Ducklings do take longer to hatch, keep checking it. Can you remember how long it's been pipped? As you say I believe it's to do with moisture also. A hen does not go back to the nest wet as a duck would. The moisture a duck would take back to the nest would soften the shell but a hen would not. Waterfowl shell seems to be like concrete, I've got three goslings hatched now but I've had to finish the zip for the last two as they just could not get through the shell
To be quite honest this hatch of goslings is driving me mad as I don't like interfering and opening the incubator all the time. I've had to put a safety air hole in number 5 as it had internally pipped but there was no external pip. Number 4 has pipped at the wrong end so that will more than likely need help and number 6 is doing nothing, no draw down of air cell and I can't see any movement so I think it will be a late death for that one.
