|Eclipse plumage is found most notably amongst ducks. It replaces the breeding plumage after the breeding season and appears at a time when the main flight feathers are moulting.
Moulting is a process where feathers are replaced. This improves flight performance and maintains the waterproofing in the case of ducks.
Eclipse plumage tends to be much duller than normal plumage which makes a lot of sense if you are a duck without fully working flight feathers and want to keep well hidden from predators.
Male ducks acquire eclipse plumage towards the end of the summer but it is a fairly short-lived phase. Some male ducks begin to regain their colourful breeding plumage before their flight feathers are fully renewed so it would appear that the advantage of developing colourful breeding plumage earlier than your rivals in terms of successfully mating can outweigh the additional danger of shedding your camouflage before you can fly properly.