Just curious, I know they swallow grit and it stays in the gizzard until it wears out. But what happens if they get too much grit in their food or swallow something small that isn't grindable (like a pencil eraser)? Will the gizzard let the unwanted "grit" pass through, and if so will it pass on an item too big for the intestines to handle?
Can a duck pass swallowed items like a mammal can?
So, I found this where a pekin managed to pass a glass marble after 24 hours.
Not a complete answer and it is just one event, but it is a good start.
I have heard of ducks ingesting bits of things here and there and being fine - but it doesn't pass. There have been necropsies that show just how much their gizzard can hold. When my ducks eat a ton of grit, you can see it about an hour later in their poop. It passes right through.