What is eclipse plumage?
After breeding, ducks moult (replace the old, worn-out feathers with new ones)
Eclipse plummage is temporary or transition plummage.
Ducks are peculiar in that they moult all their flight feathers; the long, wing feathers; at once. For about a month, they can't fly and 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.
Once the flight feathers have regrown, the birds moult again, and by October the full colours are back and the various species of ducks are easily recognisable once more. Phew!