I DO give my birds all fruit and vegetable trimmings that have not started to mold or go bad, except of course the usual no raw potatoes or apple seeds or raw beans etc. but they love leftover hashbrowns and eggs from Ihop and fried rice...they will eat pizza (my Daisy hopped right up on my chair last summer and stole a pepperoni) they love cereal, they love when I give them the yogurt or sour cream tub when theres a little left, I even buy discounted produce and greens from the grocery store and they love those too. this week I bought 2 bags of cut rainbow chard and they ate both bags in a week. (Of course the snow here covers all of their forage so it's not surprising) They go bananas over bananas, and sunflower seeds and mealworms of course. The only thing I won't feed a lot of is meat. Daisy ate a mouse once because I couldn't get it away and Ginger caught a wounded sparrow (which I did manage to snatch from her) I know chickens are naturally omnivorous. I have fed them the occasional ham trimmings (and pepperoni lol) but we only buy organic meats so it isn't as bad.
What I would NOT do is feed anything that is poisonous (like apple seeds or peach pits) or anything that is going bad or moldy. I also would be hesitant to feed them restaurant scraps from an unknown source. I used to work in a kitchen and I know how much food people waste, it's awful, but I don't think I would want to take home any old anybody's food scraps...Also, I was at Fred Meyer, a local grocer here in town and asked them what they do with their old produce. The clerk said that they used to discount it first and when it didn't sell they would give it away. They did this until reports of people's animals getting sick came back and then they were forced to throw it out. Sooo, I would be a little leery about feeding any old produce...My organic fruit stand, however sells bags of old fruit for a dollar each and I bought several of those back this summer and any old melons they had because all I have to do is crack those babies into 3 or 4 pieces and the birds go crazy....they eat the seeds and all. It's a nice refreshing treat for the hot summer. I was always careful about cutting out any mold though...some varieties can make birds sick.