Out of all of the problems facing urban survivalist, fresh food and fresh water are probably at the top of the list. Sure there are lots of other problems, such as looters and other pest. But without fresh food and safe drinking water, life is going to go downhill pretty quick.
Why would chickens be a good choice for urban survival? They are easy to raise, they lay eggs just about all year long, the eggs are a good source of fats and protein, and if you need to, you can eat the chicken. The protein and the fats address at least two nutritional requirements of your long term survival plans.
Here is an interesting youtube video that talks about some of the aspects in raising backyard chickens.
Chickens are flock animals. Meaning they will not do well by themselves. If you are planning on getting some backyard chickens, plan on getting at least 3 or 4 of them. If you get 2 chickens, and 1 dies, then that puts stress on the lone chicken.