Nesting Hen Is Losing Feathers

[JT001 0]

Hi, I'm new to this forum and would appreciate some advice. My hen is nesting. She has 5 eggs and is spending much of her time in the box, as is to be expected. Lately, though, I've noticed that she's losing feathers from her chest. It doesn't look sore, just bare. There's plenty of seed and fresh water, along with a variety of fruit and a mineral block. The cage is cleaned once a week. Can anyone explain why this might be happening? Thank you.