INFLUENCE OF TOPOGRAPHY 



The relation of the world's land masses to each other and the 

 distribution and association of biotypes within these land masses 

 influence the direction birds migrate. Topography may aid, hinder, 

 or prevent the progress of a migrant depending on the bird's particular 

 requirements. Old World migrants must contend with east-west 

 tending mountain ranges and deserts, whereas New World travelers 

 can proceed north and south across a landscape with its major 

 mountain ranges and river systems oriented in the same direction as 

 the birds migrate. 



When a distinct feature in the landscape, such as borders between 

 fields and forests, rivers, mountain ridges, desert rims, or 

 peninsulas, appears to influence migratory travel, we call these 

 formations "guiding lines," "diversion-lines," "leading lines," or in 

 German, "Leitlinie." It is an observed fact that some birds in a 

 migratory movement alter their course to travel along a leading line, 

 but whether this feature in the landscape caused the migrants to 

 change their course is only theory (Thomson 1960). Besides 

 topography, many other factors can influence this type of flight 

 behavior including weather, wind speed and direction, time of day, 

 species, age, and experience of the bird (Murray 1964). 



Large bodies of water constitute real barriers to soaring birds 

 dependent on thermals and air currents. Good examples of these 

 barriers include the Mediterranean Sea between Europe and Africa 

 and the Great Lakes in North America. Because these water areas do 

 not create good thermals (generally a warm surface, such as a large 

 field on a sunny day, is needed to create the necessary rising air 

 currents for thermals to form) for birds to soar on, migrants are 

 forced to travel around them on updrafts created where land and 

 water meet. The shoreline, then, may appear to be the guiding line, 

 but more than likely the birds are simply following air currents 

 created by onshore winds replacing the rising air from the 

 surrounding warmer land surface and being deflected upward by 

 the shoreline. These conditions often concentrate our buteos (broad- 

 winged, rough-legged, red-shouldered, and red-tailed hawks) into 

 restricted areas where, on good days, numbers observed can be 

 spectacular. Similar conditions exist over the Bosphorus at the 

 eastern end of the Mediterranean Sea where literally thousands of 

 storks, eagles, and buzzards can be observed on a good day. 



While extensive water areas may alter the migratory path of 

 soaring birds, mountain ridges, especially if parallel to the line of 

 flight, are often very conducive to migratory travel. Systematic 

 coverage of the Appalachian ridges indicates all of them aid the 



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