From Cell to Barbule the Optics of Iridescent Bird Feathers

Abstract

There are brightly colored birds spanning the globe. These fabulous animals get their colors from a variety of different sources. Some birds use pigments while others have specialized shapes and forms creating optical phenomena such as thin-films, and photonic crystals among others; within the cells of their feathers to reflect and refract light to create vibrant colors. The spectacular Bird-of-paradises’ feathers are just as incredible as the birds themselves. The male Parotia wahnesi Bird-of-paradise puts on an elaborate show to attract a mate, part of this show is flashing an iridescent ornament from under his chin called a breast ornament. To the female sitting on the branch above the male, it is a brilliant yellow, but when the ornament is flush against the bird, it reflects a variety of colors from purple to green. The colors from the ornament feather, can be modeled as a structured thin film using non-sequential optics software. The models tested were created from a digitized shape of a cell and barbule of the ornament feather. There is a combination of .05µm of melanin with a refractive index of n = 2+.01i and .128µm of keratin with a refractive index of n=1.5 in 31 alternating layers overlaid on the topography of the bottom of the barbule. The outer shell of the cell is created by a single 21µm layer of keratin. Together this created a vivid color pattern which closely matches measurements and observation in the real world. In contrast to the vivid Whanes’s Parotia, the white tip of a Mourning Dove flight feather is also examined. The feather, composed of keratin and air, lacking any organization as well as any recognizable outer structure.