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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 use to achieve this (74).

It is a simple and well-known fact that pigeons home; they do so almost regardless of what we may do to them. Therefore, natural and manipulated changes of the magnetic field have been shown to affect their homing behavior to a varying degree (Dorst 66). In the year 1974, Walcott and Green concluded that artificial fields applied to the pigeon’s head on the release sight, under overcast conditions, disrupt the bird’s ability to maintain a constant compass course. The artificial field in the order of strength of the earth’s magnetic field obviously upsets the bird’s magnetic compass, either by changing its objective north direction, or by field strengths above or below the appropriate level (71).

In 1978, Kiepeenheuer proposed the inversion of the vertical or the horizontal magnetic field component during transport, results in a diverted or random orientation on release (Kiepenheuer). Aside from such effects, after severe manipulations of the magnetic field, much more subtle changes in magnetic field strengths in the order of one percent or even far less of the normal field have been demonstrated to affect the orientation of homing pigeons. Temporal fluctuations, as well as slight topographical changes, may result in a shift of the mean of vanishing bearings or even in random orientation of the pigeons on release. It appears improbable that such small variations in magnetic field strength have any influence on the magnetic compass of the bird, since, according to the results of Wiltschko dealing with robins and other small birds, the magnetic compass seems to be somewhat resistant to deficiencies of the field which is much larger than some of the ones in question. We therefore may have to conclude that the pigeon does not rely on some type of magnetic compass, but that, at least to some extent, its navigational abilities are influenced by very slight changes in the earth’s magnetic field. We might even speculate that systemic variations, or topographical peculiarities of the magnetic field might serve the pigeons as a grid or even as landmarks by which they are able to navigate (Kiepenheuer).

In this context, vector navigation faces fewer difficulties. In order to gain magnetic compass information, a pigeon would have to measure the direction of the earth’s field with far less precision than if it were using this directly as a position-fixing cue. Changes in declination would result in navigational errors, but considering compass information alone the changes are relatively small (Lincoln 102). In the Northeastern United States, declination changes one degree in about 80 kilometers and in the regional anomalies mapped by variations of more than five degrees are infrequent. At geographically small, high-amplitude anomalies, the deviation of a magnetic compass can be larger (102).

In conclusion, the interpretations of the observed magnetic effects on animal orientation seem paradoxical. Theories that might explain the animal’s extreme sensitivity appear to be ruled out by the earth’s field. Vector navigation, on the other hand, restricts the use of the earth’s magnetic field to compass information only, but this theory does not readily explain either the animal’s sensitivity to magnetic fields or sight-specific magnetic effects (Carthy 86).

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