This is a continuation of a recent post on how feather colors are achieved in birds. To recap that discussion, feather color is a product of pigments, or micro-structural features within the feather, or both processes.
Some of the most striking colors produced are those in iridescent feathers, like the crown and back feathers of Common Grackles which turn from black to blue in the right light,
or the shimmering greens, golds, and blues in peacock tail feathers, (photo from Wikipedia)
or the throat feathers of hummingbirds, which seem to light up and flash intense color as the bird turns its head (Scintillant Hummingbird photographed in Boquete, Panama, January 2012),
(Throat feathers look brown when light is not shining directly on them.)
or the brilliant contrasts in neck and wing feathers of the Nicobar Pigeon (photographed at the MN Zoo, not in the Nicobar Islands east of Sri Lanka, unfortunately).
Iridescence is a result of optical interference between two or more transparent to semi-transparent surfaces, like that produced by the thin film of a soap bubble that seems to change color as it rotates through the air (photo from Wikipedia).
The iridescent quality is believed to be caused by multiple reflections from such semi-transparent surfaces where interference between the reflected light paths modulates the color observed. In fact, microscopic studies of hummingbird feathers reveal that their surface is constructed of layers of elliptical plates resembling a tiled floor. These reflective surfaces produce the interference optics that results in the shimmering colors of iridescence. It is likely that similar structures are present in the iridescent feathers of other birds.