Why do male ducks become flightless during their annual wing molt?

Male ducks become flightless during their annual wing molt because they shed all of their flight feathers simultaneously rather than gradually, as most other bird species do.12

Following the breeding season, male ducks typically undergo a molt-migration to a separate area, where they replace their bright nuptial plumage with a dull, female-like eclipse plumage. Immediately following this transition, they experience a simultaneous molt of their wing feathers. Because the birds cannot fly until these new feathers have fully regrown, they remain flightless for a period of three to four weeks. During this vulnerable time, they typically seek protection by skulking in thick cover or remaining on large bodies of water.321