I read somewhere that a chicken is the pet with the lowest carbon footprint (although I've never been able to find the link again), presumably because they're vegetarian non-ruminants.
As Min says, you can't feed them anything that's been intended as human food (technically, the same goes for wild birds so stop putting out breadcrumbs for them now, you scofflaws). If you're harvesting veg you can give them the offcuts as long as you've not prepared them in your kitchen. Besides, if you actually want eggs from them you have to feed them layers pellets and proper mix so any eggs you get will be quite expensive but back garden hens' eggs are amazing.
You really need a minimum of 3 birds to keep them happy. Hens can't count so as long as they see two other hens they think they're in a flock and are fine. If they see none or only one, they get a bit nervous.