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Well, anything organic. Their main commercial ration should make up 90% or more of the diet, but you could supplement them with small amounts of fresh organic fruits and veggies - some great options include lettuce, tomatoes, and watermelon - chickens especially love watermelon. If you can find it, organic scratch would make a great treat, but make sure to feed it as a treat only - it's a lot like candy for chickens and chickens, like children, will eat too much of it if given the chance. You can even offer items like meat or dairy - it is actually very healthy for them to have small amounts of these high protein foods, as they are naturally omnivores. Some great examples would be ground beef, scrambled eggs, and yogurt.