Violet Starling (Cinnyricinclus leucogaster) with feather detail. Iridescence in bird feathers is due to microstructures of the feather refracting light like a prism. Fossil evidence has shown that birds have had these structures in their feathers for at least 40 million years. EDIT: Structures like this have been found in the fossils of dinosaurs such as Microraptor, pushing back the earliest known evolution of iridescent melanosomes in feather structures to roughly 125 mya.

(photo by the Field Museum, Division of Birds, John Bates)