darkness birds only moult body and head feathers, but not all of them as they do in a full normal moult, the dark ybs ive had or seen don't moult all the feathers 100%, but enough to make them look like yearlings......they don't normally moult wing flights, but occasionally they do drop a flight or two.
ive heard of fanciers giving something called butter milk which makes them moult quicker...never used it my self so I don't know how well it works.