Magnificent magpie colours by feathers with layers of hollow melanosomes

Abstract

The blue secondary and purple-to-green tail feathers of magpies are structurally coloured due to stacks of hollow, air-containing melanosomes embedded in the keratin matrix of the barbules. We investigated the feathers’ spectral and spatial reflection characteristics by applying (micro)spectrophotometry and imaging scatterometry. To interpret the spectral data, we performed optical modelling, applying the Finite-Difference Time-Domain (FDTD) method as well as an effective media approach, treating the melanosome stacks as multilayers with effective refractive indices dependent on the component media. The differently coloured magpie feathers are realised by adjusting the melanosome size, with the diameter of the melanosomes as well as their hollowness being the most sensitive parameters that influence the appearance of the feathers.