I am sprinkling oregano throughout the coop and nestboxes as a preventative. It smells good too.
I know it’s used for chickens, haven’t heard about duck use.
Years ago I had a wood duck my son found by the river, it was just a day or two old. If I told the story before forgive me... but one day (and I'm not sure how old he was now) maybe 2/3 months? I walked around the corner of the house and he lost sight of me and started flying! he went further...