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Based on behavioral and physiological data, birds have the capacity to detect features of the Earth's magnetic field that would allow them to use it for both a compass and a map or a similar geographical positioning system. However, birds are not limited to magnetic cues for navigation. They use a variety of visual and nonvisual sources of directional information. Different species appear to weight sources differently, some use the magnetic field as a primary reference to calibrate visual cues, other species calibrate the magnetic field based on celestial rotation (Able, 1993
). Further work is needed on the interaction and hierarchical relationships among the different cues used for navigation.
The cognitive mechanisms birds use to determine location and directional information from geophysical sources is unknown. Only preliminary work has been conducted on the neural integration of sensory information used for navigation. We do not know where and how the various sources of compass and map information come together in the brain. A bird must compare among the various compass mechanisms and establish the most probable direction for north; similarly, the sources of location information must be compared and the direction for flight determined (Table 1, Fig. 7).
The mechanisms by which the magnetic field is transduced to the nervous system remain incompletely determined. Magnetite seems to be involved with the more sensitive system associated with the bird determining its geographical location relative to its goal. There is evidence for the presence of single domain magnetite and superparamagnetic magnetite in birds. These two forms are not mutually exclusive but the mechanisms that have been proposed to convey information about the magnetic field to the nervous system are radically different. In neither case has a definitive magnetic receptor structure been identified, although candidate structures have been.
Several mechanisms have been proposed to account for the characteristics of the wavelength-sensitive magnetoreceptor. To date no tests have been develop to definitively discriminate among the models. At this time such tests will be difficult to construct because the general models make similar predictions regarding wavelength sensitivities. Until the photopigments that are used for the process and the cells containing those pigments are known, developing test paradigms will be difficult. Species might differ in the photopigments they possess resulting in different critical wavelengths for different species. More importantly, none of the proposed models provide a mechanism by which the change in the magnetic field affects the membrane potential of the receptor neuron. Physiological recordings from the central nervous system indicate that the response is almost immediate with a very short latency. Hence, the effect is most likely on a receptor itself rather than involving a second messenger system.
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