MAGNETIC AND RADIO FIELDS 



tion Griffin (1952a: 368) says that the "responses of birds to pulsed, high 

 frequency, radio waves (from radar transmitters) have been observed both 

 in the United States and in Europe (Drost, 1949), and these observations 

 are cited by Yeagley as 'rapidly increasing evidence' for theories of bird 

 navigation based on electromagnetic phenomena. As Schwartzkopf (1950) 

 has pointed out, however, the density of the energy flux in a pulsed radar 

 beam is probably sufficient to exceed known thresholds for biological effects 

 of electromagnetic radiation. One such effect, described by Barlow, Kohn 

 & Walsh (1947a, b), is the production of visual sensations when the human 

 eye is stimulated either by weak alternating currents (0.2 mA. at 60 c.p.s.) 

 or by the electric current induced by an alternating magnetic field of 500 

 gauss at 60 c.p.s. This magnetic-field strength is approximately 1000 times 

 that of the earth's field. In fact both the radar beams and the artificial mag- 

 netic fields employed by Barlow et al. gready exceed any known intensity 

 of terrestrial magnetism or its electrical by-products. On quantitative 

 grounds it thus appears most unsound to erect theories of bird navigation 

 based upon these observed responses of birds or men to intense electromag- 

 netic stimuli." 



Let us not be dissuaded from the continuation of intensive studies of the 

 influence of radio and radar waves on the behavior of birds; the subject de- 

 mands further investigation. In the light of current knowledge, however, any 

 electrostatic sensitivity in birds appears to be no more useful to regional 

 orientation than sound waves from a shotgun, to which they respond in a 

 similar manner. 



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