If you are using chick starter, generally it seems to me to be a good idea to supplement with B vitamins, especially niacin.
People often get confused about what to use, so please stick with me here.
You have some options.
I would go with brewer's yeast. Forgive me for using capital letters, but for anyone else who may casually look over this thread, NOT baking yeast, NOT winemaking yeast. Brewer's yeast is a nutritional supplement and it provides niacin and a few other vitamins.
Ducklings need about three times the niacin chicks do. And some ducklings get leg problems or seizures if they don't have enough niacin.
If you cannot find the brewer's yeast, or if you happen to have (here come the capital letters again, please forgive me) PLAIN niacin - NOT no-flush, NOT timed release, then dissolve 150 mg niacin per gallon of water for the duckling’s drinking water.
Keep the little one on the supplements for about 8 weeks.
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Regarding switching from starter to maintenance or grower:
I started switching after their second week. I started adding a little grower to their starter crumble, increasing the amount of grower per starter every day until I had used up all the starter and they were on grower only. That took a week or two.