Magnets glued to the backs of experienced pigeons often resulted in disorientation when the birds were released from distances of 17-31 miles (27-50 km) under total overcast, whereas no such disorientation occurred during similar releases under clear skies. The magnets did, however, often cause disorientation when first-flight birds were released under sun, and there was some indication of disturbance to experienced pigeons released under sun at longer distances.The possibility that homing pigeons may use the earth's magnetic field as one source of navigational information has been suggested for more than a century (1-4). However, several writers have objected on theoretical grounds (5), and others who have attempted to detect a sensitivity of birds to magnetic stimuli or condition birds to respond to such stimuli (6) have been unsuccessful. Furthermore, though Yeagley (2) reported that in one test release pigeons that carried magnets were disoriented, whereas controls carrying brasses were not, he did not get the same result in tests reported later (3), nor were other investigators (7) able to find any disorienting effect of magnets. Consequently, the idea that magnetic cues are used in avian orientation fell into general disfavor.Recently, however, several Russian workers (8) have noted a general increase in locomotor activity when birds are placed in artificial magnetic fields. Reille (9) has reported success in conditioning pigeons to respond by heart-rate changes to changes in the strength of an artificial magnetic field only slightly stronger than the earth's. Merkel (10) In view of these recent reports, and in view of our own (16) demonstration that there is redundancy of cues in the pigeon orientational system, it seemed appropriate to reinvestigate the effect of magnets on the homing behavior of pigeons.Each of our tests consisted of releasing, alternately, individual pigeons from two groups; the birds for the two groups were selected randomly from a flock housed in a single pen, where they had experienced identical feeding, exercise, and training schedules. The birds of the experimental group wore a magnet bar glued, just before release, to the back at the base of the neck, and the birds of the control group wore a brass bar. Both kinds of bars were about 2.5 X 0.6 X 0.3 cm and weighed 2.7-3.2 g. The strength of the magnets was about 255 gauss at the poles; the field strength at the bird's head was roughly 0.45 gauss. The bars were glued to the skin using veterinary Branding Cement (Victor Business Forms Co., Lincoln, Nebr.), which is completely nontoxic and nonirritating; only in a few cases, when birds were lost for several weeks, did a bar ever fall off.The birds were carried to the release sites in closed vehicles. They were tossed from the hand in random directions. Two observers watched each bird with 10 X 50 binoculars until it vanished from sight, and a compass bearing for the vanishing point was recorded to the nearest five degrees. The vanishing interval, i.e. the interval between tos...
