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What causes the iridescent colors in hummingbird feathers?

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Illustration of a chestnut-collared swift (Cypseloides rutilus) in flight, showing its dark brown body and wings with a distinctive reddish-brown collar around its neck. — Apodiform Chestnut-collared swift (*Cypseloides rutilus*)

The iridescent colors observed in hummingbird feathers are not produced by pigments, but rather by the physical structure of the feathers themselves. These colors result from the thinly laminated structure of the barbules, which are the minute, elongated components of the feather barbs.1

This structural effect is further enhanced by underlying deposits of melanin. These dark, light-absorbing layers of melanin intensify the iridescent displays by providing a contrast that makes the reflected colors appear more vivid. Unlike colors derived from pigments—such as the yellows and reds produced by carotenoids—iridescence is a form of structural coloration that changes depending on the angle of the light and the viewer's perspective.12