How do you know if your chicken is broody? This is classic broody behaviour. She will be flattened, fluffed up, she will squeal if you go near her, she may even turn into chickzilla and try to peck you. She will go mental if you remove her from the nest box and making a "clocking" noise. The other previously mild mannered coop mates will turn on her especially if she is not a high ranking chicken. She apparently has ideas above her station. A Broody hen is to be cherished and you can see from the pictures here our India who is an Improved Indian Game is a perfect Broody, she had a brood of 20 (mostly duped her into adoptees) which she devotedly looked after day and night for 6 weeks until we gave her the time off. Normally a broody will carry on for about 8 weeks when all of a sudden she will say, "ENOUGH IS ENOUGH - PACK YOUR BAGS AND FIND YOUR OWN FOOD" she will then have nothing more to do with them. Not very different from what we do but we have to wait 18 LONG years to get to do that.
This is what happens when you let your broody follow what Mother Nature has planned for her. The alternative is what? What if you don't have the room? She wont be laying any eggs and is upsetting the harmony of the coop and pinching the others eggs so she can sit on them.
The term here is to "break" the broody or break the broody cycle. She will lay a load of eggs and then stop and sit. She has an incredible urge to sit and will sit on anything including golf balls and similar egg sized balls. She will not like to be taken off the nest, but moved she must be. The easiest and "almost" kindest way of doing it is to prevent her getting access to anywhere she can sit in comfort or make a nest. This includes preventing her getting her in the coop, or anywhere she can make a scrape to nest in. If she cant get into her normal nest she will seek out an alternative. She will go nuts trying to find her way back in and she will pace up and down trying every weak spot she can find. She may be diverted with a bit of bribery for a while but she will resume the pacing soon after. She may do this for 2 days and then she may just be broken. She can go into the coop at night for safety unless you have somewhere else she can go but you have to turf her out in the morning. Very determined broodies sometimes take an extra day. You can test her to see if she goes straight back on the nest. If she doesn't return to her broody behaviour she can rejoin her coop mates. I normally shut my broodies out of the run. They can see the others but cannot nest.
I have tried all the other "methods" such as dunking them in cold water, and hoping they will give up naturally. Take it from me they wont give up until they have lost loads of condition and dunking actually quite awful. It is a real shock to them and it doesn't work. You can also put them in a dog crate to make them uncomfortable which does work but is worse than my method. They cant run around and is more like entrapment. Either way depriving a broody of her nesting instinct is like trying to plait fog - its impossible.