Integumentary biophotonic nanostructures are responsible for many saturated organismal structural colours such as vivid blues and greens often used in mate choice, camouflage, and aposematism. Despite a burgeoning interest on organismal structural colouration, there is very little consensus on their signal content. Here, we use the state-of-the-art materials science technique, synchrotron small angle X-ray scattering (SAXS) [1], and scanning electron microscopy to interrogate the development of the amorphous, nanoporous morphology in medullary barb cells of growing feather germs (pin feathers) of nestling Blue Tits ( Cyanistes caeruleus , Paridae). We show that the intracellular self-assembly of the barb nanostructure is consistent with a visco-elastic model of phase separation of polymerising beta-keratin from the cellular cytoplasm, given the lack of self-similarity along the proximal to distal axis of barb maturation. In addition, we document a wavefront-like progression of phase separation in the feather barb cells from the proximal base to the more mature distal tip of the feather. The final arrested nanostructure in mature barb cells is characterised by a slight increase in the peak spatial frequency likely due to volume shrinkage, suggesting that desiccation, a fundamental feature of apoptosing feather cells, plays an important role in the determination of the dominant length scale of the nanostructure. We discuss the implications for putative mechanisms regulating signal honesty.