Okay thanks!!I am kinda new too, and I had the same concern. But most hens get off the nest for no more than half an hour( enough time to eat drink and poop without the eggs getting too cold) every day, give or take, and will be fine. The first time I let my broody hatch eggs I put a small plastic cup filled with food in the nest box for her, she spilled the food and sat on the cup lol! And yes the hen will get skinny. And make sure the nest box is on or very close to the ground, is the chicks can get out when they need food and water. Most of the time you will never see her get up. I hope you have a good hatch!
