I was trying to figure that out too! I don't know if it is once they have officially pipped or when they break the air cell inside? I don't know, but I wish it were a faster process! My last 2 hatches were so fast! They were all out within one day! This group is taking FOREVER! But then again, mine all all due on different days! I didn't realize she was sitting and everyone had been adding eggs everyday. So some look about 1 week behind the others. I am hoping that having 2 broodies will help. If one takes the babies, hopefully the other will stay on the remaining eggs. If not, looks like I will have to come up with something fast!