Notched wing and tail feathers are replaced by pointed ones. Males of most ducks have a different plumage than females during the breeding season.
Males are more showy to attract the female.
Some species are not greatly different from female, but still can be identified. ALL waterfowl molt after the nesting period and are flightless for a short period. During this time they are vulnerable to predation and being caught by biologists for banding. Males and females look similar during this period. Period varies with species.