Broody hens are responding to a natural urge to sit on a clutch of eggs long enough for chicks to hatch. Although most city hatched eggs aren't fertile, it doesn't matter to the hen. An egg is an egg in her books, and she usually gets quite insistant that she's going to hatch them!
Depending on whether you want a broody hen (some people like to have their hens hatch out fertilized eggs, which you can buy), or not, will determine your response. Below are some ideas.
What To Do If You Don't Want Broody Hens
Broody hens are especially hard in the city because most cities have such tight rules on how many hens you can have in your backyard. If you're at your limit, new chickens might not be a possibility. If this is the case, here are a few ideas that may (or may not) work to help your hen get over this broody period.
Remove Any Eggs
Immediately remove eggs that your hen may be sitting on. If she lays more, remove them too. Keep her nest empty. This step should be taken regardless of what other measures you try.
Place Her in a Raised Wire Cage
The theory behind this method is that if she's in a wire cage that's off the ground, the cool air will help cool her body temperature down and allow her to get over the broodiness. Don't put anything in the cage except for food and water. (This is one of the 'old timer' solutions and it's also the one that's worked the best for me.) Depending on how many days the hen is broody before going into 'lock down' will determine how long she will have to stay before she's no longer broody. If you catch her fast, she'll only have to stay a day or two, if you wait, she might have to stay in the cage for longer.
Lock Her Out of the Coop
If you allow your hens to free range, lock the coop up when you let them out. This will keep the broody hen from returning to her nest. Sometimes this will help her forget her urges.
Place Ice Cubes Under Her
Again, this theory is based on the 'cool her down' idea. If you cool her body temperature down, she may decide she doesn't want to be broody after all. Several rounds of ice might be needed. (I tried this theory once with one of my hens, and I gave up before she did...but in the process I got a funny video...I used plastic Easter eggs to freeze the water in...but she'd just melt each new batch under her and never gave up.)
Pair Her with an Active Rooster
A rooster might not tolerate a hen just laying on a nest. His attempts at mating with her might drive her off the nest.
What NOT to Do
Don't be mean! Some people report dunking their hen in a bucket of ice water to cool her down. Think about it! It would be like jumping into a glacier lake when you have a fever! A huge shock to the system and not very nice.
How Do You Know She's Not Broody Anymore?
Your hen could look and act completely normal while she's locked in a cage and you might be excited that she's no longer broody...only to watch her puff up and get testy once she gets around the rest of the flock. Or, the second she has the opportunity, she runs for the nest boxes again...both of these are a good indication that she's still broody and will need more time.
When she's truly broken of her broodiness, she won't fluff up and the nest boxes won't have any mysterious draw to her. It will still take a bit of time for her to start laying eggs again (part of this depends on how long she was broody before she was broken of it), but other than that, she should be acting completely 'normal' again.
If You'd Like Broody Hens...
...then when you get one that goes broody, let her hatch some eggs! You can buy fertilized eggs online, but if you look around locally, you might be able to find some from a nearby farm. When one of my hens went broody, I let her hatch some eggs I got from a friend just outside the city limits that has a rooster in her flock.
Get a few more eggs than you think you'll need because they might not all hatch (the last batch I hatched out was a clutch of 9 eggs and 7 hatched which is sometimes a better than usual percentage).
If the whole egg thing seems like too much for you, then wait until she's been on her nest for approximately 21 days. Then, stick a chick or two under her. The chicks can't be any older than 1 to 2 days. If they're older than that, you run the risk of them not imprinting on their new adoptive mother, which would be a disaster.
A nice advantage about buying chicks is that you can pick both the breed and the SEX of the chicks before you bring them home. Many fertilized eggs you can buy are a mixed bag of breeds, and you run a 50/50 chance of hatching roosters (the last batch I hatched had 7 chicks, 4 of which were roosters).
Broody Hens Breeds
There are actually some breds of chickens that can't procreate without help (an incubator or a stand in hen) because they simply won't go broody. This is because as they've been bred, the tendancy to go broody has been bred out of them (if you're into egg production a broody hen is costly. She won't lay eggs for the 21 days she sits on her nest and she won't lay them for awhile afterwards as she raises her chicks).
If you'd specifically LIKE to buy hens that have a tendancy to go broody, check out the list of broody hen breeds. (Even if you don't especially want a broody hen, they're often the birds that have the best temperment for children).
Non-Broody Hens Breeds
If you'd like to steer clear from dealing with a broody hen, their might not be any guarentees, but here are some breeds that generally don't go broody. These birds are generally known to be good egg layers, but may or may not have the gentle personality of a hen that has the tendency to go broody.
Would you like to know more about chicken breeds?