My duckling have been having this issue for a month now. He walks like he has no knees, and his tail are tucked under him like a dog who had done something it should not have done. Every time he tries to go stinky, his tail covers his vent, I think, then he flaps his wings, then sits down like he is tired from all that work. He is always lagging behind everyone. I free range them. Here is my other normal ducks. What should I do with him? Cull him? Thanks for any help.