We have a blue marans hen who has been sitting in a nest box for around 12 days now. Recently, our egg collection went way down. One day when I went in the coop, another hen was in the box with her. I watched her lay an egg. When I went to get it, I felt two more not covered by the wings of the sitting one. A few days later, I finally lifted her up to find 12 eggs under her! I could tell that only two were hers due to the coloring of them. I know that these eggs would not be pure bred if hatched because our roo is a cochin. Can I buy fertile purebred eggs and exchange them out under her now -12 days into the process? Will she continue to sit until they hatch or will she realize that it is taking way too long? Thanks!