In most duck species, after breeding the birds carry out a very rapid complete moult and lose most of their flight feathers at the same time. This obviously makes flying difficult and males in full breeding plumage would be easily seen and vulnerable to predation. They therefore carry out a complete body moult into a female-like plumage and then moult back into breeding plumage in the wintering grounds. This female-type plumage is eclipse (not non-breeding plumage as it doesn't necessarily mean that the bird hasn't bred this year.
You will notice over the next few weeks/months that arriving ducks are all in female-like plumage and that the males will gradully start to grow brighter colours to attract a mate.
