Figure 2.

(a) Use of polarized light cues by birds; (b,c) potential artefacts produced by sheet polarizers on top of orientation funnels. (a) Three-dimensional illustration of the band of maximum polarization at sunset (orange band), running through the zenith and intersecting the horizon vertically, 90° from the Sun (cf. [8]). Birds are suggested to use information from the lowest 5°–10° of the polarization pattern near the horizon to recalibrate their compasses (left double arrow), and the lower 45° of the sky to determine their migratory direction (right double arrow). It is unclear whether birds use information from the skylight polarization for sun-compass orientation during daytime (double arrow over zenith). (b,c) Illustration of light intensity artefacts produced by sheet polarizers placed on top of an orientation funnel (large circle). The sheet polarizer is shown as a square, with the black double arrows illustrating the orientation of the e-vector. Light will be reflected on the white, sloping walls of the orientation funnels maximally on (light grey areas with plus signs) and minimally perpendicular to the axis of the polarizer (dark grey areas with minus signs). (b) Polarizing filter interacting with natural light first filtered by a depolarizer (light grey sheet on top of polarizing filter). (c) Polarizing filter interacting with the natural polarization pattern at sunset (orange band), and transmitted light reflecting on the funnel walls on the axis of the polarizer.