328 
been constructed. Thorpe and Wil- 
kinson (1946) have considered whether 
such a system could operate on a small 
scale and have concluded that in a 
bird’s labyrinth viscous losses would 
be large and the Coriolis flow would 
be less than that due to random ther- 
mal agitation. 
More recently Yeagley (1947) has 
surmised that pigeons, through an 
appreciation both of Coriolis force and 
of the vertical component of the earth’s 
magnetic field, can not only recognize 
bearings but can locate themselves 
uniquely or nearly so on the earth’s 
surface. The experimental evidence 
adduced in support of this surmise has 
been destructively criticized by Thorpe 
and others (1948) so it is perhaps un- 
necessary to enumerate here all the 
theoretical objections which are ap- 
parent. The suggestion that birds 
may be capable of response to the 
earth’s magnetic field, though not 
new, and not, as Yeagley presents it, 
feasible, is nevertheless worth brief 
consideration. 
It is reasonably certain on both 
theoretical and practical grounds that 
a constant magnetic field, even if very 
large, produces no effect on a living 
body which is stationary or in uniform 
movement through it. A magnetic 
field produces biological effects only 
in such circumstances that electric 
currents are caused to flow in living 
tissue, namely, when the magnetic 
field intensity changes or the tissue is 
accelerated across the field. The 
second condition is, however, realized 
when a bird is circling and it is easily 
seen that, owing to the horizontal 
component of the earth’s magnetic 
field, the current through any small 
element of volume within the bird will 
vary sinusoidally through a cycle for 
each completed circle of flight. Hence 
if the bird could appreciate the 
instants of maximum current it could 
distinguish N. and S. from E. and W. 
If it could also appreciate the sense of 
the current it could distinguish the 
cardinal points uniquely. 
As in the case of Ising’s theory the 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
plausibility of this kind of mechanism 
turns on a comparison of the magni- 
tude of the currents likely to be pro- 
duced in such circumstances with the 
magnitudes of currents known to have 
sensory effects. One of the few rele- 
vant experiments is that of Magnusson 
and Stevens (1911) who succeeded in 
producing visual sensations in man 
with a rather large alternating mag- 
netic field. Auditory sensations are 
well known to result in man from elec- 
tromagnetic radiation of sufficient in- 
tensity, provided there is modulation 
at an audible frequency. And the 
frequently reported disturbance of 
birds in the immediate neighborhood 
of broadcast transmitters during trans- 
mission is very possibly due to similar 
sensory excitation. In all these cases 
the induced current density is proba- 
bly greater by a factor of more than 
105 than that which would be induced 
in a circling bird by the earth’s mag- 
netic field. Such a gap seems too wide 
to be easily bridgeable. 
So neither Coriolis forces nor terres- 
trial magnetism are likely explana- 
tions of birds’ migratory powers. And 
it may perhaps be misguided to seek 
such exotic solutions. Better evidence 
is needed that the ordinary senses, 
together with adventitious assistance, 
such as prevailing winds, etc., are not 
enough to explain the facts. In par- 
ticular, a balanced estimate of the 
migratory achievement is impossible 
without a far greater knowledge of the 
losses en route than we yet possess. 
Nevertheless, suggestions such as 
Ising’s and Yeagley’s, have often been 
fruitful in the past, not necessarily be- 
cause they were right, but because the 
attempt to disprove or confirm them 
gave an impetus and direction to re- 
search which was previously random 
and fumbling. 
BIBLIOGRAPHY 
Boeke, J. 
1934. Niedere Sinnesorgane. Handb. 
Vergl. Anat. Wirbelt., vol. 2, 
No. 2, pp. 841-878. (Bolk, 
Goéppert, Kallius, Lubosch.) 
