PATTERNS OF MIGRATION 



Band recoveries, netting records, and personal observations help 

 us to critically examine migration routes and probe deeper into the 

 origin and evolution of these pathways. We are beginning to realize 

 certain deviations occur from the "normal" north and south 

 movements expected in most species. In the previous section on 

 routes, we touched briefly on the fact that some routes are not 

 poleward at all, but in some other direction. We know that many 

 migrants do not stop at the exact localities year after year but they 

 probably do follow the same general course each season. After many 

 years of observations a pattern emerges for that population, species, 

 or group of species. In this section we would like to take a closer look 

 at some of the interesting patterns (or "eccentric routes" as Cook 

 (1915a) referred to them) in migration that birds are annually to 

 travel from breeding to wintering grounds and back again. In many 

 cases, the causative agents are unknown or pure conjecture, but in 

 others, sound biological principles can be put forth that may indicate 

 why a particular species could have evolved the specific pattern it 

 exhibits. 



Loops 



Many species do not return north in the spring over the same route 

 they used in the fall; rather, they fly around an enormous loop or 

 ellipse. Cook (1915a) considered food as the primary factor in 

 determining the course birds took between winter and summer 

 ranges. Individuals that returned by the same route and did not find 

 sufficient food for their needs at that time were eliminated from the 

 population, and only progeny from individuals that took a 

 different course with sufficient food lived to build the tradition of a 

 loop migration. Other investigators consider prevailing winds a 

 major factor in the evolution of loop migration. Whatever the reason 

 may be, it has most likely evolved separately in each species to satisfy 

 its particular needs, and the fact that this pattern occurs all over the 

 world in completely unrelated species is a good illustration of 

 convergent evolution. 



The annual flight of adult golden plovers is so unusual, it will be 

 given in some detail. The species is observed by hundreds of bird 

 watchers every year and it well illustrates loop migration (Fig. 24). 



In the fall, the birds fatten on the multitude of berries along the 

 coasts of Labrador and Nova Scotia, then depart south over the 

 Atlantic Ocean to South America. After reaching the South 

 American coast the birds make a short stop, then continue overland 

 to the pampas of Argentina, where they remain from September to 



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