Experiments on the homing ability of birds usually take the form of displacing a bird from its home, releasing it at a known time, observing its behaviour until out of sight and then noting the time when it returns, if at all, to its home. If the interval between release and return is that appropriate to direct flight from release point to home the interpretation is obvious; the bird has flown more-or-less straight home. If, as is more usual, the interval is considerably greater than the direct flight time the question arises as to whether the bird has been flying all the time or whether some substantial fraction of the time has been spent in other ways such as resting and feeding. It is of importance to be able to answer this question: a theory of bird homing based on methodical or random search would demand a considerable time spent actually on the wing, while a theory which postulates some genuine navigational ability on the part of the bird would admit of considerable resting periods and a time actually in the air no bigger than the direct flight time. The time spent in flying is, then, a vital datum which is lacking in conventional homing experiments.

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The correction is made as follows. Suppose the bird spends a time τ1 between release and return, the calibration exposure begins a time τ2, after the release of the bird, and then it lasts a time τ3; then the apparent flight tune as deduced from the recorder should be multiplied by

If now λτ1, λτ2, λτ3< 1 this expression reduces to ⁠, λ is the decay constant of the active body used: for polonium it is equal to 0 00495 day−1, die τ’S being measured m days.

This correction may, of course, be circumvented by usmg an active body of long half life (which is equal to 0·693/λ). Use of such a body would also get over the assumption, implicit in the correction stated above, that the flying time is roughly uniformly distributed throughout the time τ1. Many such bodies of long half life are available, though none is so convenient as polonium.

It may be convenient, for some purposes, to use an α-particle emitter which decays with a half-life of only a few hours. This would then register the flying time over the first few hours after release only and speed of ultimate recovery would be unimportant. The carrying, by a bird, of two recorders, one containing a body of short and one of long half-life would then give a crude indication of whether the initial rate of search was maintained or altered in any way.

Copyright © 1950 The Company of Biologists Ltd.
1950
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