The Orientation of Pigeons at Gravity Anomalies

The means by which homing pigeons determine the direction to their home loft when released in unfamiliar territory remains a mystery. Hypotheses involving the sun (Matthews, 1953, 1955), Coriolis force and the earth's magnetic field (Yeagley, 1947, 1951), inertial cues (Barlow, 1964, 1966), infrasound (Kreithen & Quine, 1979), have been advanced but currently there is no strong empirical support for any of these hypotheses. Recently Papi (see Papi, 1976; Papi et al. 1980 for reviews) and Wallraff (1980) have proposed that olfactory cues are the basis of the pigeons' position-finding ability but not all investigators are convinced by the evidence (see e.g. Able, 1980; Keeton, 1980; Schmidt-Koenig, 1979; Walcott & Lednor, 1983). One cue from which pigeons could, in principle, derive position-finding information is gravity. Due to the spheroidal shape of the earth and the centrifugal forces associated with its rotation there exists a regular gradient (of 0-73 mgal km" at 32 °N) in the force of gravity between the geographic equator and the poles (Dobrin, 1976). If birds were sensitive to such minute changes in gravity, and were able to compensate for the variation in gravity with altitude, they could determine their latitude. Alternatively, as suggested by Keeton (1980), birds might use the direction of the gradient of gravity as a true north-south reference which would enable them to measure magnetic declination. To test the possible role of gravity in pigeon navigation Larkin & Keeton (1978) repeatedly released the same group of birds at a particular release site. They found a correlation between the birds' mean vanishing bearings and the day of the lunar synodic month, that is, the birds' bearings were deflected as a function of the lunar cycle. Although the authors could not directly show that tidal variations in gravity (at peak approx. 0-3 mgal) were responsible, the results suggest this possibility. A second hint that gravity might be involved comes from studies of the effect of magnetic anomalies on pigeon orientation. Following up reports by Graue (1965), Talkington (1967) and Wagner (1976), Walcott (1978) and Kiepenheuer (1982) have shown that experienced pigeons released under sunny conditions are disoriented when released at localized magnetic anomalies, locations where the earth's magnetic field is distorted by underground deposits of a magnetic mineral. By releasing birds at a series of different magnetic anomalies, Walcott (1978, 1980) showed that the degree of scatter of the pigeons' bearings correlated well with an index of the magnetic