain species may show great yearly 

 fluctuations (Rudebeck 1950). 



Aldrich et al. (1949) showed from banding data great variation in 

 migration patterns between species of waterfowl. In some species 

 there was considerable diversity in direction of movement, not only of 

 different breeding populations within a species but also for different 

 individuals of the same breeding population. The impression is 

 inescapable; waterfowl migration is even more complicated than 

 originally supposed, and it is difficult to make generalizations with 



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