Electrophysiological recordings from the ophthalmic nerve and the trigeminal ganglion of the bobolink (Dolichonyx oryzivorus) indicate the presence of units (14% of the spontaneously active cells) that are sensitive to small changes in the magnetic field. The most common response was an increase in the rate of spontaneous activity. The most sensitive units responded to changes of 200 nT (less than 0.5% of the earth's total field). Other responses included reaction to a 0.5 Hz sinusoidal variation of the magnetic field, and to the movement of a hand-held bar magnet. Because of their sensitivities, the responses to small variations in the magnetic field may be involved in detection of the proposed magnetic navigation map. One possible transducer substance that could account for such a sensitivity is magnetite, which has been previously reported in the upper beak area of the bobolink.