Fig. 2. Spectrometric measurements of the full-spectrum and monochromatic light used in the experiments, and the spectrum of an incandescent light bulb and the natural spectrum at sunset for comparison. The black line indicates the spectrum of the full-spectrum light. The dark green (560.5 nm), yellow-green (567.5 nm) and red (617 nm) lines indicate the spectrum of the monochromatic light as seen by the birds sitting under one opaque Plexiglas sheet. The composition of the measured full-spectrum light passing through neutral density filters of different transmissions and multiple layers of opaque Plexiglas sheets does not deviate substantially from the spectrum shown in black. For comparison, the orange line shows the spectrum of an incandescent light bulb, and the violet line shows the natural spectrum 14 min after sunset (reproduced from McFarland and Munz, 1975; 300-700 nm only). All spectra are standardised by taking the maximum value within the measured wavelength interval as 10.