What if a female duck is sitting on rocks and thinks their her eggs?

It used to be a common practice to put stones under hens and ducks to encourage them to go broody. It will also keep them broody longer, and stones are good temporary substitute if you want to inspect a fertile egg away from the nest. Of course they cannot hatch stone eggs no matter how long the poor duck sits on them. In my experience, they do eventually get tired of the whole thing and will leave the nest after abut 45 days of broodiness (normal time from brood to hatch is 28 days in ducks). The unfortunate part is that once the duck is satisfied with the number of "eggs" under her, she will not lay eggs during the broody state, so there's no fresh eggs for breakfast and desserts.