QUICK SEARCH:   [advanced] Author: Keyword(s):
Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents

This Article Summary Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Related articles in JEB Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Osorio, D. Articles by Ham, A. D. Search for Related Content PubMed PubMed Citation Articles by Osorio, D. Articles by Ham, A. D.

Spectral reflectance and directional properties of structural coloration in bird plumage

D. Osorio^* and A. D. Ham

School of Biological Sciences, University of Sussex, Brighton BN1 9QG, UK

View larger version (18K): [in a new window]   Fig. 1. Viewing geometry. (A) Reflectance spectra were recorded from a 1 mm diameter spot on the feather surface, with the vane perpendicular to the line of sight. This spot was on the horizontal axis of rotation of the specimen holder, which lay perpendicular to the line of sight. The orientation of the feather surface and the position of the light source are specified relative to the line of sight. Surface orientation, O, is the rotation of the surface relative to the perpendicular to the line of sight and is measured clockwise in this figure (i.e. at O=90°, the surface faced upwards). The light source moved in azimuth and elevation, but we refer mainly to effects of varying elevation, with the angle E being given by clockwise rotation from the line of sight (i.e. at E=90°, the light source was directly above the specimen). (B) Feathers were mounted in two alignments. At O=0°, when vertically mounted, the apex was vertically above the base; when horizontally mounted, the vane was horizontal.

View larger version (9K): [in a new window]   Fig. 2. Reflectance geometry. (A) Directionality. A directionally iridescent feather can be understood as an imperfect spectrally tuned mirror. Light from a point source, I, incident at angle , is reflected in a diffuse beam centred on axis R at 180°-. The width of this (approximately circular) beam at 50% of maximum intensity is . The structurally reflective surface is not always in the same plane as the feather vane, and its tilt, t, out of this plane can be calculated from the angles I and R with respect to the feather surface. Tilt was predominantly away from the proximo-distal axis. For t>0°, light from directly above the feather is directed towards its base, and for t<0° towards the apex. Unlike the iridescent reflection, the direction of non-spectrally selective specular reflection was consistent with the reflective surface being in (or close to) the plane of the feather vane. Spectral tuning of laminar interference reflectors varies with angle of incidence, (see Fig. 7 in Land, 1972). (B) The spectral location of the reflectance peak, max, varied across the reflected beam. For a fixed observer (see Fig. 1A), max was independent of surface orientation, O, and a linear function of the angle, E, separating I from the line of sight. i.e. max=a-bE, where a is max at E=0°, and b is a constant. (C) We can see why max might be independent of O because, on the axis of reflection, the angles of incidence (I') and reflectance (R') are equal, here being . If O varies and E is fixed, then the angle of incidence and reflectance shift from by equal and opposite amounts, ±ß. Consequently, spectral shifts in max for the incident and reflected light are approximately equal and opposite, compared with the value of max, where angles of incidence and reflectance are equal and can be expected to cancel each other out.

View larger version (53K): [in a new window]   Fig. 3. Photographs of feathers indicating how colours alter with varying surface orientation (O) and illumination elevation (E). The feathers are aligned vertically (Fig. 1); see Materials and methods for details. (A) Magnificent hummingbird cap. (i) O=0°, E=5°; the feather is black; (ii) O=45°, E=5°; (iii) O=0°, E=-75°. (B) Feral pigeon. (i) O=0°, E=5°; (ii) O=30°, E=80°; (iii) O=45°, E=100°. (C) European jay. (i) O=0°, E=5°; (ii) O=5°, E=5°; the feather shows a specular highlight; (iii) O=45°, E=70°. (D) Common kingfisher. (i) O=30°, E=5°; (ii) O=30°, E=45°; (iii) O=30°, E=90°. (E) Indian roller. (i) O=0°, E=5°; (ii) O=45°, E=5°; (iii) O=45°, E=90°.

View larger version (28K): [in a new window]   Fig. 4. Reflectance spectra of the magnificent hummingbird crown. This had directional coloration, with a single main reflectance peak and a reflective mechanism tilted relative to the plane of the feather vane. All data, except in E, are from a vertically aligned feather (see Fig. 1B). (A) Reflectance under localised illumination for four viewing geometries: (i) E=5°; O=30°; (ii) E=5°; O=45°. (iii) E=5°; O=55°; (iv) E=40°; O=70°. The left panel gives reflectance relative to diffuse white standard, and the right panel gives reflectance curves normalised to the maximum. The spectral location of the reflectance peak (max) is dependent on the value of E but independent of O. (B) Relative reflectance maxima versus O for E=5° (solid line) and E=40° (dotted line). These data imply that the reflective structure was tilted at approximately 40° to the plane of the feather vane (see Fig. 2A) and that light was diffused into a cone approximately 40° across. (C) max versus illumination elevation, for O=50°. (D) Reflectance of the vertically oriented feather under diffuse lighting for three orientations: (i) O=0°; (ii) O=30°; (iii) O=60°. The reflectance peak was broader than under a point source, and reflectance was greatest when the feather was viewed obliquely. (E) Reflectance of the horizontally oriented feather under diffuse lighting for three orientations, as in D. The feather remained dark over all orientations. O, surface orientation; E, illumination elevation.

View larger version (29K): [in a new window]   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.

View larger version (13K): [in a new window]   Fig. 6. Reflectance spectra of a European jay wing covert, which had diffuse coloration with a specular highlight. (A) Reflectance under localised illumination for three viewing geometries: (i) E=5°; O=0°; (ii) E=5°; O=5°; (iii) E=90°; O=45°. Curve ii is elevated across the spectrum, consistent with a specular highlight. Curves i and iii are alike, implying that the structural colour (rather than the highlight) was insensitive to viewing geometry. (B) Reflectance under diffuse illumination for two orientations: (i) O=0°; (ii) O=60°. At O=30°, reflectance resembled that at O=0°. These reflectance curves had narrower peaks than for localised illumination, probably because they were recorded from different locations on the blue coloration band (this feather grades from white through black and is most saturated at an intermediate location). Otherwise the reflectance was unaffected by the difference in lighting. O, surface orientation; E, illumination elevation.

View larger version (21K): [in a new window]   Fig. 7. Reflectance spectra of the kingfisher crown. Data are for a vertically aligned feather, but the horizontal alignment spectra are the same. This feather had diffuse coloration, with a spectrally non-selective specular reflectance. Variation in reflectance across the spectrum was greatest with the surface perpendicular to the line of sight. (A) Spectra for illumination near co-axial with the line of sight, E=5°: (i) O=0°; (ii) O=8°; (iii) O=45°. (B) Spectra for oblique orientation, O=45°: (i) E=5°; (ii) E=90°. Spectrum i is the same as spectrum i in A. (C) The spectral location of the reflectance peak (max) versus E. Values of O are given in degrees. (D) Reflectance under diffuse illumination. (i) O=0°; (ii) O=60°. At O=30°, the spectrum was intermediate between the two illustrated. O, surface orientation; E, illumination elevation.

View larger version (23K): [in a new window]   Fig. 8. Reflectance spectra of the turquoise patch on an Indian roller tail. This had diffuse coloration, which looked most `colourful' when viewed obliquely with the light behind the observer. Data are for a feather aligned vertically (see Fig. 1B), but rotating the feather to a horizontal alignment had no effect on reflectance. (A) Reflectance with O=70°: (i) E=5°; (ii) E=45°; (iii) E=70°. (B) Reflectance with E=5°: (i) O=0°; (ii) O=45°; (iii) O=70°. Spectrum iii is the same as spectrum I in A. (C) The spectral location of the reflectance peak (max) versus E. Values of O (in degrees) are given beside the cross to which they refer. There is a consistent shift to longer wavelengths with increasingly oblique viewing angles. (D) Reflectance under diffuse lighting: (i) O=0°; (ii) O=30°; (iii) O=60°. O, surface orientation; E, illumination elevation.