As mentioned before, duck identification at this time of year can be difficult due to their eclipse plumage. After the breeding season all adult ducks have a complete moult including both body and flight feathers and, for a short time, are flightless. During this time the drab plumage of the females provides good camouflage from predators but the brighter males are obviously more conspicuous and therefore they initially moult into a female-style plumage so that they can hide away when necessary. This is called eclipse plumage.
Ducks such as Gadwall and Mallard are early nesters and therefore start their eclipse moult as early as June or July and are already mainly finished at this time of the year and sporting their new breeding plumage as can be seen by the following photos.
However, later nesters such as Teal and Shoveler are still in the middle of their eclipse moult and are only just starting to show signs of male plumage once more.