Anomalies in the Earth’s Magnetic Field Increase the Scatter of Pigeons’ Vanishing Bearings

Abstract

Horning pigeons were released under sunny conditions at places with large variations in the total intensity of the Earth's magnetic field. At five out of six such places, pigeons were more scattered in their homeward orientation than at magnetically normal sites. This suggests that some aspect of the magnetic field affects either the pigeon's sun compass or its navigational system. A. Introduction Since Keeton (1971, 1972) first showed that homing pigeons carrying bar magnets were often disoriented under overcast skies, the evidence that magnetic fields playa role in pigeon homing has been increasing. Walcott and Green (1974) reported that reversing the polarity of the magnetic field produced by a pair of Helmholtz coils around a pigeon's head often results in pigeons flying in a direction away from the loft when the sun is not visible. Under sunny conditions, the same coils cause an increase in the scatter of vanishing bearings and a small difference in the angle of the vanishing directions of the two groups (Walcott, 1977). However, Keeton et al. (1974) showed that the vanishing bearings of pigeons also vary with natural variations in the Earth's magnetic field. These fluctuations in the Earth's field are much smaller than those caused by either magnets or Helmholtz coils and are far weaker than local changes in the field at many magnetic anomalies. Graue (1965) and Talkington (1967) report that pigeon orientation was affected by anomalies in the Earth's magnetic field, but the data on which these abstracts were based have never been published. Recently, Wagner (1976, Frei & Wagner, 1976b) has shown that pigeons released near a magnetic anomaly orient to the left of horne in roughly the direction of the magnetic gradient. However, these anomalies were rather weak-only about 80 gamma or 0.1% of the normal Earth field. Perhaps some of the stronger local magnetic anomalies in New England would have an even greater effect on pigeon orientation: this paper summarizes the results of releases at six such sites. B. Materials and Methods Trained, experienced homing pigeons from lofts in Lincoln, Massachu-setts were released under sunny skies and radio tracked at each of the magnetic anomalies. Each bird was released only once at each anomaly and these releases were either preceded or followed by a release at a magnetically normal control site. Anomalies were located using K. Schmidt-Koenig et al. (eds.), Animal Migration, Navigation, and Homing