My ducklings also picked up everything with their mouths. I think it is normal baby behavior as I have observed it in birds, humans, puppies, and kittens.
My ducklings repeatedly tasted EVERYTHING, including poop. And they were very attracted to shiny things like zippers. We regularly get reports of ducks ingestion metal objects and dying, so it is important to keep a duckling proof environment. Other things to watch out for are poisonous plants, sharp things, things that could cause impaction (like sand), and spoiled/moldy feed.