A HISTORY Of VERTEBRATES.- BIRDS AND MAMMALS 



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problems of animal behavior. Obviously the birds must know where they 

 are going; there must be some feature of the environment that is related 

 to the goal of the bird, and the bird must have some way of perceiving 

 this feature. Theories of navigation based on magnetic fields of the earth, 

 visual landmarks, celestial points of reference and other aspects of the 

 environment have been proposed, but no single one explains all of the 

 facts. The magnetic field theory is weakened by our inability to demon- 

 strate that birds are sensitive to magnetic fields. Visual landmarks are 

 certainly used in some cases, but apparently not in all. Dr. Griffin of 

 Harvard University, in a study of the related problem of homing, re- 

 leased sea birds (gannets) in unknown territory one hinidred miles or 

 more inland from their nests and followed their return from an air- 

 plane. The birds did not head straight for home, but Hew in widening 

 circles over large areas, apparently in an exploratory fashion, until they 

 came into familiar territory, and then they headed directly home. Dr. 

 Matthews of Cambridge University questions the significance of explora- 

 tory flight and has suggested that birds use the position of the sun, a 

 sense of time, and a knowledge of the position of the smi at different 

 times in their home territory to determine their position and to find 

 their way home. This would be analogous to a mariner who uses a 

 sextant, a chronometer and a knowledge of the latitude and longitude 

 of his destination to find his way. The fact that birds released in un- 

 familiar territory find their way home better on sunny days than on over- 

 cast days lends support to his hypothesis. Both landmarks and the sun 

 may be used in homing, but it is still difficult to explain many of the 



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Figure 24.11. The northward migration of the Canada goose keeps pace with 

 spring, following the isotherm of 35° F. (Modified after Lincoln.) 



