Have you been supplementing with any form of niacin? very important that waterfowl babies get niacin from hatch till around 10 weeks of age. especially feeding chick starter but even feeding the best duckling starter there can be one that will come up with a deficiency. Not saying this is what it is.
What is the temp in your rabbit hutch? at 3 weeks ducklings should be kept at around 75*
What is the temp in your rabbit hutch? at 3 weeks ducklings should be kept at around 75*