Your ducks are still young! - Wait for the remnants of a huge grasshopper meal to re-emerge. Or after they have raided your tomato plants…I can’t really find a smell associated with their poop at all!
If you want to see some really disgusting poop, feed them some shredded red cabbage. The stains from that poop are even resistant to bleach and the stench make squirrels drop dead out of the trees around the duck-house. - But red cabbage makes the eggs look so much better - and who cares about squirrels anyways…