This just my observation. Delfin's first moult made him look ratty, in poor health and a little grumpy. But I noticed that there were certain areas of the body such has the head and chest which seem to replace a large areas of feathers in a short period. Yet the wings and tail feathers were replaced in smaller numbers over a longer period.
This could be that the body feathers have a smaller impact on the birds flight ability and more on the birds camouflage, whereas the wings and tail feathers have the opposite, so if they lost a large amount of the wing and tail feathers then their flying ability could be comprised and if they looked ratty for too long a period then their camouflage ability becomes comprised, and this wouldn't be good for a bird in the wild if they needed to hide from or out-fly a predator.
During the Moult, Delfin really loved his showers and enjoyed me giving his head, neck and face a hand with preening.