A cross section of the rami from head feathers in this species (Hudon 2005) shows a basal layer of dark melanin around the medulla, surrounded by a wider layer (described as a "homogenous cloudy cell"). Interference of light through this cloudy layer, when backed by the dark pigment, results in the blue that we can see. When feathers are green, they have a thin layer of yellow pigment in the cortex outside of the cloudy cell layer. Blue feathers lack pigment in the cortex layer.
Authors have suggested that the yellow pigment wears away, so that the bird turns blue over time. Why are other green birds not prone to this color change? Perhaps the layer of pigment is particularly thin in Cissa species or perhaps there is some other reason why the pigment is more easily lost.
- Hudon, Jocelyn. Considerations in the Conservation of Feathers and Hair, Particularly their Pigments.â CANADIAN ASSOCIATION FOR CONSERVATION, 31st Annual Conference, Fur Trade Legacy Workshop. Jasper, Canada, 2005. 127-147. http://www.royalalbertamuseum.ca/natural/birds/birdlist/_pdf/Hudon1.pdf
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