I've got a black runner duck from this year's hatch and I'm new to ducks. He's limping- I looked at the pad of his foot and I see one little scab that hasn't gotten better or worse with vetricyn treatment. It does not match the bumblefoot description. He's with three other ducks and two geese and they are all eating game bird food and mixed veggies and greens, no one else has problems so I don't think it's dietary. What else should I check?