Cockatiels diet is mainly sunflower seeds, millet, canary seed, wheat, peanuts, husked & unhusked oats, small amounts of hemp and a few others. To go with your cockatiels seeds, it should also be fed fruits and green food.
Fruits such as berries, pears, apples, citrus, bananas and other tropical fruits are enjoyed by cockatiels. They also enjoy dandelion leaves, chickweed, carrots, spinach & lettuce (mainly firm lettuce like chicory & endive). Your cockatiel can get a lot of nutrients from branches that you can use for perches by chewing at the bark to get important vitamins.
Around the time when your cockatiel is breeding, laying and rearing young, you can supplement the birds diet with special mixtures of feed that you can buy from the pets store than has added vitamins. You can also feed your cockatiel treats like, hard boiled egg yolk, cottage cheese, wheat bread that has been soaked in water and honey sticks, some of these foods will need to be removed from the cage as they start to perish.
Dosage of vitamins should only be given to your cockatiel if it is recovering from an illness or lowered immune system, you should always consult a veterinary professional about the correct doses.
Your cockatiel will also feed on grit, which helps the bird to process and grind up husked seeds in their gizzard. Cuttle bones and grated shells where they will source minerals helpful to their bones
Do not offer your cockatiel foods that you would eat such as cheese, chocolate, biscuits and other junk food, these can cause diarrhoea and make your bird unhealthy.