d swamp any measurement of that induced by the movement of the body through the earth’s field (71). By using only ten Pigeons treated in this way and ten control birds with copper bars, Yeagley claimed to have established that the magnets had a strongly deleterious effect on homing (73). To the contrary of Yeagley’s findings, many retests that involved the variables of his experiments proved countless times that his hypothesis was completely unacceptable. Unacceptable not only because of its theoretical impossibility, but also because the massive field experiments have produced entirely negative results (Yeagley 1036). At the same time it is well to be careful of dismissing possible extensions of known senses. In the year 1951, Lissmann demonstrated a remarkable form of proximate orientation in certain fish. These fish set up a weak electrical field around themselves and apparently were able to detect not only their surroundings, but also their prey by changes in impedance. It has been proven that fish of this nature will react to a moving magnet. In 1953, Griffin showed that a form of echo-sounding is used by a bird nesting in dark caves (Lissmann 201).Inspection of maps of the United States shows frequent anomalies in the horizontal and vertical components, having strengths of several hundred gamma and extending over several hundreds of kilometers. There seems to be no reason for one to suppose that regional anomalies in other parts of the world would be of any different character. This evidence suggests that position-fixing by bicoordinate navigation using any of the magnetic elements may be possible (Griffin 73). If no regular gradients exist, over the distances which pigeons navigate, it is difficult for one to imagine how the strategy would be successful. No physical mechanism is able to separate the earth’s main field from the anomaly field, and it is not clear what signal-processing a pigeon could us...
