I so wish I had taken notes when I experimented with chicken and duck eggs;
I found 8 chicken eggs in nest boxes, then a few days later while pulling weeds and trimming trees I found 6 duck eggs scattered all over in the tall grass, a young duck was laying but not sitting. I thought it would be fun to see if any of them hatched. I didn't really know any better and just put the duck eggs in with the chicken eggs. I would turn eggs twice to three times a day, made sure the were laying sloped just a bit. Reduced the temp once I started noticing pipping, I was not sure when any of the eggs where laid. They all could have been anywhere from 2-3 days for the chickens and a week or more for the ducks, so I had no idea when to start "lockdown". Anyway, I reduced the temp, if I remember correctly ducks like a lower temp anyway(?? do I remember correctly??), and would continue to turn eggs that weren't hatching. Made sure to keep water levels good for humidity and set the temp at a duck setting and wouldn't you know it, everyone of the eggs hatched!!!! All of them..... I couldn't believe it. Eggs found around a farm with no mommas setting on them and all hatched. I lived in a very small place at the time and with all these eggs I ended up overrun with babies.
The key I think, in part, was when I removed any chicks I waited until they were full hatched and mostly dry. Then I would barely crack the incubator and slip the chick out. I would also put a towel over the incubator to help cover it when I turned eggs, that helped keep the heat in. I can't remember everything I did, and that is why I wish I had taken notes and paid more attention, but I really didn't think any ducks would hatch and figured maybe one or two of the chickens, but not everything.
Maybe the entire deal was beginners luck, who knows. But I was pretty careful with the heat and humidity, timing and very careful handling.
Good luck with your ducklings, they are some of the cutest little buggers when just hatched
