Investigating the Origin of Reddish-Pink Coloration in Male Anna's Hummingbird (Calypte Anna)

Coloration in organisms is important for visual communication, natural selection, camouflage, and more. The colors can be produced by absorbing light with pigments or reflecting selective light by nanostructures. Structural colors are classified as iridescent colors and non-iridescent colors. Iridescent colors in birds are produced by crystalline arrangements of melanin granules (melanosomes) surrounded by keratin and/or air. Hummingbirds are famous due to their bright, diverse, and iridescent plumage colors. Anna's hummingbird (Calypte anna) has distinctive iridescent reddish-pink gorget and crown feathers that reflect both red and slightly blue light. The color of Anna’s hummingbird feathers is generated by stacks of hollow, platelet-shaped melanosomes that function as optical multilayers. The advantage of hollowness is that air has a low refractive index, which enhances brightness and saturation of colors. The outer keratin cortex and small, superficial melanosomes are considered to contribute to secondary reflectance peaks in the short wavelength range. The influence of structural parameters on hummingbird coloration has been studied primarily by inferring the optical effects of structure through optical modeling or cross-species comparison. These approaches still require further understanding of how these structural components affect coloration.
In this work, we investigated the optical mechanisms behind the reddish-pink coloration in Anna’s hummingbird. We focused on the optical effects by dividing two layers: the top layer is composed of the cortex and the uppermost smaller melanosomes, and the underlying layer is a stack of larger melanosomes. To understand the effect of the top layer, oxygen plasma etching was performed on gorget feathers, which resulted in the shift of color from reddish-pink to reddish-orange and eventually to yellowish-orange. We confirmed that only the top layer was etched using scanning electron microscopy with a back-scattered electron detector (SEM-BSD). Therefore, we deduced that structural changes of the top layer after the etching process affected the color change. We used the finite-difference time-domain (FDTD) method to gain a deeper understanding of how structural variations in various parameters, including melanosome and air cavity thickness in the top and underlying layers, affect optical properties. Additionally, the internal nanostructure was reconstructed in 3D tomography using serial-section scanning electron microscopy and analyzed to expand our understanding that was previously restricted to 2D imaging. Consequently, optical interaction between the top and underlying layers is crucial to the unique color of Anna's hummingbird. These findings provide new design strategies for modulating color and enhancing the performance of various nanophotonic structures.