I’ll admit to being a bit confused. I think you put two duck eggs under a broody last Monday and you want to add some chicken eggs under the broody so they hatch together.
Different breeds of ducks hatch in different time spans. Most hatch in 28 days but Muscovy hatch in 35 days. Chicken eggs hatch in 21 days. You’ll have to figure out that math depending on your breed of duck.
You’re right. These are just approximate times. Any of them can be off by days depending on a lot of different things, heredity, humidity, how and how long they were stored before you started them, and just basic differences in the individual eggs. Average incubation temperature is an important variable but it’s not the only one. If the eggs hatch with 24 hours either side of the target tine period I consider them right on time. I’ve had some that were not within that window, even under a broody.
Hatching is not an instantaneous process either. It can easily take a day or two between pip and zip. It’s not an exact science.
As far as setting the second set of eggs, just do the best you can. Don’t worry about hitting the time of day exactly. If it’s within a few hours you should be good to go. There is that much uncertainty I when they will hatch and how long the hatch will take once it starts.
As far as when they will hatch I can’t tell you well enough so that you can be home for the process. There are just too many variables. Most of the time the hen will stay on the nest for at least a night after they start to hatch. If you have the flexibility in taking your days off, you can probably wait until you see one hatching and plan to take the next day off. It’s always possible she will come off early or she may wait another day. I can’t give you any guarantees here.
Good luck!
