Because I can reflect light off my bright iridescent gorget to attract females and/or to warn off males.
Iridescent is an adjective describing surfaces that display lustrous, shifting, rainbow-like colors. The color display changes depending on the angle from which the surface is viewed or the angle of the light
Biology (Ornithology): Gorget : A vividly colored, often iridescent patch of feathers on the throat of certain birds, most commonly seen in male hummingbirds.
