How Celestial Polarization Changes due to Reflection from the Deflector Panels Used in Deflector Loft and Mirror Experiments Studying Avian Orientation

In bird experiments which investigate the role of optical orientation cues associated with sunset it is a common method to reflect the celestial sunset factors by vertical deflector panels. Similar panels are also used in the deflector loft of homing pigeons that rotate wind and light cues in a clockwise or counterclockwise direction. The responses of test birds in these behavioural experiments often appear to be contradictory, because the deflectors produce a polarized optical stimulus that differs both qualitatively and quantitatively from the natural skylight polarization. This paper gives a quantitative account of the change of celestial polarization in such situations. The polarization pattern of skylight reflected from different vertical deflector panels is computed as a function of the elevation of the sun from the horizon. It is shown that the reflection polarization of a deflector panel composed of transparent glass with or without white or black background cannot simulate the distribution of polarization of light in clear sky. On the other hand, if the deflector consists of a metal plate or metal-glass mirror, it can mimic the celestial polarization pattern well, because it changes the polarization of incident light after reflection only slightly.