Fig. 5. Reflectance spectra of the feral pigeon nape. This had directional coloration, with multiply peaked reflectance. The bird was in poor condition, which may explain the generally low reflectance. (A) Reflectance versus wavelength at E=5°. (i) O=0°; (ii) O=22°; (iii) O=45; (iv) O=60°. The feather was aligned horizontally. (B) As in A plot, with reflectance plotted versus frequency (as wavenumber). The period is 0.5 cycles µm^-1. (C) Reflectance range versus O from the peak near 440 nm to the trough near 500 nm for the spectra in A (solid line) and for illumination at an elevation of 45° with a vertically oriented feather (dotted line). (D) The spectral location of the reflectance peak (_max) versus E for the peak below 450 nm. Values of O (in degrees) are given on the plot. Data are from both horizontally and vertically aligned feathers. (E) Reflectance spectra under diffuse lighting for three orientations of a vertically oriented feather: (i) O=0°; (ii) O=30°; (iii) O=60°. Curves for orientations of -30° and -60° were virtually the same as for +30° and +60°. The peaks were more widely separated than with a point source. As expected, the modulation depth of the reflectance spectrum was smaller than under a point source. (F) _max versus O under diffuse lighting for the reflectance peak between 450 and 550 nm. _max for -30° and -60° are the same as for +30° and +60°. The variation in _max is discussed in the text. O, surface orientation; E, illumination elevation.

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