migration of soaring birds. Apparently the highest and longest 

 ridges deflect the horizontal winds upward better than the shorter 

 ridges less than 1,000 feet high, and more birds are seen, on the 

 average, along the higher ridges (Robbins 1956). 



In general, nocturnal migrants are not influenced by topography 

 as much as diurnal travellers. Radar observations have played an 

 important role in establishing this difference. Bellrose (1967) found 

 that waterfowl migrating at night through the Midwest were not 

 influenced by major river systems, but in the evening or after 

 daybreak ducks and geese tended to alter their course along the 

 rivers. Drury et al. (1961) recorded massive fall and spring 

 movements from the New England area out over the Atlantic Ocean 

 without any apparent regard for the coastline. Until nocturnal 

 migration could be "watched" on a radar screen, many bird observers 

 assumed the guiding effect of the coastline on migratory travel was 

 more restrictive than it really is. 



In summary, topography may help or deter a migrant in its 

 passage. It affects different birds in different ways. In North 

 America, migratory movements are continent wide, and no evidence 

 has indicated any particular part of the landscape influences all 

 birds in the same manner. Certain bird populations may use general 

 areas in migration, but they are usually not rigidly restricted to them 

 because of topography. 



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