I would recommend you feed your duck either all flock, chick starter, or a duck feed depending on what you have available to you. The duckling needs lots of protein and vitamins, especially naicin. Getting the duck on proper feed will be the first thing you need to do. If you provide a pellet feed, the duck will need grit on the side.
Do you have an outdoor space for this duckling to live in once it is ready to go outdoor?
Do you have an outdoor space for this duckling to live in once it is ready to go outdoor?