MIGRATIONS OF WATERFOWL 



may give the migrating bird an advantage that the man going short dis- 

 tances on foot cannot even comprehend. 



The sun, of course, is not always available for reference in travel. We 

 know that some birds migrate through regions covered by cloud both by 

 day and night, so that the success of migration may not hinge entirely on 

 sun navigation. But from the evidence of Delta observations (mass migra- 

 tions starting under clear sky), it appears that at the start, and at some 

 periods during residence at the destination or at stopping places along the 

 way, the sun is visible during segments of the long span of spring and fall 

 migration and serves as a cue to the direction of home. 



In summary, the discussion amounts to this: the distance of travel is 

 relative to the height, breadth, and velocity of travel. We know this applies 

 to our own human lives as between man on foot and man in an aircraft. 

 Avian behavior suggests that this same relativity of motion applies to bird 

 migration, but that distance is relative not only to height, breadth, and 

 velocity of travel; problems of orientation and retentiveness are also rela- 

 tive to these coordinates. If the relativity of motion is applied to bird mi- 

 gration, the problems may be faced in a truly objective manner. A com- 

 parative examination makes it clear that the complexity of migrational 

 orientation is not fairly judged by the miles of travel. 



While the four coordinates of travel are always relative, men and birds 

 may use the sun as a reference for compass direction in local travel, and as 

 a reference for longitude and latitude in longer journeys. The traveler is 

 cued by the arc and schedule of the sun and may appraise the general di- 

 rection of home even when displaced from familiar surroundings. An aware- 

 ness of the changes in the arc and schedule of the sun varies directly with 

 the distance and velocity of travel; travelers moving swiftly over long dis- 

 tances could perceive these changes (hence their position relative to home) 

 more keenly than those moving only short distances. 



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