I'm sorry I don't have an answer for you. As I read, I thought perhaps the duckling was being plucked by the others, but then I realized you had considered and rejected the possibility.

What are you feeding them? According to the Storey Country Wisdom Bulletin on Raising Ducks and Geese, ducklings need a feed with 22 - 24 percent protein for the first two weeks and then 16 - 18 percent from two to twelve weeks. Growing feathers takes a lot of protein... It is JUST possible that if there is a bit of a protein deficiency that it could cause a problem with feathering out.

Ducks are not my birds (prefer geese and also have chickens) but I thought I'd throw in my 2 cents worth on the off chance that it is helpful.

My thought precisely - thanks for the reply. It came to me when I was mowing the lawn, focusing on the green clippings spray through the ole reel mower - not behavioral, aint catchy. Must be an individual problem. Eureka - nutrition! I have them on duckling pellet from the feed store. Wanted to stay away from mash or chick feed. Technically, he's a week away from green stuff, but I'm going to push him along a little, grit first, then maybe spinich (good for protein and more). Hope it's the right decision. The touchy part is that this one has such a magnanimous personality... walk right over to my busy hands inquisitvely while the others huddle in a corner. Makes us want to work that much harder to find a solution for him. Will keep ya posted.

Yes, please post again to let me know how he is doing. The fact that he is so friendly will make it easier for you to give him special treatment. Would some bugs or earthworms perhaps be good sources of protein for him? I'm glad to hear that you will be offering grit. I think it is important. Good luck with him!