Have you looked at the bottom of his or her feet to check for bumblefoot? She doesn't have bumblefoot, it looks like normal and the same as the others.
What are you feeding the ducklings? They're on milled corn and layers pelletts for the hens
How old are they and how long has the duckling been limping? I noticed it over the weekend so about a week if not more
What kind of housing, bedding and pen do they have? They have a cat box that they hardly go in with straw bedding in their run
Any swelling of joints? Not that i've noticed
Just hate the idea of her in pain
I think mine is just an odd one thats slow to develop. I'll try be with her as much as i can and let her swim whenever. I was kinda hoping it was just something to do with the breed and she hasnt got any serious developmental issues. Shes so sweet and i've just names her Millie so shes a keeper