Computer Simulation Explains Iridescent Colours of a Bird-of-Paradise

The silvery neck feathers and the orange-green-blue iridescent breast feathers of the male bird-of-paradise Lawes's parotia form a sharp contrast to their otherwise jet black plumage. The role of these coloured feathers in the mating dance has only recently been unravelled, but their coloration mechanism remained poorly understood. In a recent PNAS article, Kristel Michielsen from JSC and her colleagues from the University of Groningen, the Netherlands, give an explanation for the various optical mechanisms causing the feathers' colours.

The coloration of both feather types is structural: the colour is produced by melanin rodlets arranged in layers, together acting as interference reflectors. The light reflection by the silvery neck feathers is unidirectional and well-described by multilayer theory. The reflection by the richly coloured breast feathers is three-directional and much more complex.

To unravel the reflection properties of the breast feathers, Kristel Michielsen and her colleagues applied finite-difference time-domain (FDTD) modelling. Transmission electron microscopy sections of feather barbules, the smallest branches of feathers, were greyscaled, extended into a three-dimensional simulation volume, and, subsequently, complex refractive index values were assigned to the different greyscale values. The FDTD modelling confirmed and refined the multilayer treatment of the neck feathers, and the modelling approach allowed for the first time a detailed explanation of the multicoloured breast feathers. It thus was found that the boomerang-shape of the barbules of the breast feathers enables the birds to change the colour of their chest plumage abruptly while dancing.

TDME3D, a massively parallel Maxwell equation solver, was used for the simulations, which were performed on the IBM Blue Gene/P of the University of Groningen. The calculation for one structure, one wavelength, one polarization state, and one incidence angle required a memory of approximately 150 GB. More information can be found in the PNAS article (DOI:10.1073/pnas.1323611111).
(Contact: Prof. Kristel Michielsen, k.michielsen@fz-juelich.de)

from JSC News No. 222, 16 May 2014