How do iridescent colours in hummingbirds and butterflies change with viewing angle?

In hummingbirds and butterflies, iridescent colours are produced by structural features rather than pigments. These colours arise from thin-film interference, where light reflects off multiple, ultrathin, and regularly layered structures—such as the barbules of feathers or the scales of wings—and interacts to reinforce or cancel specific wavelengths.1234

Because this phenomenon depends on the precise path of light through these layered films, the perceived colour is highly dependent on the angle of incidence and the viewer's orientation. As the viewing angle changes, the distance light travels through the layers shifts, causing different wavelengths to undergo constructive or destructive interference. Consequently, the reflected colour appears to change or "shift" as the organism moves or as the observer changes their position.1352

In many biological systems, these iridescent structures are backed by a dark layer of melanin, which absorbs nonreflected light and intensifies the brilliance of the colours. This dynamic optical effect is frequently utilized in animal behaviour, such as in the courtship displays of male hummingbirds, which may orient themselves specifically to ensure the light reflects the vibrant colours of their gorgets toward a potential mate.1467