Structural colours from the feathers of the bird Bostrychia hagedash

Iridescence in the bird Bostrychia hagedash is interesting in the sense that instead of the usual single, fairly narrow, reflection peak seen for many birds and insects, up to six strong peaks were found covering the spectral range from the near ultraviolet (UV) to the near infrared (IR). By recording reflectance spectra over this range and comparing it to model calculations based on a structural analysis using electron microscopy, the mechanism for iridescence as well as the optical constants of the materials comprising the coloured feathers, could be established. It was discovered that iridescence is mainly based on a relatively thick but uniform keratin outer layer on the feather barbules. In contrast to iridescence in most other birds, the melanin-containing platelets inside the barbules only serve to define the layer thickness of the keratin without contributing to the iridescence in any other way. Unlike previous work where keratin was assumed to be transparent, it was established that both keratin and melanin are absorbing and that keratin exhibits an anomalous dispersion in the green, blue and near UV.