Upper tail coverts are the feathers that cover the base of the rectrices, the actual tail feathers. In most birds they are unremarkable. In the male resplendent quetzal, two of them grow enormously long, arching well beyond the tail and trailing behind the bird in flight. They are grown for the breeding season and shed afterwards.
The colour is structural rather than pigmented. Melanin-containing layers within the feather barbules are arranged so that interference produces green or blue depending on the angle of view, which is why a quetzal shifts between emerald and blue-violet as it turns.
Nest cavities are excavated in trunks so rotted that they crumble, and pairs sometimes lose a nest when the trunk collapses. The male's streamers are folded forward over his back inside the hole, and in some nests the tips project from the entrance while he incubates.