Chickens are omnivores. When free-range, they eat seeds, berries, worms, slugs and insects. Our domesticated chickens have lost their instinct to know what is edible or not.
Complete food: Commercial food is available in mash (which is ideal for chickens in a small run as it takes an effort to take it, preventing boredom) and the easy-to-eat pellets (for the free-range chickens). It is best practice to feed them in their roost to stop mice and birds eating it.
Grain feed: Mixed grain should be provided as an additional food. Chickens love grain and it is an ideal way to make them get used to you and tame them.
Mixed grit: This should be always available. It is made of oyster shells, which contain high levels of calcium, which is important for the production of eggshell.
Green stuff: A daily small portion of greens will be appreciated by your chickens, suitable greens/fruits include: blackberries, lettuce, broccoli, carrots, young nettles, shepherd's purse, grass, dandelion, apples and pears. Always remove the leftovers as mouldy or withered greens can cause severe gastrointestinal complaints.
Proteins: Rain worms and mealworms can be given now and then.
Water: Fresh drinking water must always be available. Laying hens drink more water as an egg consists largely of water.