Despite common misconceptions, chickens are NOT vegetarians. Chickens will of course eat grains and seeds, as well as vegetable peelings and such. But they will also eat small insects, worms, mice, lizards, small snakes, etc.
As stated previously, chickens are not vegetarians. So like humans, you want to ensure they have a well-rounded diet. If you have plenty of land, you can directly pasture your hens and just supplement them with some grain and oyster shell. Most of us aren’t that lucky so you’ll have to let your hens scratch around for whatever bugs and worms they can find and give them feed daily. What the composition of the feed is can be up to you and requires a bit of research on what is currently available in your area. Generally I recommend avoiding corn, soy, cottonseed, and distillery by-products as well as GMO foods.