Fig. 2. Perception of the magnetic field as a visual pattern of light intensity and/or color (after Ritz et al., 2000). The inclination of the magnetic field in the northern (A) and southern (B) temperate zones will cause this pattern produced by the magnetic field to be `tilted' at a steep angle, with one end of the pattern viewed against the sky and the other viewed against the substrate. Because of positive inclination in the northern and negative in the southern hemisphere, respectively, the upward end of the magnetic axis (where the portion of the pattern is visible against the sky) will be towards geographic South (gS) in the northern hemisphere, and towards gN in the southern hemisphere. Consequently, when the magnetic field is rotated 90° counterclockwise (CCW), the portion of the pattern viewed against the sky will be 90° clockwise (CW) of gN (east) in the northern hemisphere and 90° CCW of gN (west) in the southern hemisphere. Similarly, when the magnetic field is rotated 90° CW, the portion of the pattern viewed against the sky will be 90° CCW of gN (west) in the northern hemisphere and 90° CW of gN (east) in the southern hemisphere. Consequently, lateralization of the light-dependent magnetic compass in the right eye of birds (Wiltschko et al., 2002), or the gradients of light intensity and spectral content associated with the sky at sunrise or sunset, may cause differences in the response to CW and CCW shifts of the magnetic field (see text). In either case, any difference in the birds' response to CW and CCW shifts in experiments carried out in the northern hemisphere, should be reversed in the southern hemisphere (see text).