Open Access

Jan Dyck

• The morphology of green and blue feathers of the Rose-faced Lovebird (Agapomis roseicollis) is described from light-, fluorescence-, and electron microscopical findings and discussed in relation to earlier works. The description is intended to provide a basis for future comparative studies. Special attention is given to the colour-producing elements (pigments and the short-wave reflecting spongy structure ('Blaustruktur', 'cloudy medium') of specialized medullary barb cells (spongy cells, box cells)), and the findings are correlated with macro- and microspectrophotometric measurements. Green barbs differ from those of blue ba rbs in having their cortex yellow pigmented, but are further distinguished by their spongy structure which is denser (wider keratin rods and correspondingly narrower air-filled channels) than that of blue barbs. This difference corresponds to the wave-length of maximum reflectance being shifted c. 30 nm towards longer wave-lengths compared to that of blue barbs. Thus green barbs are not the same as blue barbs only with a yellow pigmented instead of an unpigmented cortex, as usually stated. Dark green hack feathers reflect approximately half as much light throughout the visible spectrum as do green belly feathers. This difference is due to variations in yellow and black pigmentation of the barbules. These variations are described quantitatively and the importance of barbules for the resulting feather colour is stressed. Variation in size and shape of barbs and barbules are discussed, principally in relation to their optical efIects and the presumed functions of the colours. The colour produced by the spongy structure cannot be explained by Tyndall (Rayleigh) scattering as is usually done. This follows from the shapes of the barb reflectance spectra which are not in agreement with the Rayleigh equation (scattering inversely proportional to lambda4). A new model for colour production is forwarded. It is based on a model of the spongy structure in which this is considered to consist of short hollow keratin cylinders (diameter 0.3-0.35 ft) with air-filled cores. Backscattering from these cylinders is considered responsible for colour production and good agreement is obtained between values of lambda max calculated from the model and those measured spectrophotometrically. The backscattering from the Individual cylinders can be regarded as an Interference phenomenon. The colour of the spongy structure thus is an interference colour. That it appears diffuse and not iridescent, as is generally the case for interference colours in feathers, is due to the presence of many hollow cylinders oriented in all directions in the spongy structure.