Multi-periodic photonic mechanism for non-spectral magenta coloration in Anna's hummingbird ( Calypte anna ) feathers

Abstract

Hummingbird feathers generate iridescent colours through light interference within multi-layered arrays of platelet-shaped melanosomes embedded in corneous beta-protein. In Anna's hummingbird (Calypte anna), the gorget feathers produce a non-spectral magenta hue with multiple reflectance peaks in the red and blue spectral regions. The primary red peak has been attributed to the melanosome stack; however, the optical mechanism underlying the secondary blue peak remains unexplained. Here, we identify a previously uncharacterized intermediate-sized melanosome layer and demonstrate that it functions as part of a multi-periodic multi-layer reflector, establishing its essential role in secondary blue peak formation. Electron microscopy and tomography revealed a continuous melanosome size gradient across the barbule depth, with a superficial small melanosome, two intermediate-sized melanosomes and progressively larger melanosomes in deeper layers. Optical simulations indicate that the secondary blue peak depends on optical interaction between the top and middle layers, while the deeper multi-layer strengthens and sharpens the dominant red peak through multi-layer interference. Oxygen plasma etching showed that removal of the superficial layer eliminated the blue peak, further supporting this mechanism. Our findings reveal that the non-spectral magenta coloration arises from hierarchical optical interactions across graded melanosome architecture, providing a biomimetic design principle for non-spectral reflectors.