Yesterday I didn't see one of my hens all morning, so at lunch I checked on her and found her sat happily in the nest box. She'd pulled a load of feathers out and was sat on an egg. I moved her and removed the egg. she joined the others for the rest of the day. Today again, she sat in the nest box on an egg, she'd pulled some more feathers. She's happy enough to come out when I move her and take the egg away... how can I stop her?