62 Special Uses of Hearing and Vision /3 : 5 



internal "clock" to find direction. Another, based on the behavior of 

 untrained birds in new territories, ascribed homing to flying in random 

 circles until some feature of the terrain was recognized. A third theory 

 assigned homing to an ability to detect the vertical component of the 

 earth's magnetic field and the Coriolis force (experienced by bodies 

 moving at an angle to the earth's axis of rotation). None of these has 

 ever been conclusively disproved. However, experiments to verify any of 

 these theories have all been inconclusive. 



It is the author's guess that pigeons use strictly visual clues of a very 

 ordinary kind in homing. If this is true, pigeons must be unique in 

 their ability to see a limited number of features of the skyline from a 

 long distance. Not only must they be able to see these features, but 

 they must also possess the ability to learn these features well enough to 

 orient themselves, even if released at a long distance from their origin. 

 This guess has been strongly conditioned by experiments on bird 

 migration. 



Birds migrate as far as 15,000 km over territory they have never seen 

 before and yet manage to return to their own nesting areas of a kilometer 

 or so in radius. They thus have a tremendous precision of migration. 

 It is possible that most are led on their initial flights by other birds who 

 have flown the "course" before, but nonetheless they must either be 

 born with or acquire a tremendous store of visual memory with which to 

 compare their surroundings. This visual memory must be very precise 

 to keep them on course for 8,000 miles. At the same time, it cannot be 

 too precise or rigid, lest the birds be confused by changes in the terrain 

 which occur from year to year. The large number of birds killed by 

 flying into radio towers and monuments for many years after their con- 

 struction attest to the fact that birds only observe a limited number of 

 features of their terrain and discard other information. Likewise, the 

 ability of various species to migrate at night indicates that the position 

 of the sun is at best only one of the visual clues used during migration. 



Man is born with comparatively little information inherited at a con- 

 scious level. By analogy, one might suspect, therefore, that birds had to 

 acquire their knowledge of the terrain on the first migration or two. 

 However, very few people could learn so many landmarks so quickly; 

 by analogy again, this type of learning would also seem unlikely for birds. 

 Perhaps a better comparison than a human is a self-controlled (that is, 

 internal radar controlled) airplane which can fly from the west coast of 

 the United States to the east coast and land on the proper runway with 

 only a few feet margin of error. Such planes have been built with a 

 radar memory of the terrain imprinted on their magnetic tapes. The 

 same problems of precision while ignoring fine details affect the self- 

 controlled plane and the migrating bird. 



