J. exp. Biol. (1977), 70, 105-123 IO tj With 13 figures Printed in Great Britain MAGNETIC FIELDS AND THE ORIENTATION OF HOMING PIGEONS UNDER SUN CHARLES WALCOTT Department of Biology, State University of New York, Stony Brook, New York, 11794, U.S.A. (Received 1 March 1977) SUMMARY Applying a magnetic field of o-1 Gs (1 gauss = io~*T) to the heads of pig- eons homing from unfamiliar release sites significantly increased the scatter of the birds' vanishing bearings. A magnetic field of 0-3 gauss caused no difference between the orientation of birds wearing coils with either their north or south pole up. But a field of o-6 gauss (equal to that of the Earth) produced a small difference in the vanishing bearings of the two groups. Since an applied magneticfieldhas an effect on pigeon orientation under sun, it appears that pigeons do not simply switch between a magnetic and a sun compass, but that there is some interaction between the two systems. INTRODUCTION The evidence that at least some animals can detect magnetic field is now quite convincing. But the role, if any, that magnetic fields play in the orientation of birds is less clear. Merkel, Wiltschko and their colleagues (Merkel, Fromme & Wiltschko, 1964; Merkel & Wiltschko, 1965, 1966; Merkel & Fromme, 1958; Wiltschko, 1968, 1972; Wiltschko & Wiltschko, 1972; and Emlen et al. 1976) have shown that the orientation of caged European Robins can be altered by changing the direction of the applied magnetic field. Southern (1969, 1972) has reported that Ring-billed Gull chicks are disoriented when the earth's magnetic field is disturbed. And Keeton (1971, 1972) has convincingly demonstrated that homing pigeons returning to their lofts under overcast skies are often disoriented when small bar magnets are glued to their backs. Reviews of this work have been published by Emlen (1975) and Keeton (1974a). The magnetic field of a bar magnet is uniform in only a very small area, and to investigate the effect of a well-defined magnetic field on bird orientation it would be highly desirable to surround the bird's head, or wherever the sense organ might be located, with a uniform field that could be accurately controlled in both strength and direction. My colleagues and I have developed a small Helmholtz coil which can be glued to the pigeon's head and produces a relatively uniform field which can be easily controlled by varying the current flowing through the coil. Experiments using the above uniform-field under total overcast are reported by IValcott & Green (1974). But what would be the effect of uniform magnetic fields if