MAGNETIC AND RADIO FIELDS 



another circumstance making the theory less attractive. Even if a bird 

 could sense magnetic force, successful orientation would still have to de- 

 pend heavily upon visual geographic orientation. A Canvasback traveling 

 to Delta from the Chesapeake Bay, for instance, directs its flight first to 

 intermediate points such as Lake Erie and Lake Christina. Migrants from 

 the Atlantic Coast would be obliged to appraise magnetic force in terms of 

 directions relative to Lake Erie and each of the other stopping places on 

 the long route homeward. 



A more elaborate theory has been set forth by Ising ( 1945) and Yeagley 

 (1947), who propose that birds may be aware of the Coriolis force. Yeag- 

 ley's hypothesis implies an avian sensitivity "to the effect of motion through 

 the vertical components of the earth's magnetic field and to the effort ex- 

 erted to overcome the Coriolis forces due to the earth's rotation. Each of 

 these influences provides a set of lines joining points of equal intensity and 

 together forming a navigational gridwork. The theory is that by correlating 

 its instantaneous land speed with the two effects, a bird can fly to its home 

 which is a unique point in this gridwork, or to related conjugate points ex- 

 isting in the gridwork, at a position other than its home" (Thorpe, 1949:92). 

 While Yeagley believes that this theory clarifies the "age-old mystery of bird 

 migration" and considers it "the first working hypothesis to explain homing 

 pigeon navigation," his experiments seem most important because their 

 results appear to be essentially negative.* 



Watching the homing behavior of his pigeons from an aircraft, Yeagley 

 (1951:755) observed that they often detoured to fly over towns, usually 

 circling the larger towns several times. He suggested that this town-interest 

 might be due to the birds' response to power stations located there. Even be- 

 fore electricity arrived in Delta, however, and when the greatest electrical 

 force was in automobile batteries, racing pigeons were seen each year over 

 the village, the birds usually dropping down to alight on a house or barn. 

 A displaced pigeon might be expected to react positively to a town as a lost 

 duck turns to a lake or marsh. 



Griffin (1944:25), in his classic discussions of the sensory basis of bird 

 navigation, remarks that "there has recently been much rumor in popular 

 publications about interference with the homing of pigeons and migrations 

 of wild birds by radio stations (Brown, 1938; Casamajor, 1927; Darling, 

 1940; Aymar, 1935; and Maurain, 1926). The writer knows of no statistically 



• For reviews of the Yeagley studies, see Slepian, 1948; Varian, 1948; Davis, 1948; Odum, 

 1948; Thorpe, 1949; Wilkinson, 1949; Drost, 1951; Matthews, 1955. 



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