'Fixed direction'-responses of birds in the geomagnetic field.

IN A RECENT PAPER, WE DESCRIBED THE ORIENTATION BEHAVIOR OF TWO PASSERINE MIGRANTS UNDER DIM RED LIGHT: the birds headed westward in spring as well as in autumn, displaying a 'fixed direction'-response. 'Fixed direction'-responses in other directions were observed under abnormal light regimes. Here, we point out the characteristic features of the 'fixed direction'-responses, in particular their differences to normal compass orientation, and discuss their implications. The conditions under which they are observed suggest complex interactions between magnetoreception and the visual system.