Have you ever wondered why sometimes you can see the brilliant red gorget (throat feathers) of the male Ruby-throated and Allen’s hummingbirds, and sometimes not at all? Or why iridescent feathers appear green, and then blue, or possibly purple, and then in the next moment look drab and dreary? I think about this when photographing birds such as grackles, buffleheads and hummingbirds. Most recently, the turkeys in our community are currently displaying their wildly varying iridescent feathers when in full courtship mode.
Iridescent red gorget in male Allen’s Hummingbird; same bird, different angles
There are two types of structural color, layering and scattering. Iridescence in bird feathers is created by layering. Bird feathers are made of a translucent protein called keratin, which is a very rugged substance. Not only are the feathers made of keratin, but keratin coats the bird’s claws, legs, and bill. Because of the structure of the feather, with its microscopic barbules, when light hits the feather it causes the wave lengths to bend, or refract. Keratin reflects short wave length colors like purples, blues, and violets. The other colors are absorbed by the underlying layer of melanin. The refraction works like a prism, splitting the light into an array of colors. As the viewing angle changes, because of the viewer’s movement or because the bird is moving, the refracted light displays a shimmering iridescence, or none at all. Beautiful color combinations are created when iridescent layers are combined with pigments present.
Keratin is interspersed with tiny pockets of air of within the structure of the feather filament (called barb). Scattering is created when light hits the pockets of air, which results in specific, non-iridescent color. The color blue in feathers is almost always created in this manner. Feathers of Blue Jays, Bluebirds, and Indigo Buntings are prime examples of scattering.
Here are two graphics found online from Cornell that I found very helpful in trying to visualize the difference between layering and scattering. The first shows how iridescence is produced and the second, how blue scattering is created.