Fig. 1. Setup of the light equipment used in this study to produce full-spectrum and monochromatic light. We used two xenon arc lamps, each lamp distributing light to three orientation cages. The light first passed through a column of double-distilled water and then entered a trifurcated glass fibre bundle, at which end a filter holder contained interference filters and/or neutral density filters to produce monochromatic light and/or to reduce light intensity, respectively. Before entering the orientation cage, the filtered light passed through 1-4 opaque Plexiglas sheets that acted as diffusers and that were placed directly on the top of the orientation cage so that the birds could not see the light source as a point.