We have 15 birds in our henhouse, one of the barred plymouth rocks is spending all her time in the nest box. It was one of the nest boxes that all the other ones like to use, (now they changed up and are using other nest boxes) as far as I can see she is just spending all her time in it all by herself. I cant tell if it is one of the new rocks or old ones. Is this normal behavior? The new ones might be getting ready to lay for the first time?
Some chickens do it at a very early age. It mean they want to hatch eggs.If she is broody and you want some eggs to hatch put them under her. If you don't want her broody just put her in a wire cage. Some other posts have ways to break her.
Define all day. I have hens that are relatively new to laying and sometimes they sit in the nest boxes for 2 to 3 hours before producing an egg. Others that aren't laying yet have sat in the nest boxes for awhile as if they're practicing. A broody hen, which is not all that common in new layers, will fluff up and might even growl at you when you approach the nest. You might check that hen to see if she's maybe egg bound. Hold her in your arms and feel for a firm mass low in her abdomen.
Does she have eggs under her? Is she real protective of those eggs?? I have had some hens that seem to be spendiing all day in the nest box, but as soon as you try to see what is going on she jumps off the nest and leaves.