My other ducks don't have a natural pond either, but maybe snails are their substitute. They're always looking for things on the ground and in the mud; I just don't know what they're finding most of the time 🤷♀️.
I just noticed that the serrated edge of my duck Emma's bill has grown longer than usual. I couldn't get a good picture; it's more noticeable in person. She has been penned in a small area to heal from her broken legs, so I think she must not be able to forage enough to keep her bill worn down...