"Nature is pleased with simplicity and effects not 

 the pomp of superfluous causes." Sir Isaac Newton 



"We are accustomed to regard as real those sense 

 perceptions which are common to different indi- 

 viduals, and which therefore are, in a measure, 

 impersonal." Albert Einstein 



11 



The Dimensions of Travel 



T. he problems of avian migration are often ap- 

 proached from the standpoint of human experience; gauged thus, the 

 enigma of orientation is insurmountable. A major barrier to objective think- 

 ing about migration seems to be the great distances that birds travel, dis- 

 tances that we apprehend as human travelers. We have an understanding 

 of a bird's movements about the narrow limits of its home range, but its 

 manner of orientation in migration is unknown. Rowan (1947) has ex- 

 plained that "when a flicker or a bluebird returns from the south year 

 after year to the same nesting box, there is no essential difference between 

 this performance and the return to the same box after a feeding foray, 

 except in the matter of scale; in the one case the bird travels hundreds (or 

 thousands) of miles, in the other merely yards." The depth of the mystery 

 of migrational orientation is in direct proportion to the distance traveled. 



We think of migration, and talk and write about it, in terms of miles or 

 kilometers. Yet a bird has no awareness of a mile as such; neither can a mile 

 be the same for two species, nor equal for man and bird. The distance of 

 travel is relative to the traveler : "all movement is relative" ( Bergson, 1911b ) ; 

 "distance is relative to a particular velocity of the observer" (Eddington, 

 1930). A journey across a field for a Meadow Mouse is far greater than for 

 a man, although by yardsticks the trip is precisely the same for both. The 

 mile for a walking man is longer than for a flying Canvasback; this unit is 

 simply an arbitrary term of convenience that should be regarded as a rela- 

 tive rather than absolute measure of travel. 



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