I have a broody sitting on 5 eggs. Today is day 19 and broody left the nest to eat. She only left the eggs for about 3-4 minutes. I thought that from day 18 they sat and didn't move till the eggs hatched?
I agree with CTKen, as usual. God made chickens dumb as dirt, at least mine. But the one thing they excel at is hatching eggs. They know when it is okay to get up and move around and exactly how long before they need to go back. My main broody hen may get up just long enough to eat, or she may stay off the nest for a long time, even up to day 21. They know what they are doing. Trust nature.