Nanostructural basis of rainbow-like iridescence in common bronzewing Phaps chalcoptera feathers

Structural colors are common in nature. Generally single feathers or other integuments contain only one structural color, but those of the common bronzewing display a consistent color gradient from blue to red (462-647 nm) over the proximo-distal length of individual barbs. We used optical microscopy and macro- and micro-spectrophotometry to characterize this color gradient, and transmission electron microscopy to investigate the nanostructure. Combining optical modeling and experimental results, we demonstrate that the rainbow-like iridescence is caused by multilayer interference from organized arrays of melanosome rods in a keratin matrix and that the color gradient results from subtle shifts in both diameter and spacing of melanosome rods. This result illustrates tight developmental control feathers and may provide inspiration for the design of multi-colored coatings or fibers.