Figure 4.

(a) Predicted reflectance spectra (colour bars) from spongy tissue of eastern bluebird (S. sialis) feathers. (b) Measured (blue) and predicted (black circles with Gaussian fit) reflectance spectra from spongy tissue of eastern bluebird (S. sialis) feathers. The reflectance prediction is in per cent visible Fourier power and is based on an azimuthal average analysis of the three-dimensional Fourier power spectrum data performed by defining concentric spherical shells corresponding to equally spaced wavelength intervals. Some bins, especially the higher wavelength ones (particularly visible in (a)), contain no Fourier power owing to ‘leakage’ as their power overlaps with neighbouring bins creating a jagged reflectance profile (Briggs & Henson 1995).