Except for the flight feathers on wings and tail, the top third of hummingbird feathers lack barbicels and hamuli. Instead, they have elliptical structures called platelets (utterly different from the clotting cells in our blood of the same name) filled with microscopic air bubbles. The shape and thickness of the platelet and the amount of air determine the color seen. These air bubbles diffract light into colors that reflect back in a flash of iridescence.
