When the sun hits the feathers on an adult ruby-throats’ gorget they appear brilliant red. Then when he shifts his head a little the gorget can appear black. That’s because the iridescent feathers on the throat lack true pigmentation. The ruby red color our eyes see is actually the product of the feathers’ structure.
The iridescence is caused by refracting light waves off minute air sacs or platelets in the feathers, sort of like when light hits a soap bubble. The different colors produced are often a result of pigmented melanin within the platelet.
Possible functions for the flashy feathers may be to attract a mate, signal social status among males or communicate a threat. Young hummingbirds, which need to intrude on adult territories to feed once they have fledged, all lack gorgets. This may help to make them less visible or less threatening to adult birds.