Are both knees swollen, what are you feeding, have you checked the leg for any sign of Bumblefoot, how old is the duck, does the leg feel warm, how long has the duck been immobile, and is a vet an option? Please answer all the questions.
Both the knees are swollen, I’ve been feeding it shredded corn, chick food and Purina duck feed pellets, it doesn’t look like it has bumble foot(I will double check tomorrow), the duck is going to to be 2 mouths old in 2 days, the leg feels warm, it’s been like this for 2-3 weeks, and the vet might not be an option
The timeframe the duck started showing symptoms leads me to believe it may have been niacin related, could you read this thread, and tell me if the symptoms match up with your duck?