Why do hummingbird and butterfly colors appear to change with viewing angle?

The colors of hummingbirds and butterflies appear to change with the viewing angle due to a phenomenon known as iridescence, which is a result of thin-film interference.

Unlike colors produced by pigments, which absorb specific light frequencies, these iridescent colors are created by the physical structure of the animal's surface, such as the feathers of a hummingbird or the scales of a butterfly. These surfaces feature multiple thin layers of reflecting material.123

When light strikes these layers, it reflects off both the front and back surfaces. Because the layers have a specific thickness, certain light wavelengths are reflected out of phase—meaning the crest of one wave coincides with the valley of another—causing them to cancel each other out. The remaining light waves, which are in phase, are visible as intense, metallic colors. Because this interference depends on the angle at which the light hits the layers and the angle from which the observer views them, the perceived color shifts as the animal or the observer moves.314