It’s a characteristic of many ducks that after breeding season they lose most of their colorful plumage, and molt into drabber feathers. The Wood Duck demonstrates the difference very well.
In breeding plumage, the Wood Duck is pretty spectacular:
By contrast, a Wood Duck in eclipse plumage is a considerably less spectacular.
The reason for the drab plumage is interesting. After breeding ducks shed all of their feathers, not all at once but over a few weeks, including their wing feathers. For a brief period, they are flightless. The drabber appearance makes them a little less easy for predators to find while they are flightless or their flying is impaired. Once the flight feathers are grown back in, a matter of a few weeks or months – it varies some by species – the duck undergo a second molt, back into breeding plumage.
So eclipse plumages are a survival strategy. It comes at a considerable energy expense. Growing feathers is a major energy expenditure. Presumably, in the scales of evolution, birds that go through two molts survive a little better than those that do not, making that second molt worth the expense.
The main problem WC has with eclipse plumage, of course, is that it means winter is that much closer…