You can try pine shavings, in my experience many ducklings don't actually eat them, but they pick them up, realize it's not tasty and spit them out (lots of head shaking with this realization.)
I imagine they are somewhat similar to babies and puppies in that they don't really have a way of interacting with the world other than with their mouths.
Of course, some could just straight up be eating pine shavings, I've never really been that impressed with my ducks smarts.
Mine would eat shavings as ducklings. sigh.