Eurasian Wigeon (Mareca penelope) I think this is a male in eclipse because although it looks like a female, it still has the white wing flash. This one is hanging out with the mallards at my local reservoir and has been around for a couple of weeks now, obviously unable to fly away.
Eclipse plumage is temporary or transition plumage. After breeding, ducks moult replacing their old, worn-out feathers with new ones.
Ducks are peculiar in that they moult all their flight feathers at once. For about a month, they can't fly and are very vulnerable to predators.
To provide some protection, particularly for the brightly-coloured males, the moult starts with their bright body feathers. These are replaced by dowdy brown ones, making them look much like females.
This eclipse plumage is why in mid-summer, it seems as though all the drakes have gone.