No evidence for polarization sensitivity in the pigeon electroretinogram

The electroretinographical response to flashes of linearly polarized light directed at the pigeon's yellow field was compared with that to flashes of unpolarized light. This was carried out for white light and for monochromatic light of various wavelengths, including ultraviolet. In addition, responses to slow rotation of the E-vector of polarized light were measured. Neither the presence or absence of polarization, nor the orientation of the E-vector, influenced any of the electrophysiological variables that were monitored in these experiments.

The photopic sensitivity of the yellow field of the pigeon's retina to ultraviolet light.

The photopic spectral sensitivity of the yellow field of the pigeon's retina to UV light was determined electrophysiologically. The sensitivity curve could be approximated with a model in which the activity of only two cone types were incorporated. In this model, the first type of cone had a maximum sensitivity at 366 nm and was combined with an oil droplet that is completely transparent in the UV wavelength range. The second type had a sensitivity maximum at 415 nm and was associated with an oil droplet cutting off light below 390 nm.

The relation between celestial colour gradients and the position of the sun, with regard to the sun compass.

Colour gradients along the sky caused by atmospheric scattering were measured on sunny days. It is concluded that whereas the shape of the spectral intensity distribution in the short wavelength range is stable, the distribution at longer wavelengths depends on the direction of measurement. We expressed these relative intensity differences as a spectral contrast. This contrast plotted as a function of angular difference with respect to the position of the sun establishes a smooth gradient. We suggest that the pigeon's UV sensitivity is part of a colour processing system, which is well adapted to employ these gradients in order to derive the sun's position.

THE ORIENTATION OF THE E-VECTOR OF LINEARLY POLARIZED LIGHT DOES NOT AFFECT THE BEHAVIOUR OF THE PIGEON COLUMBA LIVIA

Orientation with reference to the time-compensated sun-azimuth compass has been established for the homing pigeon Columba livia. Previous qualitative studies claim that pigeons are sensitive to the orientation of a polarizer and it has been suggested that these animals are able to use sky-light polarization as an indirect reference to the sun's position when the latter is shielded from view. We report experiments which were undertaken to quantify the sensitivity of the homing pigeon to the orientation of linearly polarized light. The results of our initial experiments suggested that the animals responded to secondary cues. Further experiments were carried out to avoid such artefacts. Under circumstances where secondary cues were rigorously avoided, we were, however, not able to demonstrate any directional response that was caused by the E-vector orientation of the illumination. These results throw doubt on the suggested polarization-sensitivity of birds in general.