camp. Biochem. Physiol. Vol. %A, No. 4, pp. 66st-671, 1983 Printed in Great Britain 0300-9629183 $3.00 + 0.00 cc 1983 Pergamon Press Ltd HOMING PIGEON NAVIGATION: THE EFFECTS OF IN-FLIGHT EXPOSURE TO A VARYING MAGNETIC FIELD A. J. LEDNOR and C. WALCOTT Laboratory of Ornithology and Section of Neurobiology and Behaviour, Division of Biological Sciences, Cornell University, Ithaca. NY 14850. USA (Receiwd 2 March 1983) Abstract-l. As a test of the idea that under sunny conditions pigeons use magnetic cues as part of their navigation system, homing pigeons were released while carrying packs that generated a varying magnetic field across their heads. 2. Neither of the two types of pack tried produced any consistent significant effects on the birds’ initial orientation although one treatment reduced the birds’ homing success. 3. These results contrast with those obtained by flying pigeons at magnetic anomalies. The significance of this, and whether pigeons use magnetic cues in their position-finding is discussed. INTRODUCTION Homing pigeons released several hundred miles from their home loft routinely manage to find their way home, but despite the many experiments carried out over the last thirty years, their ability to navigate remains only partially understood. It seems clear that pigeons have at least two types of compass mech- anism, one based on the sun, the other on the earth’s magnetic field (for reviews see Keeton, 1974; Able, 1980). But as Kramer (1953) pointed out, knowledge of compass directions would not enable a bird to find the direction of home when released in unfamiliar territory; the bird must also be able to determine its position relative to home. It is the basis of this position-finding mechanism or “map” that is the central problem in avian navigation research. Over the last hundred years it has been suggested many times that homing pigeons might obtain position-finding information from the earth’s mag- netic field. Viguer (1882) and Thauzies (1898) were among the first to propose the idea, which was later taken up by several other authors (see Keeton, 1974 for references). Little data were available to support these early theories, and it was not until 1947 that the first critical experimental tests of the idea were con- ducted. As a test of his theory that pigeons might navigate by using a grid formed from the vertical component of the earth’s magnetic field and the Coriolis force, Yeagley (1947) released pigeons to which he had attached either small magnetic or copper plates. He reported that the birds carrying magnets were less well oriented and had a poorer homing success than the control birds, but in sub- sequent experiments neither he nor other workers could replicate the results (Gordon, 1948; Matthews, 1951; Yeagley, 1951; Van Riper and Kalmbach, 1952). Talkington (1964) proposed a further theory of pigeon navigation based on cues from the earth’s magnetic field, but with the lack of any convincing evidence the theory attracted little attention. A revival of interest in the possible role of magnetic cues in pigeon navigation began when Keeton (197 1) demonstrated that experienced pigeons flying under complete overcast were often disoriented by small bar magnets glued to their backs, whereas controls wear- ing brass bars of equal size and weight were not. Walcott and Green (1974) performed a similar experi- ment but used paired coils to achieve a more uniform magnetic field across the pigeon’s head. They found that under overcast skies the birds’ orientation was correlated with the polarity of the magnetic field; with one polarity the pigeons flew towards home, but with the artificial field pointing in the opposite direction many of the birds oriented 180” away from home. This experiment has been successfully repeated in Italy (Visalberghi and Alleva, 1979), and the results suggest pigeons can derive compass information from the earth’s magnetic field. Keeton (197 1) and Walcott (1977) further showed that magnets or paired coils have only a slight effect on the orientation of experi- enced pigeons under sunny conditions. It seems then that pigeons do have a magnetic compass, but that it is only used when the sun is not visible. Further evidence suggests that pigeons may obtain more than compass information from the earth’s magnetic field. Graue (1965) and Talkington (1967) both reported, in short abstracts, that the orientation of homing pigeons was affected when they were released at or near magnetic anomalies, places where the earth’s magnetic field is distorted by deposits of magnetic minerals. Wagner (1976) and Frei and Wagner (1976) released pigeons at several sites within a magnetic anomaly in the Swiss Jura mountains. This anomaly extends over a region about 30 x 40 km and has the form of a rather regular plateau with a relief of about 200 nT (1 nT = 1 gamma = IO 5 gauss). The pigeons’ bear- ings were deflected to the left of home direction, and they tended to fly down the magnetic gradient. In a further study of the orientation of pigeons at mag- netic anomalies Walcott (1978, 1980a) released pi- geons at a series of anomalies which differ from the Swiss Jura anomaly in a number of ways. They are far more localized, extending normally only over a few kilometers, and are of much higher amplitude (up to 3500 nT). The magnetic topography within these 665