Cu is the symbol for the element Copper. Taken on an overcast day, you can see the detail, and color variations, clearly in the gorget of this male Allen's Hummingbird. On a sunny day, what is copper colored here, would be bright red. It would be so bright that most detail would be lost. This guy likes my yard. He has been hanging out for months. Usually males stop by for only a couple days. There are a lot of Allen's girls in the yard these days, so maybe it's a 'birds and bees' sort of thing.