Usually the heat makes them hatch early. If it’s happened more than once I would get more than one thermometer and see what each of them says. I use glass thermometers not digital. Digital ones tend to be off and the thermometer on the incubator is usually never spot on.BYM.
Ducks are pure Saxony, blue Swedish, Rouen & Welsh Harlequin.
Drake: Rouen mix
What’s your humidity at till lockdown?
Sorry if you already answered that