The foam isn't white, it's the brown color of his poop. It's not really like foam, but there is little to no liquid in his poop, but he drinks the same as he normally does.
I have a few week old duckling and have a question about his poop. He recently has been switched over to pellet food from a feeding store. He used to eat scrambles eggs with some finely chopped fruit and water. Before, his poops were liquidy with a solid part in the middle. Now, they are solid...