Dyck, J. Z. Zellforsch. (1971) 115: 17. doi:10.1007/BF00330211
The spongy structure in medullary cells responsible for the colour of blue barbs in rump feathers of Agapornis roseicollis and back feathers of Cotinga maynana is studied with the scanning and the transmission electron microscope. The Agapornis structure is an irregular three-dimensional network of connected keratin rods which in many places form rings with outer diameters 0.25–0.3 μ. The air-filled space likewise consists of an irregular network of connected channels. The Cotinga structure consists of spherical cavities fairly evenly distributed in a keratin matrix.
Earlier spectrophotometric measurements on the Agapornis structure have shown that the colour production has to be ascribed to the interference of light, not to Rayleigh (Tyndall) scattering as previously held. The optical path lengths corresponding to the outer diameter of the Agapornis rings and to the Cotinga cavities including their horny walls both are c. 0.4 μ. It is suggested that the colours are produced by the interference of light reflected from the front and rear surfaces of the rings, respectively the spherical cavities.