I have been learning so much here. I have 14 call ducks just starting today. In some photos I see the circles on the eggs at day 1,11, and 18 or so. Should I do this and what or why are you doing the circle?
I think that's the air cell, at the round side of the egg. It is to monitor the growth of the air cell, during incubation it will get bigger and for ducks it will be around one forth of the egg at lockdown.
If you incubate with too high humidity the air cell won't get big enough and you risk that the ducklings get sticky or drown. If the humidity is too low other problems can occur. Humidity should be around 40-45%.
It's like Kroelies says, we trace around the air cell during each candling to monitor the size of the air cell. That tells you if your humidity level is correct or not without having to weigh the eggs and calculate the proper weights they should have during incubation. Air cell size is the easiest way to monitor how they are doing.
Here's an example chart on how air cells should grow during incubation: