Chickens can work over kitchen scraps and garden weeds like nobody's business. If the kids don't finish all the food on their plate we usually just say, "Well the chickens have to eat, too." But I don't give them egg shells; I've read that encourages cannibalism, meaning they will eat the eggs they lay. And since we hope to get eggs in return for feed and water, eating the eggs before we get to them kind of defeats the purpose of having chickens in the first place. I also don't give them coffee grounds, although I'm not sure why.
I throw out a lot of egg shells and coffee grounds and it dawned on me there must be a good use for both. So I googled it and here are a couple of interesting links. I was surprised as to all the uses for coffee grounds. My favorites are garden fertilizer and ant repellent.