Keeping chickens in the back yard of a Brooklyn brownstone makes sense. You might not want to let them roam the whole yard since they do eat everything and leave the ground barren. A little pen where they have space and are protected from predators is good.
Here are some web sites where you can get more info:
My Pet Chicken – buy chicks and get breed info
NYC Chicken Meetup – chicken people and info on community gardens
Back Yard Chickens – national forum
Hand Crafted Homes for Happy Hens – nesteggcoops at gmail. They build coops.
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Mother Earth News has some very good chicken coop plans. movable etc. All online.