Maybe the ducks are early bloomers? Two of my chicks hatched on day 20. Did they pip and just quit? Maybe they are the type to hurry and externally pip but will take forever to zip?
Easy. Don't set up ducks and chickens together! Lmao.
For real though, no clue. I always assumed ducks laid slightly larger eggs than chickens? So maybe size? 🤔
Another way to tell is to mark them with a pencil ahead of time. Like D for duck and C for chicken. And the date on the bottom for...